Mountain Flying

March 25, 2013

AOPA's A Pilot's Guide to Mountain Flying

Mountain Flying

BY THE AOPA AIR SAFETY FOUNDATION

Introduction

On behalf of the AOPA Air Safety Foundation staff, thank you for your interest in, quite literally, expanding your horizons by learning the techniques of mountain flying. Operations in the rapidly changing weather conditions and the thin air of the high country differ significantly from normal flight operations. However, Federal Aviation Regulations don't require a special rating, a CFI endorsement, or even preparatory training for a pilot to fly in this challenging environment. It is left to our good judgment and sense of professionalism to develop the appropriate skill and knowledge base before venturing into the high country.

Unfortunately, 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.

The Air Safety Foundation is pleased to be your partner in safe flying. Since 1950, we have grown into the largest organization of our type. Our motto, "Safe Pilots. Safe Skies," well describes the mission; to provide the information you need to make good decisions and fly safely. We conduct research through one of the world's largest general aviation safety databases; publish newsletters, safety advisors, and type-specific accident reviews for various aircraft; produce videotapes and online training programs; conduct free nationwide safety seminars, flight instructor refresher clinics, and specialized training programs, such as the Pinch-Hitter(r) seminar for right-seat companions.

Our programs both serve you and are supported by you. The vast majority of the Foundation's operating budget comes from individual and corporate contributions. Your support of our efforts and your continued interest in aviation safety are sincerely appreciated. We wish you safe skies.

Bruce Landsberg
Executive Director
AOPA Air Safety Foundation


Mountain Flight Training

Operating safely in mountainous terrain requires both technical knowledge and hands-on flight training by an experienced high-country CFI. Most flight schools and FBOs in mountainous areas offer such training, and normally require completion of a mountain flying course before releasing their aircraft to renters. The objective of this publication is to provide a solid background in the procedures, techniques and limitations associated with mountain country operations. Followed by a thorough, hands-on training program, you will be prepared for the exhilaration of operating in some of the most challenging and magnificent territory in the western world.

Aircraft performance is the primary limiting factor in high-altitude operations, particularly when operating aircraft with normally-aspirated engines. As such, thorough preflight planning and the determination of performance data are essential. The preflight phase also needs to include a complete weather briefing and a review of charts and airport facility directories for the area in which operations are planned. During hands-on training, don't be surprised if your instructor seems to spend more time on "ground" issues than flight time. Preparation is important to any flight; it is essential in mountain flying.

Section 1: Planning

First, the Pilot

Any substantial increase in altitude is likely to result in a decrease in human performance, both physiologically and psychologically. Pilots should be aware of the potential for reduced functioning, both on the ground and in the air, while conducting high-country operations.

Humans can acclimate to altitudes of 20,000 feet, as evidenced by the Andean natives living and working at these extreme elevations. However, they benefit from generations of adaptation. For those living in lower elevations, the body's internal adaptation to high elevations normally requires six to eight weeks. Upon returning to lower country, the acclimation is quickly lost.

Altitude tolerance varies from person to person, but everyone is affected. Pilots who claim immunity from high-altitude illnesses have either not flown very high, or have likely fallen victim to the most insidious type of illness: hypoxia, or oxygen deprivation. Since loss of judgment is one of the first and most pronounced symptoms of hypoxia, the remaining symptoms are easily ignored.

In simple terms, hypoxia is the result of insufficient oxygen in the bloodstream. In addition to altitude, many drugs, alcohol, and smoking diminish the blood's ability to absorb oxygen and the brain's ability to tolerate this condition.

As a general rule, flights below 10,000 feet msl can be conducted by most pilots without the use of supplemental oxygen. However, night vision is profoundly affected by oxygen deprivation. As such, supplemental oxygen should be considered for all night flights above 5,000 feet msl. A thorough check of oxygen system operation should be added to the preflight.

The onset of hypoxia is insidious. Often, the condition causes a sense of well-being or euphoria that masks its slow progression. Impaired reactions, confused thinking, poor judgment, unusual fatigue, and dull headaches are other common symptoms. Hypoxia may offer physical clues, including a blue coloration of the extremities, such as fingernails and lips.

The cure for hypoxia should be self-evident: do what's necessary to provide the body with more oxygen. If supplemental oxygen is available, use it. If not, select a lower altitude or land.

