mountain flying
NOTICE: The following is a summation of the minimum knowledge areas needed to fly safely in the mountains. Further study and instruction (from a knowledgeable instructor) are required prior to flying the mountains. This minimum knowledge information is intended to supplement, not replace, your preparation for flying in the mountains. basic premises Without exception, you must adhere to the two basic premises of mountain flying, whether flying "with the mountains" or over the mountains.
Always remain in a position where you can turn toward lowering terrain The novice mountain pilot should plan to fly 2,000 feet above the terrain along the route of flight. When approaching within ½ to ¼ mile from the mountain ridges, turn to approach the ridge at a 45-degree angle. This permits an easy escape with less stress on the airplane if downdrafts or turbulence are encountered. Never fly in a canyon where there is not room to turn around.
Never fly beyond the point of no return.
When flying upslope terrain, the "point of no return" is defined as the position where, if you reduce the throttle to idle, you can lower the nose for a normal glide and perform a 180-degree turn without impacting the ground. At or prior to this point, circle away from the mountain to gain additional altitude before proceeding.
mountain meteorology
A complete check of the weather is necessary t o develop a go/no-go decision. Stay out of marginal weather areas. Winds aloft greater than 30 knots at cruise altitude usually means the novice pilot should delay or postpone the flight until more favourable conditions prevail.
Airport (TEX) at Telluride, Colorado, perched perched 1,000 feet above the San Miguel River on Deep Creek Mesa (about 4 ½ miles west of town. It is the highest altitude commercial airport in the U.S. Sinking air is a problem when surface winds reach 15 knots or more.
density altitude Density altitude is the altitude the airplane thinks it is at and performs in accordance with. High, hot and humid conditions may raise the effective physical altitude of an airstrip to a performance altitude many thousands of feet higher than its actual elevation.
runway length A handy rule-of-thumb for operating from a short runway is that if you obtain 71 percent of the speed necessary for rotation at the halfway point of the runway, you c an take off in the remaining distance. Note: This rule of thumb guarantees takeoff performance, but not rate-of-climb after the takeoff.
When flying upslope terrain, the "point of no return" is defined as the position where, if you reduce the throttle to idle, you can lower the nose for a normal glide and perform a 180-degree turn without impacting the ground. At or prior to this point, circle away from the mountain to gain additional altitude before proceeding.
mountain meteorology
A complete check of the weather is necessary t o develop a go/no-go decision. Stay out of marginal weather areas. Winds aloft greater than 30 knots at cruise altitude usually means the novice pilot should delay or postpone the flight until more favourable conditions prevail.
Airport (TEX) at Telluride, Colorado, perched perched 1,000 feet above the San Miguel River on Deep Creek Mesa (about 4 ½ miles west of town. It is the highest altitude commercial airport in the U.S. Sinking air is a problem when surface winds reach 15 knots or more.
density altitude Density altitude is the altitude the airplane thinks it is at and performs in accordance with. High, hot and humid conditions may raise the effective physical altitude of an airstrip to a performance altitude many thousands of feet higher than its actual elevation.
runway length A handy rule-of-thumb for operating from a short runway is that if you obtain 71 percent of the speed necessary for rotation at the halfway point of the runway, you c an take off in the remaining distance. Note: This rule of thumb guarantees takeoff performance, but not rate-of-climb after the takeoff.
Short-field takeoff
leaning the mixture For density altitudes of 3,000 feet or greater, lean the mixture for takeoff according to the airplane manufacturer's recommendation. Do not lean turbocharged or supercharged engines for takeoff. Lacking any recommendation, lean the mixture during the takeoff roll slowly until encountering engine roughness, then enrich for engine smoothness.
approach ridges Turn to approach ridges at a 45-degree angle to provide you the option of escaping toward lowering terrain. Begin this turn to approach t he ridge at the 45-degree angle when you are about 1/2 to 1/4 mile from the ridge. The visual aspects of mountain flying can be deceiving, but if you can see more and more of the terrain on the other side of the ridge you are approaching, you are higher t han the ridge and can probably continue. As you near the ridge, arriving at a position where the power can be reduced to idle and the airplane will glide to the top of the ridgeline, a commitment to cross the ridge can be made. At this position, the airplane is close enough to the ridgeline not to experience an unexpected downdraft of a nature that will cause a problem. If a downdraft is encountered, keep th e power on, lower the nose to maintain airspeed and the airplane will clear the ridge.
flying canyons Until you have the experience of flying canyons with a knowledgeable instructor, do not fly up canyons. If it is necessary to fly in a canyon, gain altitude, fly to the head of t he canyon, then fly downslope terrain.
airspeed control Landing at a short mountain strip requires exact airspeed control to eliminate float. A 10-percent increase in the proper approach speed results in a 21-percent increase in landing distance. Use the same indicated airspeed for approach when landing at a high-elevation mountain strip that you would use for the approach at a sea level airport. The thin air at high altitudes affects the airspeed indicator.
A rule-of-thumb states that the airplane flies faster than indicated airspeed at altitudes above sea level by approximately 2-percent-per-thousand feet above sea level. This is a built-in compensator for reduced lift caused by the thin air at higher altitude airports.
darkness Allow a minimum of an extra half hour of daylight if your destination is a mountain strip without runway lighting. There may be plenty of daylight at cruise altitude, but darkness may exist because of shadows at the valley destination.
Landing at Possum Creek airstrip, 10,010-foot elevation.
gross weight The takeoff distance varies with the gross weight. A 10-percent increase in the takeoff gross weight (while not exceeding the maximum allowable gross weight) will cause a: •
5-percent increase in the speed necessary for takeoff;
•
9-percent decrease in acceleration to takeoff speed, and
•
21-percent increase in the takeoff distance.
climb out The first consideration for takeoff from a strip surrounded by mountains is terrain clearance. A considerable amount of time may be required to circle, c limbing to the en route altitude prior to turning on course.
downdrafts Use visualization to determine possible downdraft areas. Air behaves like water. Ask yourself, "What would water do if it were flowing like the winds aloft?" You can then picture areas of downdrafts, updrafts and splashes of turbulence. If you encounter unexpected downdrafts, diving–away from the visualized downdraft–to maintain airspeed will generally lessen the total displacement effect of the downdraft (altitude loss). Although the rate of descent is greater at the higher airspeed, you will be under the influence of the sink for a shorter period of time.
course reversal Everyone flying in the mountains will encounter situations when i t becomes necessary to make a 180degree turn. Forget hammerhead turns, wingovers, chandelles and the other fancy manoeuvres. By the time you figure out you are in trouble and need to turn around, there is insufficient speed to perform these manoeuvres. To turn around, slow down. This will decrease the radius of turn. Pull back on the control wheel to trade airspeed for altitude if you have extra speed. Then make the steepest turn you can comfortably make, up to 60 degrees.
arrival The mountainous terrain surrounding many air strips prevents a normal descent from cruise altitude to pattern altitude. It is necessary to m ake progressive power reductions to prevent thermal stresses from being induced in the engine. This allows the engine to cool slowly, preventing not only thermal shock, but also preventing de-tuning. Always make smooth power changes when adding or reducing power.
c a u t i o n : This is not the total information you need to fly safely in the mountains. It is merely an outline of the minimum information that should be studied.
the psychology of mountain flying
True mountain flying—that is, terrain, contour or drainage flying, as opposed to flying well above the mountains—can be done with total safety only when the pilot becomes conditioned to apply the basic premised during flight, without having to think about them.
Always remain in a position where you can turn toward lowering terrain. This axiom also encompasses the idea that you will not e nter or fly in a canyon where there is not sufficient room to turn around. Another way of stating this truth is to have an escape route in mind and be in a position to exercise this option.
Do not fly beyond the point of no return. This is the position when flying upslope terrain where, if you reduce the throttle t o idle and begin a normal glide, you will have sufficient altitude to turn around without impacting the terrain.
C o n s t a n t l y e v a l u a t e where you are and decide if you can lose altitude before having to turn the airplane. If not, you are narrowing your options substantially. What happens when the pilot flies beyond the point of no return? First, and usually the less serious consequence, involves landing the airplane straight ahead into whatever terrain exists. This normally results in destruction of the aircraft, but with proper technique the occupants will survive. Proper technique means the airspeed is maintained to allow transition to a normal landing attitude (often upslope terrain) without stalling the airplane. The second outcome of flying beyond the point of no return involves the stall-spin accident. Because there is insufficient altitude or manoeuvring space to complete the turn around, the pilot may try to hurry the turn with excessive bottom rudder, thus yawing the airplane. This induces a stall-spin.
