Performance is the primary limiting factor in high-altitude operations, particularly when operating aircraft with normally-aspirated engines

Preflight planning and the estimating performance data are essential.

Preparation is important to any flight; it is essential in mountain flying.

High country accident records are replete with pilots who defied the golden rules of mountain flying.

With each of these accidents, the safety record of the entire general aviation industry is marred, with the associated legal, regulatory and public relations consequences.

As pilots, we have a responsibility for both our own safety and, to some degree, the public perception of general aviation.

Section 1 – Planning the Flight

Section 2 – Flight Operations

Section 3 - Cautions and Suggestions for Safe Mountain Flying

The Pilot

Weather

Atmospheric Stability,

Temperature and Moisture Content

Mountain Airflow

Mountain Waves

Aircraft Performance: Density Altitude

Performance Prediction

Hypoxia

Increase in altitude is likely to result in a decrease in human performance

Humans can acclimate to altitudes of 20,000 feet, as evidenced by the Andean natives living and working at these extreme elevations.

Altitude tolerance varies from person to person

Atmospheric Stability and Temperature

Moisture Content

Mountain Airflow

Mountain Waves

Stable air is less likely to generate intolerably turbulent conditions.

Temperature plays a role in stability,

Warm air overlying a colder layer creates an inherently stable condition.

Conversely, cold air above warm causes both to seek their proper place in terms of density, resulting in convective churning.

There are other ways to predict stability.

One of the best indicators is the lapse rate.

As a rule of thumb, calm air cools at a rate of 2° Celsius (or 3.5° F) for each 1,000 foot increase in altitude.

This standard lapse rate is for marginally stable air. If the air doesn't get as cold as quickly, or even warms with an increase in altitude, the air is in the stable range. If the temperature drops at a greater rate than standard, unstable air is present. As mentioned earlier, air stability plays a key role in the development of mountain weather systems. .

Another temperature consideration for mountain flying involves survival after a forced landing.

Climbing from sea level to an altitude of 10,000 feet, temperature decreases by about 20°C, or 35°F

At a climb rate of 500 feet per minute, our aircraft can propel us from summertime conditions to snow-ski weather in just 20 minutes.

Exacerbated by wind often associated with mountainous terrain.

A balmy temperature of 30°F will feel like 4° in a 20 mph wind.

Survival Implications:

In the event of an emergency landing in high terrain, survival may depend on the cold-weather clothing we thought to pack in the aircraft.

Essentials:

warm boots, a full-body snowsuit with face protection and good gloves. Ski apparel is well suited to most situations.

When operating in remote areas, particularly in the cooler months, a full survival kit should be a checklist item. In any season, a supply of drinking water is essential to survival.

Clouds

Moist air will generate clouds.

Dry air won't.

Cloud formation offers insights into the type of air around them.

Cumulus cloud types indicate unstable air.

Stratus clouds tell of more stable atmospheric conditions.

Imagine a slow-moving river with smooth flowing water.

Add rocks

Air flowing to a mountain or ridgeline is either deflected through passes or lifted mechanically.

Often, it gains speed and becomes less stable.

Windward Side

Lifted air tends to cool, often causing moisture to condense and form clouds.

Can create upslope fog.

The way in which clouds develop offer clues as to the properties of the air itself.

Billowing cumulous clouds hint at unstable air and similarly unstable flight conditions;

Smooth stratiform clouds hint at the opposite.

The formation of clouds on the windward side of a mountain offers an increased likelihood of precipitation there, as well.

Generally, smoother flight is to be found on the windward side, where we receive the additional benefit of orographic lift.

Leeward Side

Tends to receive less precipitation, since the descending air is warmed and dried.

Air is "shaken up" by its tumultuous ride over the summit.

Airflow on the lee side tends to be far more turbulent and unstable.

 

The Peaks and Ridgelines

As wind crosses a ridgeline, it accelerates.

Not uncommon for ridge top velocities to be twice as strong as undisturbed airflow velocities.

This acceleration amplifies the effects of updrafts, downdrafts, and the development of mini-weather systems.

Preflight considerations include:

Speed and stability of the mountain airflow. In the face of high winds and unstable air, even experienced mountain pilots in powerful aircraft will decline a scheduled flight.

During preflight weather briefings in mountainous areas, be certain to request as much information as is available regarding wind speeds and stability indices.