To ascertain individual tolerance and to experience the effects of high-altitude performance degradation, military pilots are provided with physiological training in altitude chambers. Through an agreement with FAA, all pilots are allowed to take part in these programs, which are conducted at Air Force bases nationwide. General aviation pilots who have participated in physiological training have found it a vivid and valuable demonstration of the limitations of human effectiveness at decreased pressure levels. This training is strongly recommended.

Weather

In flight planning, be meticulous in obtaining and interpreting weather information. In any operating environment, fronts, fog, thunderstorms, ice, and other significant hazards need to be taken quite seriously. Mountain country weather offers new considerations and concerns. Among these is the variability of weather in the mountains, which cannot always be predicted, but should always be expected. In addition, the sparse nature of weather reporting stations in the remote reaches of the high country can reduce the amount of information available. Also, there are fewer airports offering safe haven when adverse conditions develop. An understanding of the dynamics of airflow over mountains and ridges offers some insight into how and where mini-weather systems develop.

Atmospheric Stability, Temperature and Moisture Content

Stability, temperature, and moisture are the three primary properties of a parcel of air. How air will react to its encounter with mountains is dictated by these properties. Stable air, for instance, is less likely to generate intolerably turbulent conditions. Temperature plays a role in stability, since 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.

Moist air will generate more clouds, where dry air won't. If clouds do form, they usually offer insights into the type of air around them. Cumulus cloud types indicate unstable air, while stratus clouds tell of more stable atmospheric conditions.

Aside from cloud reading, 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. When climbing from sea level to an altitude of 10,000 feet, the air temperature can be expected to decrease by 20�C, or 35�F With an average climb rate of 500 feet per minute, our aircraft can propel us from summertime conditions to snow-ski weather in just 20 minutes. This change is exacerbated by the windy conditions often associated with mountainous terrain. A balmy temperature of 30�F will feel like 4� in a 20 mph wind.

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. Among the essentials are 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.

Mountain Airflow

Imagine a slow-moving river with smooth flowing water. Where we find rocks in the stream, the water finds less tranquil passage. Restricted by the rocks, the same volume of water is forced to pass through gaps or lunge over the obstructions. Speed increases and the flow is so turbulent that it earns a rather foreboding name: whitewater.

The same can be said of air passing through mountainous terrain. Air flowing to a mountain or ridgeline is either deflected through passes or lifted mechanically. Often, it gains speed and becomes less stable. This is the atmospheric equivalent of whitewater.

Lifted air tends to cool, often causing moisture to condense and form clouds. Under certain conditions, this action creates upslope fog. In others, developing clouds 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.

By contrast, the lee side tends to receive less precipitation, since the descending air is warmed and dried. That air has, however, been "shaken up" by its tumultuous ride over the summit. As such, airflow on the lee side tends to be far more turbulent and unstable. Generally, smoother flight is to be found on the windward side, where we receive the additional benefit of orographic lift.

As wind crosses a ridgeline, it accelerates. It is not at all uncommon for ridgetop velocities to be twice as strong as undisturbed airflow velocities. Of course, this acceleration amplifies the effects of updrafts, downdrafts, and the development of mini-weather systems.

Preflight considerations include the 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.

Mountain Waves

When airflow over mountainous terrain becomes strong enough, a "mountain wave" or "standing wave" may form. These waves 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.

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.

Mountain waves can be predicted with some degree of accuracy. However, in the absence of forecast information or pilot reports (pireps) for a given area, pilots should 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.

The visual signatures of a mountain wave 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.

Lenticular clouds are lens- or almond-shaped altocumulus clouds found in the upper regions of the mountain wave, anywhere from just above ridgetop level to more than 40,000 feet. They 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.

The cap cloud, 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.

The rotor cloud is a self-descriptive phenomenon 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. These clouds are found on the lee side of a ridge, where turbulent air could be expected regardless of mountain wave activity. Rotor cloud bases are generally just below ridgetop level, with tops sometimes reaching almost to the base of low-lying lenticulars.

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.

Aircraft Performance: Density Altitude

Put simply, density altitude is where your airplane thinks it is. An airport with an actual field elevation of 2,000 feet may have unusually thin air, caused by a warm temperature and low barometric pressure. Your airplane's takeoff and climb performance may be significantly reduced. If the pressure/temperature combination causes the air to compare to standard conditions at 5,000 feet, that airport has a density altitude of 5,000 feet. Your aircraft will take off and climb as if departing a field with a 5,000-foot elevation.