It is necessary for you to constantly think about the axioms of flight until you become conditioned to unconsciously remain in a position where you can turn toward lowering terrain and never fly beyond the point of no return.
problems in the mountains Occasionally the wind defies all common sense reasoning and visualization. When this occurs it is usually due to one or a combination of the following: subsidence inversion terrain modification valley breeze mountain breeze
Circulation (This discussion is limited to the northern hemisphere) A quick review of some basic weather phenomena helps make the point. Circulation refers simply to the movement of air about the earth's surface. The sun heats the Earth's surface unevenly. The most direct rays strike near the equator, heating the equatorial regions more than the polar regions. The equatorial region re-radiates to space less heat than is received from the sun, while the reverse is true at the poles. Yet the equator does not continue to get hotter and hotter, nor does the polar region get colder. The only explanation is that heat is transferred from one latitude to another by the actual transport of air. Warm air forced aloft at the equator begins to move north at high elevation. Coriolis force turns it to the right (east). This turning develops a strong band of winds, "prevailing westerlies," at about 30º north latitude. Similarly, cool air from the poles begins a low-elevation journey toward the equator. It is also deflected to its right by Coriolis force creating a belt of low-level "polar easterlies." The result is to create an temporary impasse that disrupts simple, convective transfer. The atmosphere seeks stability and in an attempt to reach equilibrium, huge masses of air overturn in the middle latitudes. Cold air masses break through the barriers, plunging southward. The result is a midlatitude bank of migratory storms with ever-changing weather.
Air Mass The large air masses are high pressure areas. In the northern hemisphere, high pressure areas circulate in a clockwise direction. The high pressure system depicted on weather maps should be visualized as a mountain of air. The mountain is composed of isobars or lines of equal pressure. Consider the isobars as topographic in nature. If they are far apart, the high pressure area has a shallow topography. When close together, there is a very steep slope to the mountain of air. Where isobars are close together it indicates the air is squeezed into a smaller, more confined area with a steep slope creating a rapid flow of air and strong surface winds. Between the high pressure areas will be areas of low pressure where the air flows counter-clockwise. Visualize the low pressure area as a valley between air masses. None of the pressure areas are stagnant. The earth's atmosphere is in a constant state of imbalance, but there is always a tendency to regain a state of balance.
Wind Three forces act on wind. The pressure gradient force drives the wind. Pressure gradient is the decrease of pressure with distance and is in the direction of greatest decrease, thus, pressure gradient is from higher to lower pressure and perpendicular to the isobars. If pressure gradient was the only force acting on the wind, wind would always blow perpendicular to the isobars. Rotation of the earth generates a force t hat deflects from a straight path any mass moving relative to the earth's surface. Coriolis force is zero at the equator and increases with latitude to a maximum at the poles. It is at a right angle to wind direction and is directly proportional to wind speed. Air in motion, due to pressure gradient, blows straight across the isobars from higher to lower pressure. When the air is in motion, Coriolis force begins to act at right angles to the wind, turning it to the right. Coriolis force continues to deflect the wind until is is blowing parallel to the isobars. Coriolis force and pressure gradient force balance, and above surface friction (about 2,000 feet), causes the wind to blow parallel to the isobars. The winds at the earth's surface do not blow parallel to the isobars. Instead, they cross the isobars at an angle from higher to lower pressure.
Frictional force always acts opposite to wind direction. As friction slows the wind speed, Coriolis force decreases; however, friction has no effect on pressure gradient force. Pressure gradient and Coriolis forces are no longer in balance. Above 2,000 feet AGL the wind blows parallel to isobars. Below that altitude, friction causes the surface wind to blow 45º inward toward a low pressure area and 45º outward from a high pressure area. Subsidence Variations in temperature and humidity create a contrast in pressure and density. The pressure differences drive a complex system of air currents in a never-ending attempt to attain equilibrium. Suppose an air mass (high pressure area) arrives over the plateau area of the upper Arkansas River Valley near Leadville, Colorado. The down flow, sinking are may be a stronger force than the prevailing winds aloft. The pilot departing Aspen and flying up the Roaring Fork River toward Independence Pass will be hard pressed to find an updraft in the face of this down flow. Yet it's always been there before. This pilot may be an accident waiting to happen. According to Aviation Space Environment Medicine, 232 airplanes crashed within 50 nautical miles of Aspen, CO, between 1964 and 1987. A t otal of 202 people died and 69 were seriously injured. This points out the need for better training in mountain flying.
Inversion Often there is a layer of air within the troposphere that is characterized by an increase of temperature with altitude. It is called an inversion and is usually confined to a shallow layer. Widespread sinking air (subsidence) is heated by compression and may become warmer than the air below it causing the inversion. The most frequent type of inversion over land is that produced immediately above the ground on a clear, still night. The ground loses heat rapidly through terrestrial radiation, cooling the layer of air next to it. Frontal inversions are also found in association with movement of colder air under warm air or the movement of warm air over cold air. In a valley, expect the prevailing westerly winds to flow down the east-facing side of the mountain on the downwind side, pass through the valley and flow up on t he west-facing upwind side of the next mountain. An inversion may place a cap over the area preventing the wind from flowing down the mountain. But when the wind strikes the terrain on the downwind side of the valley, it may tuck and move down the mountain side. With enough velocity, it may continue across the valley and up the other side.
Terrain Modification
fig.1
Figure 1 – Anabatic Lift Uneven terrain features may cause the air flow to be deflected downslope on what is considered the updraft side of the mountain. In the absence of wind, the sun's heating of the surface will produce convection currents known as anabatic lift.
fig. 2
Figure 2 – Valley Breeze During the day, the sun warms the valley walls and its adjacent air. The heated air being less dense will— lacking strong prevailing winds—rise gently upslope and is known as a valley breeze. The east facing mountain will receive the benefit of the sun's rays first and may cause a downslope wind on the westfacing slope as air rushes down to fill the evacuated air. The valley breeze begins early in the morning and depending on the elevation of the mountain and the heat of the sun, may reach a peak speed of around 10 knots by noon. The significance of t his is that when
landing on an airstrip in a drainage, there will be a tailwind to contend with. The average wind speed is 68 knots.
fig. 3
Figure 3 - Mountain Breeze During the late afternoon and evening the valley walls cool quickly, cooling a layer of air next to the slope. This more dense air moves downslope into the valley causing the mountain breeze (gravity or drainage wind). The slopes cool at a rate faster than they heat up, so t he mountain breeze may be stronger than the valley breeze, averaging 10-12 knots. Departing downslope will mean the airplane may be subject to the tailwind.
Coping We tend to think in constants when contemplating the weather and associate whatever is happening as affecting a large area. Often a phenomena is isolated or may crop up in various isolated areas. Despite what is happening or where it is happening, it is important to visualize what is going on. Air is fluid, similar to water—although less dense. Ask "What would water do in this situation?" More often than not the picture becomes clear, you will know where there are areas of lift, sink and turbulence. So what will happen to the pilot heading up the Roaring Fork River toward Independence Pass? As long as he remains in a position where he can turn to lowering terrain and does not fly beyond the point of no return, Mother Nature will not have a chance to perform a "got-cha." The "point of no return" is defined as a point on the ground of rising t errain where the terrain out climbs the aircraft. The turn-around point is det ermined as the position where, if the throttle is reduced to idle, the aircraft can be turned around during a glide without impacting the terrain.
Never fly beyond this point of no return. Turn around and manoeuvre for additional altitude prior to continuing.
density altitude 'Experienced pilots sometimes get into trouble with density altitude. It's not that they don't know what it is, it's just that they become complacent.'
It is essential that a pilot check the density altitude. Before any flight check runway lengths at airports of intended use, and takeoff and landing distance information ... also ensure that the aircraft will be able to perform with an adequate safety margin under the expected values of airport elevation and runway slope, aircraft gross weight, and wind and temperature. Density altitude is a term that sometimes causes confusion. A high density altitude is NOT a good thing. Density altitude is defined as the pressure altitude corrected for non-standard temperature variations. And while this is a correct definition, my definition is perhaps more appropriate: DENSITY ALTITUDE IS THE ALTITUDE THE AIRPLANE THINKS IT IS AT, AND PERFORMS IN ACCORDANCE WITH. Density altitude can be computed on a density altitude chart, flight computer, electronic flight calculator or by rule of thumb. Density altitude gives us some idea about the expected performance of the airplane, but only if you apply the information to the performance charts. An accurate rule of thumb (usually any error will be less than 300 feet) for determining the density altitude is easy to remember. For each 10-degrees Fahrenheit above standard temperature at any particular elevation, add 600 feet to the field elevation. ( And, conversely for each 10-degrees F below standard temperature, subtract 600 feet.) Standard temperature at sea level is 59-degree Fahrenheit. For elevations above sea level, subtract 3.5 degrees per thousand feet of elevation from the sea level temperature of 59 degrees. For example, at Jackson, Wyoming the elevation is 6,444. Multiply 6.444 times 3.5 for 22.55. Subtract this from 59 (5922.55) for 36.45. The standard temperature at Jackson is 36.5 degrees. If the existing temperature is 80 degrees, subtract (80-36.5 = 43.5). Divide t his difference by 10 degrees (for each 10-degrees F above standard), and multiply 4.35 times 600 (600 feet per 10 degrees) equals 2,610. Add 2,610 to t he field elevation (6,444) for a density altitude of 9,054. Under the existing conditions (of our example), the airplane will perform as it would on a standard day at 9,054 feet elevation. Density altitude not only affects the takeoff distance and rate of climb, but also applies to the service ceiling of the airplane while en route. A simple rule of thumb for determining t akeoff distance exists that helps you deal with density altitude during takeoff. The only problem is that it does not guarantee rate of climb after takeoff, but it in sures that you will be able to takeoff in the distance available for the runway involved.