If a "go" decision is made, continuously look for signs of adverse conditions, which we will explore in the following pages.

Formed by strong airflow over mountainous terrain.

Also called a "standing wave."

Form on the downwind side of a mountain or ridgeline, and can extend for 100 miles or more.

The intensity of the wave action is determined mainly by mountain height, slope and wind velocity.

Can be predicted with some degree of accuracy.

Absent forecast information or PIREPs be alert for the presence of the conditions necessary for mountain wave formation:

Wind flow perpendicular to the ridgeline, with velocities of 20 knots or more at mountaintop level.

A wind profile marked by an increase in velocity near the summit. A strong, steady airflow is usually found in the upper atmosphere.

An inversion layer near the mountaintop.

Visual signatures include:

lenticular clouds,

cap clouds and

rotor clouds.

These clouds are peculiar in that they appear to hold perfectly still in the roiling airflow.

In fact, the clouds are anything but still; the passing airflow offers moisture for condensation at a given location, then continuously dissipates the visible moisture at the trailing edge of the cloud. These clouds are constantly forming and dissipating in the same place.

Lens- or almond-shaped altocumulus clouds found in the upper regions of the mountain wave, anywhere from just above ridge top level to more than 40,000 feet.

Always form downwind from the ridge responsible for the wave, and may form in bands or as a single cloud. Although the smooth form of lenticular clouds indicates stable, laminar airflow in the vicinity of the clouds, very unstable air is a virtual certainty in the area immediately below.

Or "foehnwall" has been described as resembling a fur cap sitting on the crown of a mountain.

The largest part of the cloud hangs over the upwind side of the mountain, with finger-like extensions running down the ridge on the lee side.

Often described as a "horizontal tornado."

Rotor clouds mark the core area for violent updrafts and downdrafts, which have been measured in velocities exceeding 5,000 feet per minute.

Found on the lee side of a ridge, expect turbulent air regardless of mountain wave activity.

Bases are generally just below ridge top level, tops sometimes reach the base of low-lying lenticular.

As with all clouds, those marking mountain waves are visual signposts, offering valuable insights into atmospheric conditions. "Reading" clouds is a particularly valuable skill in mountainous areas.

Mountain waves usually offer visible clues of their existence, as well as predictable atmospheric influences.

The existence of a mountain wave is a near certain indicator of turbulence.

This varies from light to extreme, but generally tends toward severe.

The degree of turbulence varies with wind velocity, air stability and location within the wave.

As with all clouds, those marking mountain waves are visual signposts, offering valuable insights into atmospheric conditions.

"Reading" clouds is a particularly valuable skill in mountainous areas.

Density Altitude

Predicting Performance

Density altitude is where your airplane thinks it is.

Pressure Altitude corrected for non-standard temperature.

Why it’s important.

Why it’s important

Both lift and engine output are affected by changes in density altitude.

Combined, these changes can be significant.

In the summer months, it is not uncommon for density altitude at high-elevation airports to exceed the service ceiling of many normally aspirated aircraft.

During the cool hours of early morning and evening, the cooler temperatures may lower density altitude to a reasonable level.

The less turbulent air and lighter winds associated with these times of the day are added benefits.

Review the essential elements of temperature and pressure.

Ambient Condition Standards:

Temperature 15 degrees Celsius or 59 degrees Fahrenheit.

Pressure – 29.92 inches Hg

Lapse Rate:

-2 degrees Celsius/1,000 feet of elevation

-1 inch Hg/1,000 feet of elevation

Armed with pressure altitude and outside air temperature, we can determine density altitude from a chart or by using a slide-rule or electronic computer.

The most profound effect of elevated density altitude values is a decrease in take-off and climb performance.

In hot and high conditions, density altitude consideration is vital for several reasons.

First, even if the available runway is adequate for takeoff, conditions may not allow a reasonable climb rate.

As a rule, it takes 40 to 80 percent more distance to clear a 50-foot obstacle than it does to get the wheels off the ground.

This assumes that the aircraft is actually flying and not just mushing along in ground effect.

First, even if the available runway is adequate for takeoff, conditions may not allow a reasonable climb rate.

As a rule, it takes 40 to 80 percent more distance to clear a 50-foot obstacle than it does to get the wheels off the ground.

This assumes that the aircraft is actually flying and not just mushing along in ground effect.

