July 2000Features

Above It All

Flying High In The Mountains


The United States, as well as all of our neighbors - Canada, Mexico, the Caribbean, and even Greenland and South America - have plenty of mountainous geography. Even with the significant area the Great Plains occupies, the map of North America is covered with mountain ranges like the Rockies, Sierras, Smokey Mountains, Appalachians, Adirondacks, Ozarks, Coast Range, Brooks Range, Black Hills, White Mountains, Blue Mountains, and Green Mountains. Many of these mountains are among the highest in the world. Denali (also known as Mount McKinley) in Alaska is more than 20,000 feet high, and Mount Whitney in California is more than 14,000 feet high.

Even when a pilot does not learn to fly in a mountainous area, the time will surely come when he or she wants to fly in the mountains. And for good reason. Spectacular scenery, skiing, camping, and scores of other events, activities, and attractions can be reached only by flying into airports in mountainous areas. If your goal is not in the mountains themselves, a flight over the mountains often can shave hours from a roundabout, terrain-avoiding route.

But just jumping into the airplane and thinking that you can fly over and near mountains as you would fly down around sea level is a very serious mistake.

Your Cessna or Piper will jump right off the runway and climb away from a low altitude airport. Your runup, leaning technique, climbout, and many other performance factors will all be straightforward and routine at altitudes around sea level, too. Exotic terms and concepts such as mountain obscuration, density altitude, normally aspirated engine, and leaning technique may be only faint memories from flight training now. And terms and concepts that you use every time you fly, such as rotation speed, weight and balance, and best rate of climb, take on a whole new meaning in the context of operating around mountains and at high altitudes.

But if you've never flown in mountainous terrain, how do you know just what constitutes high altitude? Of course, that depends on the aircraft being flown. Large twins operate in the 15,000-foot to 30,000-foot range, jet airliners are normally cruising in the 35,000- to 41,000-foot range, and business jets can be found at these same altitudes. A few military aircraft routinely operate at altitudes above 60,000 feet. But for most light general aviation airplanes, about 5,000 feet above mean sea level (msl) is the point at which altitude begins to have a noticeable effect on aircraft performance. Other factors such as whether the flight is conducted during the day or at night, terrain in the vicinity, humidity, and temperature, all factor into the definition of "high altitude operation." For that reason, many pilots drop their personal high-altitude floor down to 3,000 feet msl.

How about the term mountainous? Just when are you in mountainous terrain? Although it usually conjures up majestic alpine scenes, the word mountainous can apply to just about any terrain that deviates from a flat plain. A 1,200-foot-tall hill on the low coastlands does not present a performance problem, but it can become a fatal mountain for a pilot skimming the coast in the fog.

Pilots have two primary issues to consider when they are operating in mountains and at high altitudes - altitude, which affects the performance of the airplane, and terrain, which can affect the airplane's safe passage because it is an obstacle. Terrain and altitude can both influence the weather as well, but we're getting ahead of ourselves.

As an aircraft climbs, its performance - both in terms of aerodynamics and powerplant output - is negatively affected by the thinning atmosphere. The pilot may also become less effective physiologically speaking. Less oxygen means less physical and mental energy and ability, fatigue, and potentially dangerous medical conditions like hypoxia that can put the flight in jeopardy.

The federal aviation regulations, specifically FAR 91.211, place altitude restrictions on pilots flying unpressurized aircraft. Of course, some general aviation aircraft are pressurized so that, like airliners, the cabin pressure can be regulated to allow passengers to breathe without supplemental oxygen, even when the air outside the airplane is too thin to breathe. Aircraft may have turbocharged engines that allow the airplane to climb above safe breathing altitudes. Such airplanes are typically equipped with onboard supplemental oxygen systems for pilots and passengers. But these aircraft tend to be larger complex and high-performance machines requiring special checkouts, insurance considerations, and pilot currency. Chances are that most of the aircraft you fly will be of the simpler unpressurized, normally aspirated types that do not have a built-in supplemental oxygen system.