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.

Such conditions usually are temporary. 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.

Before further exploring the effects of density altitude, let's review the essential elements of temperature and pressure.

Conditions are said to be standard at a sea-level airport if the atmospheric pressure is 29.92" Hg and the temperature is 59�F. It also is standard for these values to decline by 1" Hg and 3.5�F, respectively, for each 1,000-foot increase in altitude. Therefore, an airport with a field elevation of 1,000 feet is experiencing standard conditions if the ambient pressure is 28.92" Hg and the temperature is 55.5�F. At a 2,000-foot elevation field, 27.92" Hg ambient pressure and 52� F would be standard.

Before going further, note that the pressure values above are ambient pressure. The altimeter setting is not the actual barometric pressure at the field, but the pressure adjusted to sea level. Thus, when we set the sea level pressure into our altimeter, it reads the altitude above sea level. If we set the actual, or ambient, pressure into an altimeter, it would read "0" on the ground.

A common and more convenient standard for gauging air pressure is pressure altitude. This is simply the altimeter readout when 29.92" is dialed into the pressure window. If the ambient pressure is low, pressure altitude will be high. For example, at an airport with an elevation of 300 feet, you set 29.92" into the altimeter, and the altitude readout is 1,000 feet. Your altimeter has just informed you that, as compared to standard pressure, the air outside feels like air should feel at 1,000 feet. You've just determined that the air pressure is relatively low. In this thin air, the aircraft cannot be expected to perform as well as it would in denser air.

Of course, pressure is not the full story. As noted previously, an increase in temperature also decreases performance. An accurate prediction of aircraft performance must consider both pressure and temperature influences. That's density altitude. The pure definition of density altitude is "pressure altitude corrected for nonstandard temperature."

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.

Also, performance charts in many operating handbooks allow us to select the ideal speed for rotation and climbout. These numbers become increasingly important when the aircraft is operated at the edge of its performance envelope. A number of high-density altitude takeoff accidents have resulted from rotation at too high a speed, which consumes too much runway; or at too low a speed, which induces drag and prevents efficient acceleration to a safe climb speed.

Another performance value of interest is the highest altitude at which the powerplant is capable of producing maximum cruise power, which is usually about 75% of rated horsepower. At this altitude, the aircraft can produce its fastest level-flight true air speed. By determining this value, we can select our fastest cruise altitude, which, incidentally, will be lower on hot days.

Performance Prediction

The best source for aircraft performance data is its Pilot's Operating Handbook (POH). Generally, the more complex and more recently manufactured the aircraft, the more comprehensive is the POH. Unfortunately, this leaves some older aircraft without much information. Fortunately, we can compliment our POH data with estimates from other sources.

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. Other factors, such as wind, runway slope, gross weight changes, and runway surface type are not considered by the Denalt. Once sold for less than $1.00 by the government printing office, Denalt computers now are difficult to find.

A more sophisticated computer, the TOPComp, does allow a wide range of performance-changing variables to be factored into its final estimate of aircraft performance. The TOPComp, too, uses sea-level, gross-weight performance figures as its basis, since these values are available in the most rudimentary POHs.

A useful capability of this device is the opportunity to trade performance factors to arrive at acceptable takeoff distance and rate of-climb values. For example, a pilot could determine whether a downhill takeoff with a tailwind was preferable to an uphill takeoff with a headwind. TOPComps are available from a number of commercial sources.

Because takeoff and climb performance are so critical in high-country operations, it is a very good idea to use more than one source for calculation. The value obtained by the aircraft POH is, of course, considered the most accurate. However, a significantly longer ground roll computed on a slide rule computer should be taken quite seriously.

The performance values obtained from the POH should be considered very optimistic in nature. Because the original figures were obtained in a new, perfect-condition aircraft with an experienced test pilot at the controls, calculated performance figures should be padded to allow for less definitive variables. At a minimum, required runway values should be increased by 50%. When operating an older aircraft in an unfamiliar environment, it is more prudent to double the estimated values.

