FLIGHT SAFETY :: 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.

1. 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.

2. 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 to 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.

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 can take off in the remaining distance.

Note: This rule of thumb guarantees takeoff performance, but not rate-of-climb after the 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 the 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 than 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 the 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 the 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.

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, climbing 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 it becomes necessary to make a 180-degree 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 make 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.

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 enter 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 to idle and begin a normal glide, you will have sufficient altitude to turn around without impacting the terrain.

Constantly evaluate 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.

(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 terrain, he has experienced a phenomena known as "short arm" effect. The self-preservation instinct causes a pilot to unconsciously pull back on the control wheel to avoid the rising terrain. The airplane slows down and this 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."

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 mid-latitude 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 that 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 total 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 the 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 west-facing 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 this is that when landing on an airstrip in a drainage, there will be a tailwind to contend with. The average wind speed is 6-8 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 the 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 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.

Never fly beyond this point of no return. Turn around and manoeuvre for additional altitude prior to continuing.

CAUTION: 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.