do's and don'ts of mountain flying
DON’T fly into unimproved mountain strips without a minimum of 150-hours total flight experience. Even then, be proficient at slow flight manoeuvring and the spot method for landing. DON’T plan a cross-country flight into the mountains when the wind at mountaintop level exceeds 30 knots unless you are experienced in this t ype operation (strong updrafts, strong downdrafts and moderate or greater turbulence). This does not preclude taking a “look-see.” Often with a stable air mass the air will contain very little turbulence during these high-wind conditions. Expect the wind velocity to double or more in mountain passes and over the ridges due to a venturi effect. DON’T choose a route that would prevent a suit able forced-landing area. DON’T leave the airplane without a compelling reason if you have executed an emergency or precautionary landing. Temporary evacuation may be necessary if a fire hazard exists. DON’T go if the weather is doubtful or “bad.” DON’T become quiescent with weather reports of ceilings of 1,000-2,000 feet. The ceiling is reported above ground level. Often, in the mountains, the weather reporting facility will be surrounded by mountains that extend thousands of feet higher than the facility. Clouds may obscure the mountains and passes in the vicinity. DON’T fly VFR or IFR in the mountains in an unfamiliar airplane make and model. It is required that you learn the flight characteristics, slow flight and stalls in various configurations, beforehand. DON’T make the landing approach too slow. Some pilots feel they have to make a low approach on the backside of the power curve to get into a mountain strip. This “hanging on t he prop” is a dangerous operation. Use a stabilized approach for all landings. DON’T operate low-performance aircraft into marginal mountain strips. If in doubt about your takeoff, use the “sufficient runway length” rule of thumb. DON’T rely on cloud shadows for wind direction (unless you are flying at or near the cloud bases). Expect the wind to be constantly changing in direction and velocity because of modification by mountain ridges and canyons. DON’T fly close to rough terrain or cliffs when the wind approaches 20 knots or more. Dangerous turbulence may be encountered.
DON’T fail to realize that air, although invisible, acts like water and it will “flow” along the contour of the mountains and valleys. Visualize where the wind i s from and ask yourself, “What would water do in this same situation?” DON’T slow down in a downdraft. By maintaining your speed, you will be under the influence of the downdraft for a lesser period of time and lose less altitude overall. DON’T forget or fail to realize the adverse effect of frost. Less than 1/8 inch of frost may increase the takeoff distance by 50 percent and reduce the cruise speed by 10 percent. Often, if the airplane becomes airborne, the smooth flow of air over the wings is broken up by the frost and the ext ra drag prevents the airplane from climbing out of ground effect. DON’T give insufficient attention to the importance of fuel and survival equipment. It is important to keep the airplane light, but don’t skimp on these items. DON’T fly the middle of a canyon. This places you in a poor position to make a turnaround and it subjects you to shear turbulence. DON’T fail to use the same indicated airspeed at high-altitude airports that you use at low-altitude or sea level airports for the takeoff or for the approach to landing. DON’T be too proud or too vain to check with experienced mountain pilots concerning operations to and from unfamiliar fields. DON’T attempt VFR flight in mountainous terrain unless you have the minimum visibility you have established as a personal safety standard. DON’T become complacent about the horizon when flying with outside visual reference. A gentle upslope terrain may cause an unknown constant climb with t he possibility of an inadvertent stall. The horizon is the base of the mountains some six to eight miles away. DO file a flight plan for each leg of your flight. Also, make regular position reports to allow search and rescue personnel to narrow down the search area if you are overdue on the flight plan. DO familiarize yourself with the high-altitude characteristics and performance of your airplane. This includes the takeoff and landing distance and rate of climb under various density altitude conditions. DO spend some time studying the charts to determine the lowest terrain along the proposed route of flight. If possible, route the flight along airways. DO have confidence in the magnetic compass. The compass (unless it has leaked fluid or someone has placed interfering metal near its magnets) is the most reliable instrument. Charts will show the areas of local magnetic disturbance that may affect the accuracy of t he compass reading. DO plan the fuel load to allow flight from the departure to the destination airport with a reserve to counter unexpected winds. DO fly a downdraft, that is, maintain speed by lowering the nose of the airplane. Unless the airplane is over a tall stand of trees or near a shear cliff, the downdraft will not extend to the ground (exception: microburst). DO use Sectional Aeronautical Charts instead of World Aeronautical Charts (WAC) because of the greater detail (8 miles per inch). DO approach ridges at an angle. The recommendation is to use a 45-degree angle approach when in a position of one-half to one-quarter mile away. This allows an escape, with less stress on the pilot and
airplane, if unexpected downdrafts or turbulence are encountered. Flying perpendicular to the ridge, rather than at a 45-degree angle, does not mean you cannot escape the downdraft or turbulence by making a 180-degree turn. But, it does mean the airplane will be subjected to the effects of the downdraft and turbulence for a greater period of time. Usually, a steeper bank will be required to make the 180-degree turn. This will increase the g-loading stress on the airplane.
DO use common sense when performing takeoffs or landings at mountain strips. If you have any doubt about the operation, confirm the aircraft performance using the Pilot’s Operating Handbook or Owner’s Manual. If the physical conditions are adverse and compromise the operation, delay the operation until conditions are better. DO count on the valley breeze (wind blowing upstream during the morning hours) and the mountain breeze (wind blowing downstream during the evening hours). In an otherwise calm wind condition the valley breeze will create an approximate 4-knot tailwind for landing upstream. The mountain breeze will cause an approximate 8-knot to 12-knot tailwind for takeoff downstream. DO make a stabilized approach for landings. Since the late ‘ 60s the power-off approach has been discouraged because of thermal shock to the engine. DO remember your study of aerodynamics. It is possible to stall the airplane at any airspeed and any attitude (providing you are strong enough and the airplane doesn’t break first). If a stall is entered in the same manner, for example, with a slow deterioration of the airspeed, it will stall at the same indicated airspeed at all altitudes.
mountain wave
mountain wave formation perpendicular wind flow increasing wind velocity stable layer or inversion
The U.S. Aeronautical Information Manual states, “Your first experience of flying over mountainous terrain, particularly if most of your flight time has been over the flatlands of the Midwest, could be a never-to-be-forgotten nightmare if you are not aware of the potential hazards awaiting … Many pilots go all their lives without understanding what a mountain wave is. Quite a few have lost their lives because of this lack of understanding. One need not be a licensed meteorologist to understand the mountain wave phenomenon.”
The most distinctive characteristic of the mountain wave is the lenticular cloud. This is a "signpost of the sky" indicating that mountain wave activity is present. there are several terms for mountain wave:Mountain wave Standing wave Lee wave Gravity wave Standing lenticular ACSL (altocumulus standing lenticularis) Or just plane "wave" The wave that forms over the mountain is more properly called the "mountain wave." The waves downwind from the mountain are the "standing wave" or "lee wave." Pilots have come to accept all of these names for wave activity, regardless of position of the lenticular clouds. To set up a mountain wave condition three elements are needed: Wind flow perpendicular to the mountain range, or nearly so, being within about 30 degrees of perpendicular. An increasing wind velocity with altitude with the wind velocity 20 knots or more near mountaintop level. Either a stable air mass layer aloft or an inversion below about 15,000 feet. Because of these elements, the weather service is able to predict the mountain wave condition with over 90-percent accuracy.
Figure 1
In figure 1 , we have likened an atmosphere with low stability to a flimsy spring that offers little resistance to vertical motion. So while the lower coils move easily up and over the mountain, the jolt received at ground level is not transmitted very far upward.
Figure 2
Figure 2 represents a stable atmosphere that is similar to a tough, heavy spring. This air, when it strikes the mountains, tends to suppress i nternal vertical motion. It is essentially too tough for oscillations to be set up.
Figure 3
In figure 3 we have an arrangement of a strong coil sandwiched between two weaker springs to simulate an atmosphere with a stable layer sandwiched between areas of lesser stability. With this arrangement it is conceivable that the strong spring will continue to bounce up and down for some time after the parcel of air has crossed the mountain ridge. With a stable layer (or inversion aloft) the air stream is both flexible enough to be set in vertical m otion and elastic enough to maintain t hat motion as a series of vertical oscillations. As the air ascends, it cools and condenses out moisture, forming the distinctive lenticular clouds. As it descends, it compresses and the heat of compression reabsorbs the moisture. It goes through this up and down action many times forming a distinctive lenticular cloud at the apex of each crest, providing t here is sufficient moisture present for the cloud formation.
Wave length •
directly proportional to wind speed
•
Inversely proportional to stability Intermountain West - averages 4 miles Appalachia Wave - averages 10 miles
•
•
Lee Wave Variation •
Diurnal variation: in the summer early morning or late afternoon is best for formation
•
Seasonal variation: winter is the best time for formation (jet stream, snow covered ground = no convection, stable layer aloft)
The up-and-down action forms a trough at the bottom of its flow and a crest at the top of the flow. The distance from trough to trough (or crest to crest) is called the wave length. The wave length is directly proportional to wind wind and inversely proportional to stability. The wave length is used for visualization. In the area from the trough to the crest is an area of updrafts. The area from the crest to the trough is predominately downdrafts. In the intermountain west the wave length can vary from about 2 nautical miles to over 25 nautical miles. It averages 8 miles and extends downrange about 150-300 nautical miles. Satellite photos have shown the wave capable of extending over 700-nautical miles downwind from the mountain range.