As noted previously, climbout should occur at a calculated airspeed, rather than a "ballpark" figure. 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. In routine, flatland operations, where our aircraft is blessed with abundant reserve performance, the sea-level numbers are fine targets. 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 (Vx), on the other hand, increases as altitude increases. The change is not as great as the Vy change. The rule of thumb here is to allow for just less than a .05% increase per 1,000-foot increase in altitude.

The precise 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. Contrary to popular belief, our sample aircraft, flying at absolute altitude at 65 knots, is not wobbling on the edge of a stall. It is at its most efficient airspeed in terms of lift compared to drag. Any change in airspeed results in increased drag and a descent. Operating at UD max to squeeze the last bit of performance from an aircraft is nothing new to experienced glider pilots. It is, however, why hands-on training is useful for new high-country pilots.

You can create a Vx/Vy chart for your aircraft. You'll need POH values for Vx and Vy at different altitudes. If these are unavailable, use the rule-of-thumb methods described above. The end result is a revealing graphical display of proper airspeeds for optimum climb performance.

Another useful chart is one that plots the best rate of climb for various density altitudes. This, too, can be developed from POH data, or home grown by actually recording your aircraft's climb performance at various altitudes.

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, Vy and Vx each decrease about .5 knots for each 100 pounds below maximum gross weight. If your aircraft is 400 pounds under maximum gross weight, selecting a target speed two knots slower than usual should improve climb performance.

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.

Section 2: Flight Operations

Routing and Terrain

Any high-country flight must begin with a thorough study of the terrain along the proposed route. Of course, the highest elevations along that route will determine the cruise altitude requirement, which must be considered against aircraft performance and oxygen availability. In addition, available fuel can be a constraint, since long climbs-to-altitude and circuitous routings can substantially increase fuel use.

Very remote and precipitous terrain should be avoided for several reasons, running the gamut from the desire to have good navigation signals to survival considerations in the event of a forced landing.

Filing a flight plan should be considered mandatory in the high country. In the "Route" portion of the plan, be sure to give a detailed description, including any passes or valleys you intend to traverse.

Armed with the ability to visualize airflow over terrain, a pilot can select a route that minimizes the effect of turbulence and maximizes the benefit of available updrafts. Flying along the upslope side of a ridgeline not only provides free energy, it can be a lifesaver if a 180� turn is necessary. That turn would be made into the wind, toward lower terrain and with the full width of the canyon available. Compared to flying up the middle of a canyon, the advantages of this technique are abundant.

The ability to turn toward lower terrain is considered essential by experienced mountain pilots. Narrow canyons and box canyons (those with a closed end) are common traps for pilots who fall short on their homework assignment. Deteriorating weather and cable runs across canyons present additional snares. Discussing routing and deviation options with local pilots is the best way to avoid such unpleasant surprises.

When crossing a ridge, you should do so at an approximate 45� angle to the ridgeline. If a downdraft or other conditions require you to abandon the crossing, a 90� turn will take you away from the ridge. If you approach the ridge at a 90� angle, turning away from the ridge would involve at least 135� of turn. As you approach any ridge, look at the terrain beyond it. More of the scenery beyond should become visible. This indicates that you are higher than the ridge. If the background scenery becomes increasingly hidden by the ridge, you are below ridgetop level.

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. If the winds are greater than 20 knots or the air is unstable, an adequate altitude could be 3,000 feet or more. In any case, always recall the 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.

If an in-flight emergency requires 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.

High-Elevation Takeoffs

As we've learned, 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.

The first operational consideration is the selection of 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.

The departure flightpath is far more important in the mountains than in lower elevations. This is particularly true of airports situated in craggy terrain, where straight-out departures could result in an off-airport mountainside landing. Local pilots can be of great assistance in describing safe departure corridors. If left to your own devices, look for a path offering the gentlest turns possible, since any departure from straight flight consumes lift. Also, be certain to 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.

Prior to departing an airport with a density altitude above 5,000 feet, make a full-power runup. This procedure allows you to set the mixture for maximum power at take-off. It requires a hard surface free of sand or pebbles to prevent damage to the propeller or empennage. Often, even grass and gravel high-country airports include a hard stand for this purpose. If there is no debris-free location for a full-power runup, you have no choice but to estimate the mixture setting and tweak it during the takeoff roll.