POH/IM

Computers

Technique

The best source for aircraft performance data

Complex and more recently manufactured aircraft generally more comprehensive is the POH

Some older aircraft are left without much information

There are a number of computers that provide general estimates of predicted performance.

The Denalt computer is a simple circular slide rule capable of arriving at relatively conservative takeoff distance and rate-of-climb values. The Denalt factors ambient temperature and density altitude into sea-level performance for a given aircraft to arrive at its estimates..

More sophisticated computers allow a wide range of performance-changing variables to be factored into its final estimate of aircraft performance.

Takeoff and climb performance are critical in high-country operations,

It is a good idea to use more than one source for calculation.

The value obtained by the aircraft POH is considered the most accurate.

However, a significantly longer ground roll computed on a slide rule computer should be taken quite seriously.

Performance values obtained from the POH should be considered very optimistic in nature.

Original figures were obtained in a new, perfect-condition aircraft with an experienced test pilot at the controls.

Calculated performance figures should be padded by 50%-100% to allow for less definitive variables.

Climb out at a calculated airspeed, rather than a "ballpark" figure. (be precise)

The best-rate and best-angle of climb airspeeds that we've likely committed to memory for our aircraft are usually values for sea-level, standard-day, gross weight operations.

For high-country operations, an understanding of performance values beyond the baseline is critical.

First, best rate of climb airspeed (Vy) decreases with altitude.

For example, a Cessna 172N offers its best sea-level climb performance at 73 knots; at 12,000 feet, Vy is 67. Climbing through 12,000 feet at any airspeed other than 67 knots will result in decreased climb performance.

For aircraft without published Vy values other than sea level, it is generally assumed that Vy decreases by approximately 1% per 1,000-foot increase in altitude.

As with all rules of thumb, this is a rough estimate, which is better than no estimate at all.

Best angle of climb airspeed (V_x), on the other hand, increases as altitude increases.

The change is not as great as the V_Y change.

The rule of thumb here is to allow for just less than a .05% increase per 1,000-foot increase in altitude.

Airspeed control required for flight at the absolute ceiling is a cogent reminder that speed ranges become smaller and smaller and airspeed control becomes increasingly critical as altitude increases.

Many POHs fail to offer much guidance on target climb speeds at reduced weights.

Another rule-of-thumb is useful here. For most single-engine light aircraft, V_y and V_x each decrease about .5 knots for each 100 pounds below maximum gross weight.

The climb performance improvement available varies significantly from aircraft to aircraft, but can be as much as 100-feet-per-minute for each 100 pounds off-loaded.

Routing and Terrain

High-Elevation Takeoffs

Enroute: Leaning

Cruising Flight Cockpit Check

Rough Air Procedures

Landings

Specific Mountain Airport Peculiarities

Summary

A thorough study of the terrain along the proposed route.

Highest elevations along that route will determine the cruise altitude requirement.

Climbs-to-altitude and circuitous routings can substantially increase fuel use.

File a flight plan.

Safe crossing altitudes are determined in large part by wind and turbulence.

In placid air, a margin of just 1,000 feet above the ridge may be adequate.

Winds greater than 20 knots or unstable air, 3,000 feet or more.

Cardinal rule of operating in mountainous terrain:

Always be able to turn toward lower terrain. Note: Initially, you must be able to descend without turning; otherwise you are too low.

Routing and Terrain –
In Flight Emergency

In-flight emergency requiring a landing,

turn downhill immediately.

look for valleys or meadows or other favorable landing areas

if landing in trees is necessary, look for a stand containing smaller trees.

The landing should be made upwind and uphill if at all possible.

Contact with the ground should be made at a slow airspeed, but at a flying airspeed.

Do not try to stall out at any significant height above the ground or attempt to pancake the airplane.

Attempting to minimize forward speed often results in high vertical and horizontal impact loads.

Both the aircraft structure and your body are capable of sustaining stronger forward impact loads compared to vertical loads. The best approach involves slow, but controlled, ground contact at a minimum vertical sink rate.

Go/no-go decisions at high-country airports are influenced by:

density altitude,

aircraft weight,

wind, and

runway surface and slope.

After these variables are applied to POH or computer-generated estimates, takeoff roll and initial climb estimates are made, and a 50 to 100% safety factor is applied. If a go decision is made, the pilot then needs to apply proper high-country operating techniques to obtain the best possible aircraft performance.