Cap cloud of the Teton mountain range This cloud is mostly on the windward side of the mountain.
Foehngap The foehngap exists because moisture is reabsorbed during the down rush of air.
With sufficient moisture three typical wave clouds will form, although there are four types of clouds associated with the wave. Cap cloud (foehnwall) Lenticular Roll (rotor, arcus) Mother-of-Pearl The presence of clouds merely point out wave activity and not wave intensity at any particular level. Because moist air takes less vertical distance to reach its condensation level than does dryer air, the presence of a lenticular cloud is not necessarily an indication of the strength of the updrafts or downdrafts in a mountain wave. For example, high altitude lenticulars may indicate there is sufficient moisture at that altitude to form them, when in fact the strongest wave lift and sink occurs at a lower altitude where t here isn't enough moisture to form the lenticular clouds. This is one reason visualization is so important. The mother-of-pearl or nacreous cloud is a pancake-shaped cloud that is extremely thin and visible for only a short time after sunset or before sunrise when the sky is dark. It is normally seen i n latitudes higher than 50 degree north, or over Ant arctica. It is best seen in t he polar regions at 80,000 to 100,000 feet when the sun is below the horizon.
Lenticulars over Montana
Rotor cloud in Alaska
The lenticular cloud appears to be stationary although the wind may be blowing through the wave at 50 knots or more. The wave lift can extend into the stratosphere, more than 10 miles above sea level, so you can't escape wave effects by flying over them. What are the flight conditions in lenticular clouds? Generally the lenticular area will be quite smooth. The only danger is the magnitude of the sustained updrafts and downdrafts. Usually individual lenticulars are composed of ice crystals, but when they are composed of super-c ooled water droplets watch out for severe icing conditions.
mountain wave safety practices •
altitude 50% above terrain
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approach at 45 degree angle
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avoid ragged & irregular lenticulars
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climb in lift
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dive in sink
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avoid the area of the rotor
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visualise the wave length
Line of rotors - Calgary
Normally the rotor clouds is centred beneath the lenticular cloud. Most often it extends anywhere from ground level to mountaintop level, but is frequently observed up to 35,000 feet. Destructive turbulence from the rotor rarely exists more than 2,000-3,000 feet above mountaintop level. The rotor is described as a "dark, ominous-looking cloud with a rotating appearance." If it forms near the ground where it can pick up dust and debris, it is dark and ominous looking, but more often it looks similar to a fair-weather cumulus. Turbulence is most frequent and most severe in the standing rotors j ust beneath the wave crests at or below mountaintop level (visualization is helpful where there is insufficient moisture to form the rotor or the lenticular). The rotor area forms beneath the lee wave where a large swirling eddy forms. Sometimes with an inversion (normally stable air), turbulence succeeds in overturning t he air in the stable layer. Once warm air is suddenly forced beneath colder and denser air a vigorous convection is set up in an attempt to restore normal equilibrium. This makes the roll cloud a particularly turbulent hazard. If the top of the cloud is rotating faster than the bottom, avoid the area like the plague.
The most dangerous characteristic of the standing wave is the rotor. The rotor can be assumed to exist whenever a mountain wave forms, but a cloud will not always form to alert you to its presence. Avoid the area where the rotor will form with visualization. Often the three conditions that must exist to form a mountain wave will exist ( perpendicular wind flow, increasing wind velocity with altitude, and a stable air mass layer or inversion) ... but there is insufficient moisture for the wave clouds to form. This is called a dry wave. All of the updrafts, downdrafts and rotor turbulence exists, you just can't see the clouds. You must use visualization. Just because a mountain wave exists, it is not a sure sign that your flight must be delayed or cancelled. The degree of stability can be determined from pilot reports or by a test flight.
Mountain wave safety practices Altitude 50 percent above the terrain - Turbulence caused by extreme mountain waves can extend into all altitudes that you might use, but dangerous turbulence can usually be avoided by clearing the mountains at least half again as high as the height of the mountain. In Colorado there are 54 peaks over 14,000-foot elevation. Does this mean we have to fly at 14,000 plus one-half (7,000) or 21,000 feet? No, use the base of the terrain to begin measuring. For example, if t he surrounding terrain is 10,000 feet and the mountaintop is 14,000 feet, use one- half of the 4,000-foot value and fly 2,000 feet above the mountaintops. Approach at a 45-degree angle - The rule-of-thumb of flying half again as high as the mountain is designed to reduce the risk of entering the turbulent rotor zone, but it does not necessarily give you a sufficient margin to allow for height loss due to downdrafts. You must have an escape route. Avoid ragged or irregular-shaped lenticulars - Ragged and irregular-shaped lenticulars can contain the same turbulence as the rotor area. Climb in lift - Dive in sink - By diving in sink, rather than trying to maintain altitude, the airplane is exposed to the effects of the downdraft for a lesser amount of time. Even though the rate of descent will likely be double or more the rate of climbing at the best rate-of-climb airspeed, the airplane will loose less altitude overall. Avoid the rotor - If rotor clouds are not present, visualize the area of the rotor and avoid it. Visualize the wave length - When flying parallel to the wave, fly in the updraft area.
runway length Operating at mountain airstrips presents us with various passenger loads and different density altitude conditions for nearly each takeoff. These factors combine to provide a loss of performance, creating concerns about whether or not the runway is long enough for takeoff.
Airstrip runs NW-SE on right side
You might not be too concerned about landing at this airstrip. There are unobstructed approaches from either end. But, when it comes time to depart, you might have some doubts about the runway length. We have a rule of thumb that can determine if the runway length is adequate for the takeoff; although, it will not guarantee rate of climb after the t akeoff. The POH (pilots operating handbook) should be consulted to determine the rate of climb. The rule really is quite simple. It states: "Ten times the square root of the percentage of lift-off distance required is equal to the percentage of lift-off speed that should be attained in that distance." Because airplanes stop better than they accelerate, we can easily accelerate to the halfway point of a runway and determine if there is sufficient performance t o continue the takeoff. If there is insufficient speed, we can easily stop in the remaining half of the runway. (This really doesn't work well on downhill runways.) So we will use the halfway point of the runway for "lift-off distance required."
Rule of thumb to determine if the runway length is adequate 10 times the square root of the percentage of liftoff distance required is equal to the percentage of lift-off speed that should be attained in that distance
Remember, this rule of thumb does not guarantee that the rate of climb will be sufficient to clear any obstacles after takeoff, but it does guarantee there is sufficient runway for the takeoff. Mark the halfway point on the runway. This might require you to walk the length and c ount your steps, then walk back and determine a distinguishing characteristic or place a flag or marker at this point. Using the rule, "10 times the square root of the percentage of lift-off distance," we use 50 percent for the lift-off distance and the square root of 50 is 7.07. Ten times 7.07 equates to 70.7 percent of the lift-off speed should be obtained at the halfway point to guarantee takeoff in the remaining half of the runway. If you have the speed, continue. If you do not have the speed abort.
maximum lift flap setting There is a concern about the use of flaps for takeoff from a backcountry strip among many pilots. Should flaps be used or not, and if used, how much flap should be used? The majority of mountain pilots agree ... flaps should be used for takeoff. How much flaps? The POH or Owner's Manual may give a recommendation, in which case you are obligated to use their wisdom. But, if there is no blessing listed, the following procedure will provide the maximum lift from any particular airfoil section.
Begin by making full control deflection, aileron control (wheel or stick) moved full left in this picture. This represents the maximum lift for the airfoil design. Remember, lift and drag are directly proportional. Increase lift and you increase drag. Here the manufacturer determined the maximum lift for the aileron deflection is obtained at the particular angle formed.
Next, match the flap deflection to the aileron deflection. This provides the maximum amount of lift for the airfoil (considering the effect of drag). This works for normally aspirated engines. With the Cessnatype airplanes, it is necessary to parallel the flap deflection to match the aileron deflection since they are not side-by-side. This will result, for example, in about 12-degrees flap extension in the Cessna 150170 series airplanes. If your airplane happens to be a turbo-super charged wild duck, or some such derivative, the flaps will probably be set at 50 percent because you are not as concerned with the balance of engine power and lift.
canyon flying Most pilots don't read a book about aerobatics and go out to practice on their own. Flying in canyons is usually done after you have gained experience from someone knowledgeable in canyon flying. It is far better to go out with an experienced flight instructor first.
Generally speaking the best path to fly through a canyon will be on the updraft side (a narrow canyon is the exception). The canyon may be more or less level terrain, or it may be sloping terrain. The airplane will have to be close to the mountain side to take advantage of the potential orographic lift. Manoeuvre the airplane to within a couple of hundred yards of the canyon side if lift is desired. Beyond this area the lift is insignificant. Novice pilots often fly down the centre of a canyon. This places the m the farthest away from the sides of those scary old mountains. But, this is not the correct position for the airplane.
There are two good reasons for flying the side of a canyon. First, you will avoid the shear area caused by the mixing of air flowing down one side and up th e other side. And, second, you will be in a bett er position from which a turnaround can be safely made. You have the full canyon width to perform the turnaround manoeuvre if the terrain becomes unsuitable, adverse weather is encountered or you don't want to be there.