Another pre-takeoff consideration involves the use of flaps. Since these add drag as well as lift, their value to takeoff performance diminishes as altitude increases. A good general rule is that, 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, constraining the use of flaps.

In the takeoff performance calculations, we arrived at a best angle-of-climb airspeed (Vx) for our aircraft at the heightened altitude. Rotation speed should be approximately five knots slower than Vx. This rotation speed 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. Bear in mind that high density-altitude takeoff rolls will begin sluggishly and will involve a much higher groundspeed than low-altitude takeoffs. As a result of the increased distance and groundspeed, you may notice increased wheel noise and vibration. Be patient; rotation speed or the abort point will come along soon enough, with your actions dictated by which occurs first.

Upon rotation, it's a good idea to accelerate in ground effect, particularly if the runway is long or the air is turbulent. Since drag is reduced in ground effect, this is often the most efficient way to accelerate to best climb speed.

Don't rush landing gear retraction. Many high-country accidents involve aircraft settling back to the runway after the gear is retracted. In some of these, the additional drag associated with the retraction process may have played a role. Most light aircraft are only slightly draggier with the gear extended, so leave the gear alone until a good rate of climb is established and the wheels are no longer useful. Likewise, flaps should be judiciously retracted only after speed and altitude offer breathing room for configuration changes.

During your initial climb, you may wish to experiment with speeds slightly above and below the calculated best climb speed. You may find a speed better suited to your aircraft in the existing environment. When conducting such experiments, don't be fooled by "stick lift," which is the "zooming" ability of the aircraft when the nose initially is raised. Likewise, take time to verify that a dramatic performance change isn't the result of rising or descending air. Once the best climb speed is established, trim the aircraft. Again, since performance is speed-critical, poor airspeed control will be detrimental to your climbout.

Enroute: Leaning

To ensure best engine operation at altitude, proper leaning is essential. Specific procedures and cautions regarding engine leaning are outlined in your Pilot's Operating Handbook.

A new mindset is necessary with regard to engine leaning in the high country. Specifically, pilots should be aware that the mixture should be adjusted whenever a power setting or altitude changes. Doing so not only optimizes engine performance, it reduces internal wear on the engine components.

After startup at a high density-altitude airport, the power should be set to 1,000 RPM and the mixture leaned for maximum RPM. This process sometimes results in an increase of several hundred RPM, and graphically illustrates the additional power available from a properly leaned powerplant. Even at low elevations, running a suitably lean mixture during taxi results in cleaner, longer lasting plugs, valves and pistons.

Before advancing power for the runup, 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 runup, lean the mixture again for maximum RPM until ready for takeoff.

Virtually all 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.

During letdown, mixture control depends on pilot technique. If power settings are allowed to increase during the descent, the mixture must be enriched accordingly. If, however, power settings are continuously reduced, leaning may be in order to maintain proper operating temperature and cleanliness within the engine. Prior to landing at a high-elevation airport, a maximum power mixture setting should be made. At elevations below 3,000-5,000 feet, on the other hand, a full-rich mixture is usually recommended by the manufacturer.

After landing, the mixture immediately should be leaned to the proper taxi setting. Otherwise, the cold engine may drown in the rich mixture, resulting in engine stoppage. A busy runway or taxiway is never a fun place to try to coax a hot start.

Another leaning consideration has to do with equipment installed in the aircraft. The basic gauge for measuring the fuel/air mixture ratio is the Exhaust Gas Temperature (EGT) gauge. In general, 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. A good 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. None of this, of course, is to underestimate the value of a good ear in the engine tuning process. When used in conjunction with one another, these methods provide good quality control over this important process.

Cruising Flight Cockpit Check

At pressure altitudes above 6,000 feet, virtually all pilots begin to experience some degree of performance impairment. Even for the most acclimated pilots, altitudes above 10,000 feet cause significant functioning limitations. As such, even the legal altitudes above 10,000 feet should be considered off limits without supplemental oxygen.

Because of the potential for physiological limitations, pilots need to maintain a practiced routine for the accomplishment of cockpit chores. A basic procedure is to use a good checklist. Others use systematic or memory aid checks, comparable to the landing GUMP check.

One approach that has proven useful to many pilots is a systematic review of system "clusters." "CAN GET" is a memory aid that points to each of the six clusters that need to be monitored. Communication requirements, Avionics settings, Navigation fixes, Gyro settings, Engine Performance, and Trim are the essential clusters, with their associated subsystems fairly apparent.