Select an abort marker. This can be a crossing runway, the fifteenth (for example) runway light, or a point adjacent to an object on the ground.

This point should consider your takeoff distance estimate and its associated safety factor, balanced against the distance required to stop the aircraft.

This point should be considered a non-negotiable go/no-go decision point.

During a long, fast takeoff roll, it will prevent having to make a critical decision late in the takeoff sequence.

Departure flight path more important in the mountains than in lower elevations.

Particularly true of airports situated in craggy terrain, where straight-out departures could result in an off-airport mountainside landing.

Local pilots great help describing safe departure corridors.

Look for a path offering the gentlest turns possible, since any departure from straight flight consumes lift.

Visualize the airflow over mountains along the route. The lowest pass may not be the best if it requires passing through downdrafts.

Also ask local pilots about runway grade. In mountainous terrain, visual illusions abound, and what appears to be a perfectly level airstrip may have a substantial slope.

Run-up

Density altitude above 5,000 feet, make a full-power run-up.

Allows you to set the mixture for maximum power at take-off.

Requires a hard surface free of sand or pebbles

If there is no debris-free location for a full-power run-up, you have no choice but to estimate the mixture setting and tweak it during the takeoff roll.

Flaps.

Add drag as well as lift, their value to takeoff performance diminishes as altitude increases.

General rule - if the POH recommends 20° flaps for a short field takeoff at sea level, we would use half that, or 10°, at a density altitude between sea level and the aircraft's absolute ceiling.

At density altitudes closer to absolute ceiling, even minimal drag is undesirable, don’t use flaps.

Best angle-of-climb airspeed (V_X) for our aircraft at the heightened altitude.

Rotation speed (V_R) should be approximately five knots slower than V_X.

Offers a good balance between the too fast/too slow dilemma described in the previous section.

Pilots inexperienced in high-country operations often try to rotate early, resulting in a significant drag increase for the duration of the takeoff roll.

Proper Leaning

Most efficient, most power

Mixture should be adjusted whenever a power setting or altitude changes.

After startup at a high density-altitude airport, the power should be set to 1,000 RPM and the mixture leaned for maximum RPM.

For the run-up, it is necessary to enrich the mixture from its taxi setting.

Usually, pushing the mixture half way in from its taxi position provides a usable setting.

After run-up, lean the mixture again for maximum RPM until ready for takeoff.

Engine manufacturers require that the mixture be set to the full rich position for takeoff.

This setting is intended to augment engine cooling.

However, the "full rich" stipulation usually doesn't apply at density altitudes above 5,000 feet.

There, the reverse applies;

to obtain maximum power for takeoff, it is necessary to lean to the best power setting prior to takeoff.

During climb-out, the process of leaning for best power will continue.

Only upon reaching cruise altitude does the mixture control come to rest..

Another leaning consideration has to do with equipment installed in the aircraft.

Exhaust Gas Temperature (EGT) gauge.

The best mixture setting is obtained when the EGT on the hottest cylinder is taken to its hottest point, then the mixture enriched until the temperature cools by 50° to 100°, depending on advice offered by the manufacturer.

Fuel flow meter also provides good leaning counsel. When neither of these aids is available, engines paired with fixed-pitch props can be leaned to maximum RPM.

Use a good ear in the engine tuning process.

Performance decreases drastically above 6,000 feet.

Practiced routine for cockpit chores.

Systematic approach

Comms

Avionics

Nav

Gyro

Engine

Trim

Use V_A in turbulent conditions

Keep pitch and bank as level as possible.

Don’t fight the controls-be gentle.

Mountain waves may prevent climbs at V_Y

There are situations in which an aircraft cannot keep from climbing even while pitched nose-down and at idle power .

Accept and notify ATC if on IFR flight plan

Plan performance from POH

Expect higher than normal ground speed – its not an illusion.

Fly the approach at the minimum acceptable approach speed.

Increase approach speed only for gusty or wind shear conditions.

The Approach

Stabilized approach essential to high-altitude operations.

Adjust power for a reasonable descent profile at the planned airspeeds.

On short final, trim the airplane for five knots faster than the over-the-fence airspeed.

Once below 200 feet AGL and committed to landing, add up to full flaps.

Keep in mind, with flaps and gear extended, there may not be sufficient power to maintain level flight, let alone enter a climb. At this point, you’ve committed to land.