Always remain in a position to turn to lowering terrain; and, never fly beyond the point of no return. These two axioms encompass the idea that you will never enter a canyon if there is n ot room to turn around. Only fly in a canyon when there is adequate room to allow a turnaround. Otherwise, fly the terrain. That is, gain altitude and over-fly the canyon area from the high end to the low end.
It doesn't matter which side you fly down a canyon, either the updraft or downdraft side, because flying downhill makes it easy to transition to either side. Normally we associate updrafts with the sunny side of a mountain, but in canyons it depends on the airflow down a slope more than whether or not the sun is shining on the surface. The majority of mountain instructors will caution you when flying in canyons to gain sufficient altitude to go to the head of the canyon and then fly downslope terrain. This is sage advice. But, often it is necessary to fly up canyons (fire patrol, game and fish surveys, search and rescue, law enforcement).
flying up canyons There is nothing wrong with flying up canyons ... when you do it properly. In addition to never entering a canyon where there is not room to turn around, you must remain in a position that allows a turnaround if the canyons narrows or if the terrain begins to out climb the airplane. It is a good idea to fly at a speed faster than Vx (best angle-of-climb airspeed). Have you ever flown over water beyond power-off gliding distance from the shore? Have you noticed the engine goes to "automatic rough?" You start hearing strange noises that you haven't noticed before. The oil pressure gauge begins ticking and the engine seems to run rough.
A similar thing happens when flying upslope terrain i n a canyon. Your left arm become shorter and the airspeed decreases without you noticing it. When flying up a canyon, fly the UPDRAFT SIDE. If you can't gain altitude on one side, try the other side (Mother Nature may be fooling you about which side has the updraft).
flying up narrow canyons One thing can be said about flying up narrow canyons ... if it's not done properly, it's not habit forming. Until you are experienced (with a knowledgeable mountain instructor), stay out of these areas. Speaking of a mountain instructor, do you have to fly with a certified flight instructor to obtain mountain flight instruction? Absolutely not. Many excellent, knowledgeable pilots can provide a wealth of information about mountain flying, but they can't sign your logbook. Let's define a narrow canyon. This is one, where, if you have to turn around the turn radius exceeds one half the canyon width. This can be intimidating to experienced mountain pilots when conditions aren't perfect. Flying up a narrow canyon requires a different technique from the "regular" canyon. Rather than flying the updraft side, you are better off flying the downdraft side. This way, if you get into trouble, when you turn around you won't be getting into a worse situation. You will be entering an area of updraft during the turn. BUT REMEMBER, the turnaround will be subject to a tail wind that will increase the radius of t urn.
It is not uncommon when flying in unfamiliar terrain to encounter a blind canyon. Blind canyons leading to a dead end shouldn't be a problem, but they are. The reason is that the pilot violates the basic premises of mountain flying. To avoid potential problems stay out of canyons where there is not room to turn around, remain in a position to turn to lower terrain, and never fly beyond the point-of-no-return.
point-of-no-return Always remain in a position where you can turn toward lowering terrain. Never fly beyond the point of no return.
the point of no return The "point of no return" is defined as the point on the ground of rising terrain where the terrain out climbs the aircraft. The turn-around point is det ermined as the position where, if the throttle is reduced to idle, the aircraft can be turned around during a glide without impacting the terrain. (It is not proper technique to reduce the throttle for the turnaround. This merely denotes the point where the turnaround must be initiated.)
turn around point More important than the "point of no return " is the "turn around point." What or where exactly is this position where, if the throttle is reduced to idle, the aircraft can be turned around during a glide without impacting the terrain? The reason it is an elusive value is because of the variables that may be encountered. If the airplane is flying upslope terrain at a high speed, the turn around point will be further up the upslope than it would be if the airplane is flying at minimum airspeed. Usually, if a pilot gets into trouble while flying upslope t errain, he has experienced a phenomena known as "short arm" effect. The self-preservation instinct causes a pilot t o unconsciously pull back on the control wheel to avoid the rising terrain. The airplane slows down and t his reduction in airspeed is usually imperceptible to the pilot, who is probably directing his attention outside the airplane. As the pilot, flying at or near the minimum controllable airspeed, realizes he needs to turn around, the density altitude may preclude a level flight turn around. It becomes necessary to trade altitude for airspeed during the turn. This is the main reason for the definition of the "turn around point." One of the manoeuvres that should be demonstrated by a good mountain flight school is the "turn around point."
demonstration - turn around point NOTE: This demonstration is not required to safely fly in the mountains. Search pilots operate close to the terrain (500 feet vertically and 500 feet laterally) on a continuous basis. It is felt this demonstration, with the required steep nose-down attitude, will help prevent complacency and cause the search pilot t o continually be aware of his position and altitude.
While flying upslope terrain in a canyon, the "student" (actually, the participants are all experienced pilots) is asked to determine the t urn around point. The Cessna 182 or T-41 is used for the backcountry flying in this course. The instructor must monitor the position diligently i n order not to fly beyond the turn- around point. This is definitely a place where complacency will "get you."
This picture shows flying up a canyon after completing the last pass of a contour search
The contour search began at the top of the ridge and moved back and forth with step-downs in 500-foot intervals
The airspeed is 80 knots indicated, the speed used for the contour search technique. When the student determines the turn around point, the throttle is reduced to idle and the turn around is commenced. Because of the slow speed it is necessary to lower the nose to a position most students consider excessive; however, to maintain a constant airspeed, it is required.
This is the last pass of the contour. In this case it results in the airplane flying upslope terrain at low altitude
The student continues up the canyon until reaching the "turn around point"
If the student has judged the position pro perly, the airplane will complete the 180-degree turn just over the tree tops at 80 knots indicated airspeed.
At the "turn around point" the throttle is reduced to idle and a gliding turn is begun
Because of the slow speed the nose must be lowered to maintain 80 KIAS. Operation in a confined area may also require a steep turn. Lowering the nose further is necessary to maintain the constant 80-KIAS airspeed.
It is in this area of the turn that the 'student's' heart rate increases. The nose is pointed downward and the airplane is approaching the trees.
Most students find this demonstration quite exuberating ... and most of the time the instructor does too. This demonstration is made with the power at idle. If the student misjudges the turn-around point, power is used to get out of the situation, so it is not as dangerous as it may appear.
Required Altitude What altitude is required for the Cessna 182 t o complete the 80-KIAS turn around? It's g oing to be about 400-500 feet above ground level, probably closer to 500 feet.
box canyon turn No one should need the box canyon turn under normal circumstances. If you need this manoeuvre, you have violated the laws of mountain flying. To explain the box canyon turn it is necessary to consider two scenarios. In the first, the pilot is flying along at cruise power setting and cruise airspeed. In the second case, the pilot is flying at minimum controllable airspeed. This minimum controllable airspeed is probably not an intentional flight condition. Flying over water beyond the power-off gliding distance from the shore, sometimes causes the oil pressure gauge to begin ticking. And it hasn't done that before. Next the engine may appear to give a little shudder of roughness. This might happen several times before you again approach the safety of the shoreline. A similar phenomenon occurs when flying upslope terrain in the mountains. Your left arm becomes shorter. This is a normal self-preservation aspect of flight. You unconsciously pull away from the rising terrain and often the deterioration of airspeed goes unnoticed.
Conditioned Response Mountain flying, like Mother Nature, can be harsh and unforgiving for the novice who fails to adhere to the two basic premises for all mountain flying: It’s really a simple matter to flirt with the mountains if you always remain in a position to be able to turn toward lowering terrain and never fly beyond the point of no return. The first law, being able to turn while having some extra altitude to descend, does encompass the idea that you never enter into a canyon if there is not sufficient room to turn around. The second law, to never fly beyond the point of no return, requires the pilot to establish a turn-around point whenever flying upslope terrain. The point of no return is defined as a point on the ground of rising terrain where the terrain out climbs the aircraft. The turn-around point is determined as the position where, if the throttle is reduced to idle, the aircraft can be turned around during a glide without impacting the terrain. Obviously, the power is not reduced to idle. This merely is a gauge to judge and establish the point over the ground where an escape turn must be made. For the unconcerned aviator bopping along through the mountains at cruise power setting, it is still necessary to maintain a conditioned reflex of maintaining a position where you can always turn to lowering terrain and n ever fly beyond the point of no return. This must be a conditioned reflex rather than instinct, because instinct is often wrong in an airplane. For example, if you have ever experienced a spin, your first impression is that the airplane is pointing straight towards the ground while rotating. The Cessna 172, for example, has its nose 46 degrees below the horizon, only about halfway from the horizon to the vertical. Your instinct will be to raise the nose with back pressure. It's always worked before. But now you must use the conditioned reflex of relaxing the controls (or pushing the controls forward) to break the stall and then fly out of the resulting dive without exceeding the critical angle of attack (somewhere around 16-18 degrees). Another example of the conditioned reflex is the forced landing procedure experienced at the beginning o f the private pilot training. After several lessons, the flight instructor reaches out and pulls the power lever, stating something like, "You're engine just quit, proceed as you would in an actual emergency." To begin, your first endeavours don't provide much satisfaction for yourself or the instructor. You try to p ick out an area for a forced landing and next try to extend the glide to make it to that spot; however, without experience only luck will allow you to approach anywhere near your projected spot. If you have an excellent flight instructor, someone who teaches the spot method of landing, it is easy to determine how far the airplane will glide. Using the spot method technique allows you to look at a windscreen mark during a glide and determine the spot on the ground where the airplane will glide. By mentally subscribing a line in an arc from this point, the area surrounding the airplane within which the airplane can be landed is defined.