According to the regulations, you can fly an unpressurized aircraft at altitudes from 12,500 feet to 14,000 feet msl for up to 30 minutes without using supplemental oxygen. If you plan to stay longer at those altitudes, the pilot and any required crewmembers must have supplemental oxygen. Above 14,000 feet msl the pilot and required crewmembers must use supplemental oxygen at all times, and above 15,000 feet msl all occupants of an unpressurized aircraft must be supplied with supplemental oxygen. As the pilot, you do not need to force your passengers to use the oxygen. This oxygen is usually in the form of portable bottles with attached masks that can be purchased or rented from many FBOs.

From The Bottom Up

Keeping the rules in mind, it's time to start preparing for that first flight to a mountainous destination. A preflight weather briefing is a must. Even if you can see the mountains in the distance and the sky looks clear in all directions, many dangers can still lurk in mountain ranges. There can be mountain obscuration (mountains hidden by cloud cover), severe downdrafts, high wind speeds made worse as winds speed up through narrow passes and valleys, severe turbulence, wind shear, and temperature inversions. A high temperature at your destination airport can make aircraft performance much less than when lower temperatures prevail.

As the aircraft ascends, the thinning air contains fewer oxygen molecules, causing the engine to produce less power. On top of that, the propeller has less and less air to grab onto. The wings also lose some of their effectiveness as you climb higher into the atmosphere. The engine must be leaned much more than in a low altitude cruise. Because the optimum fuel/air ratio stays constant for efficient engine operation even as the air itself becomes thinner, a pilot who fails to lean the engine properly is slowly flooding it and making it peak at a lower rpm and, therefore, run less effectively. With the aerodynamic lift already suffering, the last thing you need is for the engine to become even less effective.

Climbing through 3,000 feet msl is a good time to begin leaning the mixture. Two easy techniques will assure peak engine performance in the climb. If the airplane is equipped with an EGT (exhaust gas temperature) gauge, lean (pull back) the mixture lever (red) slowly and watch the EGT begin to rise. Remember to scan outside while this is going on. Eventually, the EGT will begin to fall. When this happens, enrichen (push in) the mixture until you get a very slight rise. This is the most efficient setting for your aircraft at this altitude. If you continue to climb, simply repeat the process every 1,000 feet or so. If the aircraft has no EGT gauge, lean the mixture until the engine begins to run slightly roughly (the rpm will also drop) and enrich slightly.

When it comes time to descend for a landing, you have to be careful that the mixture is not too lean for a go-around. If, for example, you are cruising at 10,500 feet to cross the mountains but your destination airport is at 6,500 feet and you have not enriched the mixture during the descent, you will be too lean for a successful go-around. Fully enriching the mixture as you would normally do before landing at an airport near sea level will make the mixture too rich for a go-around. In either case, you will not be using the airplane's maximum available power. With the airplane's already weak climbing ability at high altitudes, you really need all the power that the engine can give you. To prevent this problem, just before you reach pattern altitude, go through the leaning process again until you achieve best power.

From The Top Down

When you are ready to fly out of a high-altitude airport on your way back to the flatlands, you will find that the poor performance you encountered en route is even more of an issue because of your proximity to the surface. To make achieving maximum performance even more critical, many airports in mountainous areas are surrounded by even higher peaks, and you may need to do a lot of maneuvering as you climb to avoid steeply rising terrain. If the airport temperature is above normal, the airplane will need more runway distance to achieve rotation speed, will climb more slowly, and will require an increased angle of attack to achieve that climb and thus will be flying at an attitude closer to the stalling angle of attack. (To determine what constitutes a "normal" temperature, use the standard adiabatic lapse rate which says that beginning with a standard temperature of 15 degrees Celsius at sea level, the temperature will decrease 2 degrees C for every 1,000 feet of increased altitude. In other words, if you are at 5,000 feet, the "normal" temperature would be 5 degrees C.) It is very important to note that the indicated airspeed will not change with regard to rotation, climb, stall, etc. If, for example, you normally rotate at 60 knots indicated airspeed you will still rotate at that indicated speed no matter how high the airport is above sea level or how much the temperature is above average.