Experience has shown that the use of such systematic reviews of aircraft status results in increased pilot awareness. After some conscious application of the system review, many pilots find that the process becomes quite automatic. This ongoing survey of aircraft status enhances safety in routine operations, and is invaluable in the high-altitude environment.

Rough Air Procedures

Turbulence is common in the mountains. As such, pilots should know design maneuvering speed (Va) for their aircraft. Often, this speed decreases as aircraft weight decreases, so an awareness of the adjusted Va for the aircraft is important. Before encountering turbulence, set power and trim for Va. If the turbulence becomes heavy or severe, you should attempt to keep the wings of the aircraft close to level. Chasing airspeed and heading create additional stresses on the airframe. Often, in fact, such corrections are made out-of-phase with the turbulence, making the deviations even worse. Temporarily accept the deviations while you ride out the bumps.

Mountain waves generate widespread areas of rising and sinking air. It is possible for a downdraft to exceed the climb capability of an aircraft at full power and Vy. Likewise, there are situations in which an aircraft cannot keep from climbing even while pitched nose-down and at idle power. In these situations, there is no option other than to await the inevitable reversal of the conditions. Altitude changes will occur, and pilots on IFR flight plans should explain the deviations to ATC and request an altitude block for their cruise. In the case of severe downdrafts, it may be necessary to turn toward lower terrain or seek locations in which rising air can be expected.

Landings

Landings at high-country airports also require thorough planning. A review of NOS Airport/Facility Directory information is mandatory, ideally supplemented by data from other sources, such as AOPA's Airport Directory. Like takeoff distance determination, landing distance calculation needs to consider density altitude, runway length, composition and slope, wind, and aircraft weight. Again, POH calculations should be padded with a safety margin of 50% to 100%.

After determining landing distance, it's a good idea to calculate takeoff distances for some typical density altitude scenarios at that airport. Because it takes less room to land than to take off, it is possible to be trapped at a destination.

Along with runway length determination, it's useful to note the width of the destination runway, since that often determines the error margin for landings in crosswinds or turbulent conditions.

Try to plan your arrival for a time that won't have you staring into the sun on approach. A high-altitude landing at an unfamiliar location is challenging enough when you can see the airport.

As with takeoffs, landings at high-elevation airports will involve higher ground speeds and longer ground rolls. Because even a slightly fast approach will result in a significantly longer ground roll, a proper "over the fence" approach speed is important. The indicated airspeed used at high-altitude airports should be the same speed used in similar conditions at low-altitude airports. The true airspeed, of course, will be higher. For this reason, traffic patterns at high-altitude airports should be wider.

Most aircraft have a final approach target airspeed published in the POH. If such a figure is not available for your aircraft, use 1.3 x Vso (the stall speed in the landing configuration). For example, if Vso were 45 knots, 1.3 x 45 would result in a final approach target speed of 58.5 knots. Often, this speed seems uncomfortably slow, and pilots are known to add five knots for each immediate family member. Bear in mind, however, that our example has us flying more than 13 knots above the stall speed. A more realistic danger would be to increase the approach speed to 70 knots, which would increase the landing roll distance by nearly 50 percent. A runway overrun is a tragic error at many high-country airports.

The only time airspeed should be added to the approach is during gusty wind conditions. In that situation, add half of the gust factor. For example, if the wind is 15 knots gusting to 25, the gust factor is 10. You should add half of that, or five knots, to your approach speed. If the end result is a necessarily fast approach to a short runway, other destinations should be considered. It may not be possible to land at a short strip in windy conditions.

At pattern altitude, perform a maximum-power mixture adjustment. Since this adjustment is made to ensure maximum power availability in the event of a go-around, it should be made at the lowest altitude possible. Even then, it's worth noting that the mixture will become richer as approach commences. As such, many high-country pilots give the mixture a last adjustment on short final.

While in the pattern, conduct a final evaluation of the landing runway and the surrounding terrain. Verify wind direction and speed and assess the runway condition. Determine the best departure path in the event of a go-around. Particularly at one-way runways, select a minimum abort altitude that will allow a safe retreat if the approach does not develop as planned.