The instructor continues this "conditioning," much as Pavlov conditioned his dogs, but hopefully without quite as much salivating. At some point during this process, your subconscious begins mentally picking out forced-landing areas. When the conditioning is complete, the instructor pulls the engine power and you, without really thinking or concentrating about it, head for a forced-landing spot. The spot may be ahead or behind the airplane, it doesn't really matter for your subconscious h as already made the decision. Until you have practiced the box canyon turn and understand the mechanics of and the ramifications of an unintentional stall close to the terrain, the best advice for escaping from a "tight," or rapidly rising terrain or the narrowing confines of a canyon, is to make a steep turn at a slow speed, using flaps if prudent. What possible options are available for the course reversal manoeuvre to escape the precarious position?
Hammerhead Turn Pilots, in all seriousness, have asked my advice about performing the hammerhead turn as an emergency procedure for getting out of a tight spot. There are several problems that immediately jump to mind, negating the possibility of performing the hammerhead turn. First by way of definition, the hammerhead turn is an aerobatic manoeuvre where the airplane enters a vertical climb from manoeuvring speed (or the recommended indicated airspeed for the aerobatic airplane involved). As the airplane slows, but before it encounters stall buffet, the pilot initiates the turn. For a left turn, the torque of the engine aids in making the turn. Application of left rudder is coordinated with the application of right aileron and forward movement of the control wheel (left rudder and left aileron used together causes the airplane to roll onto its back). When the airplane pivots to a nose-down position, back pressure is used to fly out of the resulting dive. Definitely it is best to avoid this manoeuvre in a "tight." The airplane is usually at a dangerously low airspeed when the pilot arrives at the "tight." This precludes even thinking about performing the hammerhead manoeuvre. Even with plenty of airspeed, it would be stupid (as in not exhibiting common sense) to try the hammerhead. The airplane used for mountain flying is probably not an aerobatic certified machine.
Wing Over The wing over is more of a fun manoeuvre than an emergency escape manoeuvre. Usually the pilot pre-plans the wing over, allowing sufficient airspeed to transition from level flight to a climbing pitch attitude of about 40 degrees. During the increase in pitch, a coordinated bank is begun. The maximum pitch is reached after about a quarter turn (45 degrees of turn). At this point the back pressure is completely relaxed, but the bank continues to increase to 90 degrees. The bank is rolled out during the last quarter of the turn and back pressure is increased to arrest the descent. The airplane should arrive at the 180-degree turn point at the same altitude at which it began the manoeuvre. Again, this is a manoeuvre that is intentionally performed for fun, rather than to escape during an emergency situation.
Steep Turn The safest and perhaps the most commonly used method of course reversal is the steep turn. It is very similar to the box canyon turn. The stall speed of an airplane increases as the square root of the wing load factor. In a 60-degree coordinated turn, regardless of airspeed, the airplane experiences a 2-g load factor. The square root of 2 is 1.41, so there is a 41 percent increase in stall speed. Most pilots don't really care how to determine the radius of turn. By formula, the radius of a turn is equal to the velocity in true airspeed (knots) squared and then divided b y a constant of 11.26 times the tangent of the bank angle in degrees. The valid information this formula provides is the fact that the radius of turn can be shortened by either reducing the true airspeed, or by increasing the angle of bank. A combination of the two provides the greatest benefit.
The ratio of turn radius to an increase in airspeed at a constant bank varies as the square of the true airspeed. If the airplane doubles its speed, it will quadruple the distance travelled. So even if the airplane is going faster (twice as fast in this case), it still takes twice the amount of time to complete the turn around (four times further travelled). What about using flaps during this steep turn? Definitely, use them as appropriate to the flight conditions. Flaps were invented to allow an airplane to increase its approach angle without an increase in airspeed. They work because lift and drag are directly proportional. If the lift is increased (by applying flaps to increase the camber of the wing), the drag is increased (and hence, no increase in airspeed). For most airplanes the addition of flaps, up to half the total available, provides more lift than drag because the drag can be “subdued” with excess power. At a high density altitude it may not be possible to use full flaps without intentionally losing altitude to maintain a safe airspeed. If a trade-off between altitude and airspeed cannot be made because of rapidly rising terrain, limit the use of the flaps to the extent that the airplane will maintain a constant altitude during the turn. Remember too that flaps reduce the structural strength of the airplane. Many of the normal category airplanes are stressed for 3.8 gs (g = gravity unit). This is the limit-load factor that should not be exceeded. Okay, you say, what about the ultimate load factor, you know, that 50-percent safety factor built into the airplane? Shouldn't the airplane be capable of flying at 5.7 gs? The correct response requires a definite and emphatically strong NO. For certification the airplane must be able to withstand the ultimate load factor for a period of fewer than 2 seconds without permanent deformation of the structure. More time than this at a load greater than the limit-load factor and the airplane may experience structural failure ( that is, the wing breaks off). Check the POH (pilots operating handbook) to determine the amount of reduction in structural strength with the application of flaps. The book may say: normal category 3.8 gs; flaps extended 2.2 gs (a 42 percent reduction).
Box Canyon Turn - Introduction The box canyon turn varies from the steep turn in that it is either performed from level flight at such a slow airspeed that an unintentional stall is imminent, or some excess airspeed at the beginning of the manoeuvre allows the nose to b e raised above the horizon prior to initiating the bank and the airspeed, during the turn, will be too slow to sustain level flight. We have learned the airplane always stalls at the same critical angle of attack. When banking the airplane, the stall speed increases (remember? it increases as the square root of the wing load factor). Whenever the airplane is banked in a coordinated turn, it is balancing the centripetal force (horizontal lift component that causes the turn) and the centrifugal force (the force of the turn). The turn takes place because the centripetal force pulls the airplane towards the inside of the turn. Without a compensating increase in the amount of total lift during a turn, the airplane will lose altitude. The total lift (lift) is divided between a vector force that sustains the weight of the airplane and its contents (weight). The portion of lift that is directed sideward (centripetal force) causes the turn. The centrifugal force acts towards the outside of the turn. To maintain level flight while turning it is necessary to increase back pressure (more lift equals an increase in angle of attack). This increases the load factor and stall speed. Some pilots get into trouble with the box canyon turn without realizing it because they have been "conditioned" to maintain level flight when performing steep turns.
Box Canyon Turn - Procedure from Cruise Flight The first time a pilot has to perform a box canyon turn in a true life situation, he may feel like the lady who climbs on a stool to avoid a mouse scampering across the floor. A little scream to get the adrenaline flowing wouldn’t hurt either.
The box canyon turn could be d escribed as a combination of the steep turn and wing over (when entered at or near cruise airspeed). The nose is raised above the horizon, but nowhere near the 40-degree attitude of the wing over. About five to 20 degrees is about right, depending on the airspeed. This does two things for you. First it trades airspeed for altitude and second, it slows the airspeed for a smaller radius of turn. At the same time, full power is added and full flaps (providing the airspeed is within the flap operating range) are applied while beginning the bank. The bank will be a minimum of 60 degrees and may approach 90 degrees. To insure that the g-load factor is not exceeded during the steep bank it is necessary to relax the back pressure once the bank passes about 45 degrees. The back pressure is not increased again until the bank passes through about 45 degrees toward zero degrees during the rollout. Initiate the turn - the procedure requires coordination to accomplish all items at the same time:
Increase pitch attitude Increase power Begin a bank Apply full flaps At approximately 45 degrees of bank increasing toward 60-90 degrees:
Relax back pressure from the control wheel Recovery - at approximately 45 degrees of bank, decreasing from 60 -90 degrees:
Increase back pressure on the control wheel to arrest any loss of altitude. When the airplane is in a position that allows, reduce flaps to one half
Box Canyon Turn - Procedure from Climbing Flight When operating near cruise airspeed the box canyon turn was described as a combination of the steep turn and wing over where the nose was raised above the ho rizon. Hopefully, the airspeed is near the best rate-of-climb speed or best angle-of-climb speed. This is usually a critical situation because the airspeed will probably be slower than Vy or Vx due to the “short-arm” effect. While applying full power and full flaps, a bank is established at a minimum of 60 degrees. Again the bank may approach 90 degrees. Previously we stated that the back pressure was relaxed to insure that the g-load factor was not exceeded. This is not as much of a problem at low speed, but it still exists. At the slow speed the airplane will probably stall before it exceeds the structural limitations. The main reason for relaxing the back pressure now is so t he airplane does not stall. The back pressure is not increased again until the bank passes through about 45 degrees toward zero degrees during the rollout. Initiate the turn - the procedure requires coordination to accomplish all items at the same time:
Maintain pitch attitude initially Increase power Begin a bank Apply full flaps
At approximately 30 degrees of bank, increasing toward 60-90 degrees: Relax back pressure from the control wheel The pitch attitude will fall below the horizon Recovery - at approximately 30 degrees of bank, decreasing toward zero degrees:
Increase back pressure on the control wheel Reduce flaps to one half
Natural Horizon The natural horizon is used to teach flying by outside visual reference. An instructor demonstrates a climb attitude at the best rate-of-climb airspeed. The student mimics this attitude. The airspeed indicator can be covered and the student, by noticing the pitch attitude in relation to the horizon (where the horizon intersects the side of the nose cowling), is able to fly at the best rate-of-climb airspeed within plus or minus one knot. Learning the “climb attitude” can provide for a very accurate climb speed, without looking at the airspeed indicator.