The Round out, Flare and Touchdown.

Entering ground effect, usually below 20 feet AGL, reduce power to idle and initiate flare per normal procedure.

At touchdown, the sensation of speed and vibration will be accentuated due to the increased true airspeed.

Use short field braking technique.

Raising the flaps, too, can reduce lift and allow improved braking.

At turn-off, you should further lean the mixture setting to a taxi power setting. Prevent over-rich engine settings on the ground to prevent spark plug fouling, make more engine power available for the next departure.

Go Around

If a go-around is necessary,

apply full power without delay

establish the airplane in the best angle of climb attitude.

Bear in mind that published best angle-of-climb airspeeds generally are intended for gear-up, no-flap, sea-level climbs. In a dirty configuration half way to the aircraft's absolute altitude, best climb performance could occur as much as five knots slower than published Vx.

Reduce flaps to the setting recommended in the POH. The landing gear should be retracted when the runway and any rollout area cease to be usable.

Big Bear and Tahoe, California, are popular high-altitude airports with excellent facilities. Both occasionally have turbulent conditions, generally associated with unstable afternoon air.

Visual illusions are not uncommon at high-country airports. Mariposa, California, features a sloped runway that appears level during the approach.

Mammoth, California, the seemingly level terrain past the end of the runway actually slopes up about 200 feet per mile. At 90 knots, an aircraft climbing at 300 feet per minute would just keep pace with the terrain gradient.

If you can recognize certain hazards and appropriately address them, the high country offers safe flying in some of the planet's most breathtaking scenery. It is a substantial reward for completing mountain flight training.

Terrain

The flight should be planned and flown to avoid terrain that would prevent a safe (survivable) forced or precautionary landing.

Sufficient altitude should be maintained at all times to permit gliding to a reasonably safe landing area.

The flight should be made along routes that include populated areas, roads and/or rivers and well-known mountain passes.

Are much better than WAC charts for detailed information useful in pilotage.

Study them thoroughly for prominent navigational information and check points useful over the route to be flown.

With proper application of variation, deviation, and wind correction information, it is the only device you can rely on to get you out of trouble.

However, be alert to compass irregularities in areas of local magnetic disturbance, usually marked on maps and charts.

Don't fly in light aircraft when winds aloft, at your proposed altitude, are reported above 30 knots. Expect winds to be of much greater velocity over ridges and through passes than reported a few miles from them.

Know the wind direction at all times; compare it to water as it flows up, over and down mountain ranges. Watch for abrupt changes of wind direction and speed in mountainous terrain.

Don't fly near or above abrupt changes of terrain such as cliffs, peaks or rugged edges. Extreme turbulence may be expected, especially with high winds.

Don't fly up the MIDDLE of a canyon at any time. It is better to fly up one side or the other to so as to be in better position to make a 180 degree turn. The sun side is the lift side (normally).

Realize the actual horizon is near the base of the mountain. The mistake of using the summit of the peaks as the horizon will result in the aircraft being placed in a climb attitude. This could inadvertently lead to a stall.

Approach mountain passes with as much excess altitude as possible. Downdrafts of 1,500 to 2,000 (or more) may be encountered on the leeward side.

Approaching the passes over a ridge will reduce this effect considerably. A clearance of 1,500 feet to 2,000 feet is preferred on windy days (at least!).

Expect winds above 10,000 feet to be prevailing westerlies in most western state areas.
Approach passes and ridges at a 45-degree angle so that you will be able to turn 90 degrees to the low country, instead of 180 degrees, if you encounter too great a downdraft.

Many experienced pilots advise that an inexperienced pilot should make a power-on approach and landing at a high altitude airport.

This procedure is definitely advisable in gusty air.

Parking on sloping terrain may cause fuel to siphon overboard. Place the fuel selector in OFF when parked or tied down. (don't forget to turn the fuel selector back to ON before departure!).

When taking off in a narrow canyon with several sharp bends, DOWN AIR may be encountered without warning.

Remember that seldom is a flight in mountainous terrain purely routine - - EXPECT THE UNEXPECTED!!

Remember that YOU, the PILOT, have responsibility for the GO/NO-GO decision based on the best information available.

DO NOT let compulsion take the place of good judgment: Know you CAN go or stay on the ground!

Enjoy the adventure!