A stabilized approach is essential to high-altitude operations. Power should be adjusted to allow a reasonable descent profile at the planned airspeeds. On short final, the airplane should be trimmed for five knots faster than the over-the-fence airspeed. Once below 200 feet agl and committed to landing, up to full flaps can be added. Bear in mind, however, that with flaps and gear extended, there may not be sufficient power to maintain level flight, let alone enter a climb. At this point, the decision has been made to land.

As the airplane enters ground effect, usually below 20 feet agl, power can be reduced to idle and the flare initiated as per normal procedure. At touchdown, the sensation of speed and vibration will be accentuated due to the increased true airspeed. For short field braking with a tricycle gear airplane, lower the nosewheel to the ground and begin heavy brake pressure. The most effective braking occurs just short of locking the wheels and skidding the tires. Adding back pressure during braking holds weight on the main wheels, reducing tire skidding tendency and allowing greater braking pressure.

Raising the flaps, too, can reduce lift and allow improved braking. However, the additional action of raising the flaps has proven distracting enough for many pilots that it actually detracts from the braking process. In addition, many gear-up rollouts have resulted from distracted pilots reaching down and pulling a lever (the wrong one, obviously) up. In light of the foregoing, it is reasonable procedure to retract the flaps during rollout only if another pilot occupies the right seat and is assigned that task.

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

If a go-around is necessary, apply full power without delay while establishing 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. Flaps should be reduced to the setting recommended in the POH. The landing gear should be retracted when the runway and any rollout area cease to be usable.

Specific Mountain Airport Peculiarities

Airports in the high country have some unique "personalities."

  • Leadville, Colorado, at 9,927 msl, is regarded as the highest paved, public airport in the U.S. Transient pilots landing there are rewarded with a certificate of achievement. Provided the aircraft has good climb performance, Leadville is a fairly conventional airport in other respects, with good approaches and an almost level runway.
  • Big Bear and Tahoe, California, are popular high-altitude airports with excellent facilities. Both occasionally have turbulent conditions, generally associated with unstable afternoon air.
  • Both Aspen and Kremmeling, Colorado, have terrain that has to be circumnavigated during the approach. Calistoga, California has a one-way runway due to situation of downtown Calistoga on its northwest end. Other airports have less colorful reasons for one-way operations, often a steep runway slope or close-in high terrain at one end.
  • Visual illusions are not uncommon at high-country airports. Granby, Colorado, and Mariposa, California, feature sloped runways that appear level during the approach.
  • At both Mammoth, California, and Boulder, Nevada, 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.

Because of such features, it's important to research destination airports before conducting operations there. Local pilots are tremendous resources, since even the best airport reference guides don't always capture the personality of a high-country landing strip.

Summary

If certain hazards are recognized and appropriately addressed, 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.

After this summary, a list of commonly accepted Do's and Don'ts is provided. Foremost among them is an admonition that cannot be emphasized strongly enough: Always remain in a position that permits a turn toward lower terrain.

Enjoy the adventure.

The Do's and Don'ts of Mountain Flying

  1. The flight should be planned and flown to avoid terrain that would prevent a safe (survivable) forced or precautionary landing.
  2. Sufficient altitude should be maintained at all times to permit gliding to a reasonably safe landing area.
  3. The flight should be made along routes that include populated areas, roads and/or rivers and well-known mountain passes.
  4. Sectional charts 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.
  5. Believe your magnetic compass. 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.
  6. 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.
  7. 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.
  8. 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.
  9. 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).
  10. Don't get excited if you get in a downdraft. It will usually cease, leaving enough altitude above the ground to maneuver safely away. DO NOT count on this in extremely turbulent air or canyon areas.
  11. When encountering a downdraft, maintain sufficient airspeed, guard against stalling and fly out of the downdraft to an updraft area or smoother air (use maximum available power).
  12. 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.
  13. 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!).
  14. Expect winds above 10,000 feet to be prevailing westerlies in most western state areas.
  15. 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.
  16. 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.
  17. 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!).
  18. When taking off in a narrow canyon with several sharp bends, DOWN AIR may be encountered without warning.
  19. Remember that seldom is a flight in mountainous terrain purely routine - - EXPECT THE UNEXPECTED!!
  20. 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!

Updated Monday, January 24, 2005 10:21:25 AM