The instructor also demonstrates where is the nose in relation to the horizon in level flight, where are the wings in relation to the horizon in level flight, and where is the nose in relation to the horizon in a steep turn (left and right turns). This natural horizon is easy to use in the flatlands as a reference for basic attitude flying. In the mountains, the natural horizon may disappear. By visualizing a h orizon, basic attitude flying can still be maintained. The base of the mountains, at least six to eight miles away, represents the natural horizon. What if the airplane is closer than the six to eight miles? Visualization must be used. Perhaps the mountains at least six to eight miles in the distance are visible out the side window. Project the same horizon visually to the front of the airplane.
Summary The box canyon turn is an emergency procedure. It is best to practice it with an experienced instructor prior to the time when it becomes necessary as a life-saving manoeuvre. Without practice it is very easy to get into an accelerated stall condition that will exacerbate the original situation.
Caveat You must exercise caution in using full flaps during the practice of the box canyon turn and for drainage searches because of the possibility of the flaps failing in the extended position. For practice you may want to restrict the use of flaps because of the real possibility of the flaps failing to retract. It is possible to demonstrate and learn the box canyon turn without full flaps where t he same technique, using full flaps, is applicable to a real "tight" or emergency condition.
crossing ridges Without experience the visual aspects of mountainous country c an be very deceptive. It is difficult to be able to look out the windshield and say with any certainty whether or not you are higher than the ridge you are approaching. In approaching and crossing ridges, the novice pilot is well advised to start out by maintaining a 2,000foot clearance over both mountains and valleys.
When a mountain ridge is approached from the upwind side there is usually a cushion of air to help you up and over the ridge, providing the wind is blowing somewhat perpendicular to the mountain. Once the wind hits about 20 knots or more at mountaintop level there will be turbulence associated with any downdrafts on the lee side, but this depends a great extent on the stability of the air. Under stable conditions, there may only be a laminar flow with smooth down air on the downwind side of the mountain. And, too, the updraft generated on the upside of the mountain may extend beyond the mountaintop to form updrafts on the downwind side (above ridge level).
If a mountain ridge is approached from the downwind or lee side, the pilot runs the risk of encountering a downdraft and turbulence. When the airplane is flown perpendicular t o the mountain there is also a possibility of encountering a downdraft that could cause the airplane to impact the mountain. I am not implying that there will not be sufficient room to turn away from the ridge if it is approached "head on, but when you approach the mountain at an angel, it will permit a safer retreat with less stress on the aircraft should severe turbulence or downdrafts be experienced. Remember that in addition to the load factor induced by the turbulence, the load factor also increases during a banked turn. These are not separate forces, but add together for a total force on the airplane.
Also, the stall speed increases as the square root of the wing load factor. In a 60-degree bank, the load factor is 2 Gs (gravity units). The square root of 2 is 1.41 or a 41 percent increase in stall speed. For this reason, ridges are always approached at an angle, a 45-degree angle is recommended by most instructors. Even when the stability of the air is such that the 45-degree angle approach isn't necessary, do it anyway (or at least remain in a position to turn to lowering terrain). The reason for this recommendation is because Mother Nature can provide some surprises. As an example of such a surprise, consider the pilot who departs Aspen, Colorado with a right downwind from runway 33. He follows the Roaring Fork River to Independence Pass. Knowing he i s in an area of prevailing westerly winds aloft, the pilot assumes there will be a cushion of air to help him up and over the pass. But, Mother Nature may have stalled a high pressure area over the Upper Arkansas River Valley near Leadville. This high-pressure area is a mountain of air t hat is creating an instability in the atmosphere. In regaining stability, it subsides, that is, the air flows down the mountain toward areas of lesser pressure. This mig ht occur in the vicinity of Independence Pass and the subsidence can overpower the westerly winds aloft, presenting a downdraft where an updraft is expected. So, for safety sake, remain in a position to turn to lower terrain!
exiting downdraughts Whenever you are caught in a downdraft, it is wise to immediately turn toward lowering terrain. Compute the rate of climb for the density altitude that you are flying. Perhaps the POH gives a value of 400 fpm rate of climb at 8,000-feet density altitude. In a strong or sustained downdraft, if the descent rate, after transitioning to the best rate-of-climb airspeed, is greater than your computed best rate of climb, transition to cruise speed to escape the downdraft. If turbulence is a concern, accelerate to the manoeuvring speed. Perhaps you are descending at 500 feet per minute and transition to cruise airspeed. The airplane may now be descending at 1,100 feet per minute. Accept this temporary increase in descent rate. Although the airplane is descending faster, it will exit the area of the downdraft in lesser time, providing an altitude loss that will be less than fighting the downdraft at the best rate of climb speed. (Steve Philipson, a co-worker in the development of the Civil Air Patrol Mountain Fury program, has run mathematical profiles to prove this point).
Although we (mountain instructors) advocate that it is best to approach mountain ridges at a 45-degree angle, it is not necessary to do so when you are four or five miles away from the mountains. Wait until you are within about 1/2 mile to 1/4 mile from the ridge, then manoeuvre to approach at the 45-degree angle. If you are crossing a series of ridges, you might consider crossing one ridge to the right and the next to the left and so on, to stay somewhat on course.
If you elect to make a flight without maintaining the 2,000-foot clearance altitude above the ridges, you can determine if you have sufficient altitude t o cross the ridge by picking two spots. The first spot is whatever you can see over the ridgeline. The other spot is an arbitrary point. In the example to the right, the first point is the bottom of the yellow arc and the arbitrary point is the top of the yellow arc. As you get closer to the ridge the spacing between the two spots will increase if you are higher than the ridge (the pick arc). If the dist ance decreases, there is not sufficient altitude to cross the ridge. don't like this method of determining sufficient ridge clearance. As an instructor I found my student s developing "tunnel vision," where they concentrated only on the points and became unaware of other things going on in and around the airplane. It is easier (and better) to just be aware of the terrain. If you can see more and more of the terrain on the other side of the ridge, you are higher than the ridge and can probably continue. If the terrain on the other side of the ridge is disappearing, get out. Turn around, gain more and try again. If this technique causes you worry or concern rather than challenging your ability, don't do it. Fly over the ridge with 2,000-foot terrain clearance.
In the photo to the left the airplane has been following I-70 (Colorado) east from the Dillon Reservoir area along Straight Creek. The Eisenhower Tunnel is below the nose to the left. Denver is ahead of t he airplane. The ridgeline ahead is the Continental Divide. Loveland ski area is visible on the right side. The wind is from the west-northwest (left rear of the airplane). The airplane is being flown up the right (south) side of the canyon to be in an area of updraft. Can the pilot make a commitment to cross the ridgeline at this point? NO. Once the pilot manoeuvres to a position (maintain the 45-degree angle approach) where the throttle can be reduced to idle and the airplane has sufficient altitude to dive and hit the top of the ridgeline, t he
pilot can make the commitment to cross the ridge. I'm not suggesting that it is proper procedure to reduce the throttle to idle -- this is how you just the proper position for making a commitment. If you have gotten to this position without e ncountering a downdraft, any downdraft experienced can be overcome by lowering the nose slightly to maintain airspeed while crossing the ridge. Once you have made the commitment it is a good idea to fly toward lowering terrain. This "safe ridge crossing" technique may be used whether approaching the ridge from upwind or downwind. A word of caution. If you are trying to cross an extended plateau as opposed to the ridgeline, this rule will not work. In this case you will need additional altitude and you must remain in a position t o turn to lower terrain.
night flying About one-tenth of all general aviation accidents occur at night, while fewer than 10 percent of the flying is done after dark. Presuming that night flying is more dangerous than day flying on the basis of statistics is a fallacy. The airplane does not discriminate between light and dark. Conditions of night flight are different from daytime flying and it is the pilot's knowledge, or lack of it, combined with a lack of visual clues that present an e xtra challenge. A pilot may be subjected to times when the destination airplane lies beyond the rays of the setting sun. Perhaps an engagement becomes a compelling reason to venture out into the black of night. Flying at night in the mountains is a matter of determining the weather condition to arrive at a personal go/no-go decision. For those who decide to fly at night, an understanding of night vision differences and adjusting the flight operation accordingly, will increase the margin of safety.
night vision Even a competent attitude-instrument pilot will need to use his outside vision during a night flight, if only in the transition during takeoff and landing. What you see, or don't see, in the dark is dependent on t he state of dark adaptation of your eyes. If you enter a dark area, your vision improves slowly. After six to seven minutes the eyes are 100 times more sensitive than when you entered the dark. Full adaptation takes 30 minutes, at which time the rods of the eye are 100,000 time more sensitive. This is due to the build-up of a photosensitive chemical called visual purple, the key to night vision. Visual purple is dependent on vitamin A (carrots, eggs, milk, cheese and most veget ables). Vitamin A cannot be stored by the body; it is necessary to eat well-balanced meals before night flying. Although it takes 30 minutes for dark adaptation t o occur, it can be lost in a second or two of exposure to bright light. Minimize the use of white light in the cockpit and keep it as dim as possible. Dark adaptation is an independent process in each eye. If you are exposed t light, close one eye to preserve half your dark adaptation.