If all of the performance numbers are "normal," what is the problem? Even though you still rotate at 60 kt, the aircraft will use up much more runway to achieve that speed than it did at sea level. If the temperature is high, it will take even more runway. Then when you reach rotation speed, the airplane will get off the runway much more slowly and require a higher angle of attack to achieve a normal rate of climb, even though the indicated best angle and rate of climb speed remain the same as at lower altitudes. This higher angle of attack can bring the airplane right to the edge of stalling, and the stall horn may be activated. Consequently, the pilot will lower the angle of attack to avoid the imminent stall, and the already miserable climb rate will become even worse. Couple that with the fact that you have already used up most of the available runway, and you can see the treetops looming in the front window. If you pull the nose up you will stall; if you stay at this altitude much longer, you will end up in the mulch. Either way, you are on the front page of tomorrow's local newspaper.

Once you get your preflight weather briefing, do a careful weight-and-balance calculation. Heavy or improperly loaded aircraft may either fail to get off the runway or be unable to climb. The same weight and balance numbers that worked back home may not work in the mountains.

This means that you must take the time to use your flight computer to find out what the density altitude (pressure altitude corrected for nonstandard temperature) is at the airport, and take that figure into account when you check the aircraft performance charts in your pilot operating handbook (POH). If you're lucky, the local automated weather may include the density altitude. Density altitude is a performance concept that basically provides the pilot with information on how the aircraft should perform at this altitude and temperature. The higher the temperature, the poorer the aircraft performance. If you are at a field elevation of 6,200 feet msl and the temperature is standard for that altitude (re- member the standard lapse rate?), the airplane will perform as though it were at 6,200 feet msl, which is a considerable depreciation from its sea level abilities. But if the temperature is above standard, your already poor performance may become deadly. An aircraft that is trying to climb out of an airport at 6,200 feet when the temperature is about 40 degrees C will perform as if it were taking off at an airport that has a field elevation of more than 10,000 feet msl.

Once you have the density altitude and weight and balance, use the POH to figure out how much runway you will need to lift off, your rate of climb, and how much distance it will take to safely fly over a 50-foot obstacle. A pilot in a light aircraft flying with a couple of buddies on a hot day may be shocked by his airplane's performance. Often, it is simply not safe to even attempt a takeoff without removing weight from the aircraft.

Since the pilot cannot control the weather, he may have to wait until sundown when the temperature decreases - but then be forced to negotiate a mountain range in the dark. Maybe the pilot needs to shift or offload people or cargo - usually not a great option because your friends won't want to take a 10-hour bus ride home, and they are not about to leave their $500 skis at the airport. Finally, you may choose to fly with less-than-full fuel tanks - but be sure that you've got plenty of fuel to get to a nearby airport at a lower altitude. Of course, proper preflight planning can solve most of these problems before you ever start your trip. For instance, you might decide to bring one fewer person and rent skis instead of carrying them with you.

If you have determined that you are within safe performance limits based on the temperature, field elevation, and weight and balance, then a few more tricks in the runup area and the takeoff/climb portion of your flight will ensure a safe trip home. When you get to the runup block, do a full power/full rich runup, then lean for highest engine rpm.

Because the airplane will require a higher angle of attack to achieve the rotation and climb, you have to be very careful when you get to rotation speed and thereafter. Instead of a normal and assertive rotation, gently and slowly pull the yoke back and let the airplane climb out of ground effect. Be patient and let the airplane move away from the surface at its own pace. Any more back pressure and you may end up in a stall with inadequate altitude to recover. It is also important to be familiar with "climbing zones" adjacent to the airport. These are areas that allow light aircraft to circle and gain altitude before they proceed over higher terrain. Most commercial airport guides list these areas, and local flight instructors will be happy to give you their insight and knowledge about the area, so always ask.

Flying to mountainous areas can be among the most exciting and memorable of all your aviation activities. If at all possible, take some instruction in mountain flying before you make your first solo trip. Consider having your instructor come along as a right-seater for your first mountain flight or, if you prefer, drive to a mountain airport and do some flying with a local instructor. The investment of time and money will pay handsome dividends in safety. Then, when you're ready to make a trip on your own, start with a very clear plan of what you intend to do. Get a thorough preflight weather briefing, carefully calculate weight and balance, study your airplane's performance figures, wait if the weather is even slightly questionable, and you will gain more confidence and experience in this exciting aspect of aviation.


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