scanning techniques
The cones of the eye, used for day vision, provide peripheral vision. Night vision uses rods. This creates a blind spot in the centre of the eye at night. If you detect something with peripheral vision, the natural tendency is to turn and look directly at it. Night vision is impossible at the centre of the eye, so a technique called "off-centre scanning" must be developed. Look six t o 12 degrees away from the object you wish to see.
autokenesis A visual illusion may occur at night if you stare at one light for a long period of time. Involuntary muscle twitches cause the light to be displayed on a different portion of the eye, creating false motion, where the light appears to move. Avoid autokinesis by the off-centre scanning technique.
night take-off and departure Before departing from a mountain airport at night, firmly fix in your mind the nature of the terrain and obstructions adjacent to the airport. Pilots have no intention of making an emergency landing shortly after takeoff, especially at night, but it does not hurt to survey the terrain during daylight conditions to form a plan of action. With the interior lights adjusted to the minimum brightness that affords instrument readability, initiate a normal takeoff. Acceleration error in the attitude indicator will cause an indication of a higher nose attitude than during a regular climb. As a consequence, some pilots have lowered the nose to the normal attitude and have flown into the ground. The moment the airplane leaves the ground on a dark night, it is en veloped in black. Outside visual reference becomes impossible. Maintain a positive rate of climb based on the airspeed indication, regardless of the attitude indicator display.
climb out If you become disoriented during the climb out, terrain clearance becomes a big concern. Fly toward the rotating beacon. At airstrips without lighting, fly toward any light on the ground.
terrain clearance Proper pre-flight planning includes studying charts and developing a plan of action. Still, at some time during a night flight, you are going to experience a moment of fear arising from your concern about terrain clearance. If there are any light around, use them. Fly directly toward the light. As you approach this light, select another light. If the light flickers or disappears, t here is something between you and the light. Immediately choose another light to fly toward. If there is only one usable light in the area, a shuttle climb in a holding pattern may be the most prudent course of action. To perform a shuttle climb, make a 90-degree t urn (direction depends on obstructions) and immediat ely turn the opposite direction for 270 degrees. This is the same as a 180-degree turn, but keeps the airplane confined to a small geographical area. Before passing beyond the ground light, perform a 180-degree turn back to the ground light. Continue this manoeuvring while climbing to a safe en route altitude before proceeding on course.
weather It is difficult to see and avoid weather at night. The first indication of a cloud may be a glow emanating from the navigation lights, or a brilliant flash of the strobes being diffused throughout a cloud. VFR pilots should do an immediate 180-degree turn. Restricted visibility conditions become apparent with the gradual disappearance of lights on the ground or when they become fuzzy and flow. Remember, the horizontal visibility through a restriction such as fog, haze or smoke is must less than when looking down through it from above. Pilots get into trouble trying to land at an airport with fog because they fly over and can see the runway, but when on final approach they can't see anything.
approach for landing Distances at night are deceptive, due to lack of illumination and inability of the pilot to judge them by the usual method of comparing the size of different objects. At night, fly towards an airport light and make a standard pattern, rather than attempting a straight-in approach. The perception of distance can be dealt with by flying the downwind leg until the touchdown point is half-way between the wing tip and tail. They turn onto the base leg. The only way to approach a runway in the mountains at night, with complete safety, is to incorporate the "spot method for landing" technique. The flare and landing is accomplished in the same manner as during the day. There may be a tendency to look too far down the runway, causing the flare to be too high.
night flying tips Always carry a workable flashlight (You can recognize the pilot who has flown at night without a flashlight. He's the one that has two or more flashlights in his bag). Close one eye when exposed to bright light. Force the eyes to view off-centre. Blink the eyes if they become blurred. Do not attempt violent or abrupt manoeuvres at night. Watch for the disappearance of ground lights or an area of glow around the navigation lights. This indicates entering instrument conditions. Remember the deceptiveness of altitude and speed at night. A normal approach looks steeper at night, creating an illusion of overshooting. Distance judgment at night is less accurate than by day. A simple visual assessment can lead to a premature descent.
scud running in mountains
Note the power lines on the ridge The US National Transportation Safety Board (NTSB) statistics for any particular year during the past two decades include a list of the most frequent causal factors for general aviation accidents . The troubling point of these statistics is that the same things are causing the same accidents year after year. This points out the need for continued refresher training to establish a higher level of flight proficiency for all pilots. Doesn’t this remind you of the saying, “Fool me once, shame on you. Fool me twice, shame on me.” It should bother us that we, as pilots, are unable to learn from the mistakes of others. The first item on the NTSB’s top ten list is “inadequate pre-flight preparation and/or planning,” that denotes we must spend time thinking about the flight. Will the flight extend into the night? Do I need a flashlight? Is the destination airport lighted? Are services available at my time of arrival? What if I run into a venturi effect over the mountains? Will I have enough fuel to continue the flight to the destination? Suppose un-forecast weather crops up, where will I go? What will I do? These examples show that pre-flight planning is not just a matter of digging out the plotter and computer. The next item on the NTSB list is “landing accidents—excessive speed on the landing roll or failure to correct for crosswind conditions.” The third item is “continued VFR flight into adverse weather conditions.” Each year some 200 accidents are listed under this category. The really sad part about this is about 65 percent of these accidents are fatal. How do these accidents occur when everyone knows that pushing the weather on a VFR flight is not wise? Is it that the pilot cannot recognize “adverse weather conditions” from a distance and unintentionally gets in the weather? Most times intentional scud running is the main culprit. Those scattered clouds that are easily flown around in open country can be a me nace in a narrow canyon. Yet pilots continue t o scud run. And, not all of them are low-time pilots who haven’t learned better. Some are high-time pilots— maybe instrument-rated—and mostly of sound judgment.
Flying above a layer of clouds with the sun at your back will produce a "glory" if the cloud is composed of liquid water. This rainbow circle is caused by a diffraction phenomenon. Beware of icing in these clouds if the outside air temperature is near freezing or below freezing.
Often the aircraft's shadow is visible in the centre of the "glory" rainbow ring.
Some pilots scud run successfully, having found it necessary to do so from time to time because of ice or turbulence aloft, strong headwinds at IFR altitudes, navigation equipment failures, or flying to a destination that does not have an instrument approach facility. Whatever the reason, they do it. But, they have disciplined themselves and learned how to scud run safely. Experienced pilots develop rules that they will not deviate from under any circumstances. Minimum weather: 2,000-foot ceiling and 5 miles visibility. If weather reports of five miles visibility or better do not exist at stations beyond the destination, don’t go. Do not scud run a route you have not previously flown at 1,500 feet AGL or less. Even so, the terrain looks much different when the weather is bad. Do not scud run toward worsening weather. The ten dency to push on for a few more miles is ju st too great.
scary practice One frightening pilot technique unconsciously practiced by many pilots when the weather is marginal is that of flying near the cloud base. This is a natural tendency since it places the airplane farther away
from the ground. But, you have t o realize the forward visibility will be severely limited near the cloud base, allowing you to fly into trouble before you can see it. An experienced pilot will fly low. Divide the area from the ground to the cloud base into thirds and fly the middle or lower third. If terrain constraints prohibit this, don’t fly.
rules Keep navigation simple by following a highway or railroad. Pay attention to your chart to make sure there isn’t a tunnel. Be cautious about following a river. That’s where the poorest visibilities tend to gather. Turn on all your lights. It’s not likely that anyone else will be out there with you, but if they are, you want them to see and avoid your airplane. When flying through a narrow canyon like The Gorge between Portland and The Dalles, Ore., it is customary t o remain to the right side, just as on a highway, to avoid pilots going the other way. Throttle back to a comfortable slow-speed cruise to keep the terrain features clearly in sight.
life saving manoeuvre It is with hesitation and trepidation that the following manoeuvre is presented. Not because it is difficult to perform, but because someone is going t o use it as a crutch t o scud run when he shouldn’t. Remember, this is an “out,” not a manoeuvre that is reserved as a routine part of flight. This is an emergency lifesaving manoeuvre. Suppose your weather check meets your parameters for safe flight. You take off and fly toward the destination and somehow get lulled into a situation where the visibility makes you a strong candidate for flying into cumulo-granite. Descend close to the ground and find a prominent landmark that can be circled without worrying about the lateral terrain clearance. Fly around and around until the visibility improves. You might be stuck there for a long time. The object is to remain flying without running into something.
Kelvin-Helmholtz Billow Clouds are similar to whitecaps on a lake and indicate turbulence.
Different form of Kelvin-Helmholtz Billow Clouds.
Scud running is, like Russian roulette, a dangerous game. With reduced visibility only the foolhardy flirt with Mother Nature.
When the visibility is unrestricted beneath the clouds, a look-see may (with a strong emphasis on may) be tried. Be careful.
