|Written by Scott Stahl|
|Thursday, 10 January 2013 08:27|
Many pilots have heard the term “mountain flying.” The expression usually conjures up images of fly-fishing trips via short fields and tundra tires. However, the FAA defines mountainous terrain as any terrain with an elevation above 5,000 feet mean sea level (MSL). This definition expands the idea to include much of the United States. While romantic images of fly fishing may be the first thing that comes to mind, the reality is many domestic trips may begin, end, or stop over in mountainous terrain. Mountain flying is a great way for a pilot to see some of the greatest beauty in the United States while providing a fantastic opportunity to enhance and improve knowledge related to weather, atmospheric conditions, aircraft performance, and terrain avoidance procedures. Few would argue the breathtaking scenic opportunities that mountain flying makes available. With careful planning and consideration, mountain flying can also be done with a high degree of safety. It is challenging for the best pilot and enlightening for the newest. The scope of this article is to discuss some of the unique considerations that apply to operations in mountainous terrain. If you are interested in learning more, several training courses are tailored specifically to flying aircraft in mountainous terrain, such as the one offered by the Colorado Pilots Association.
Like any typical pre-flight planning process, a flight in mountainous terrain should include a thorough review of factors related to the day’s trip. These include weather, terrain, aircraft performance, route planning, and even hypoxia.
Air Density Considerations
Be aware of the effects various weather conditions can have on a typical flight.
There are several factors to consider here, starting with density altitude. Most will remember that pressure altitude is MSL altitude corrected for non-standard pressure, and density altitude is defined as the pressure altitude corrected for non-standard temperature. Although pressure altitude certainly has an effect on performance, it is minor compared to the effect of large temperature changes. There are several reasons why density altitude plays such a vital role in aircraft performance. First, as the temperature increases, density decreases. This results in less power from the engine due to the reduced density of oxygen in the cylinders to be used for combustion. Assuming a naturally aspirated engine, it is a good rule of thumb to expect a 3-percent loss in engine power for every 1,000-foot increase in density altitude.
Further, less-dense air results in a higher true airspeed in order to get the airfoil to produce the same lift, and, consequently, propeller efficiency also decreases in high-density altitudes.
One of the biggest factors of mountain flying is understanding the change in performance that occurs as temperatures increase. Even on a standard day, the performance of an aircraft out of a high-altitude airport will be far less than it would be for a plane out of a sea-level airport. As an example, a Cessna 172 SP will experience a 27-percent decrease in climb performance simply by going from sea level on a standard day to a 5,000- foot elevation on a standard day. By 7,000 feet on a standard day, the performance has decreased by 42 percent. If both airports were 10 degrees Celsius above standard, the aircraft would lose 30 percent and 43 percent, respectively, from the sea-level value. To put this in perspective, the 172 SP would be down to a rate of climb less than 500 fpm on a 10-degree-warmer-than-standard day at 7,000 feet MSL. Now consider that a more modestly powered airplane, such as a 150-hp 172, might be down to essentially zero climb rate in these same conditions. With temperatures 25-plus degrees Celsius above standard in some parts of the country common during summer, even decently powered airplanes are going to have virtually zero climb performance. A good technique for mitigating high-density altitudes is to limit maximum takeoff weight. Loading the aircraft to no more than 80-90 percent of maximum gross weight will help recoup most of the performance lost by the increase in density altitude. It may also be advisable to wait for more favorable temperature conditions, such as those found in early morning or evening. The FAA recommends that 160 hp should be considered as a bare minimum for mountain operations. If that much power is not available, extreme caution should be exercised in the planning phase. Density altitude also affects the aircraft’s service and absolute ceilings, so care must be exercised when planning the climb and en-route phase of the flight.
Wind is always a major factor in any planning process, but it becomes critical in mountainous terrain. Strong wind is potentially the most dangerous factor in any mountain flight. As the air flows across mountains, it experiences several changes. First, as wind flows around mountains, it behaves the way water does in a stream. The airflow will be relatively smooth upwind of the terrain, and as it flows over the terrain, it will become turbulent, much like water flowing over rocks. Thus, it is reasonable to always expect turbulence on the leeward side of mountainous terrain. Second, wind creates mountain waves, which are essentially vertical waves created when the air flows over the top of the mountain. As the air rises over the mountain, intense updrafts, turbulence, and even cloud formation/precipitation known as orographic lifting and is a major factor in turbulence and precipitation formation in these regions. Downwind of the mountain, the air will begin to descend and cause a series of diminishing waves. These waves can persist for many miles downwind of the mountain and create severe turbulence and downdrafts. If moisture in the air is sufficient, altocumulus standing lenticular (ACSL) clouds may be present, as well as roll clouds created by the mountain waves. If the atmosphere is relatively stable, cloud caps on the windward slope or top of the mountain may indicate turbulence.
However, in the absence of visual indications, turbulence should still be expected any time wind velocities are high. If winds aloft are reported or forecast to be greater than 25 knots, extreme caution should be exercised. It may even be a good idea to consider cancelling the flight. If the flight is not cancelled, avoid flying below the tops of the mountains. Plan a route of flight that will avoid the terrain in question. Consideration should also be given to mountain passes. Much like a venture accelerates the air through a carburetor, the passes in a mountain range will also increase the velocity of the air flowing through them. Thus, it is important to consider this increased airspeed when operating in and near passes, and care should be used to avoid them whenever possible. Stay at least 1,000 feet above any ridgeline top, and if winds are in excess of 20 knots, stay 2,000 feet above any mountain top.
Another common weather phenomenon encountered in mountainous terrain is the dry microburst. This will usually be found in the vicinity of any thunderstorm clouds with bases above 3,000 feet AGL. Dry microbursts can create downdrafts of several thousand feet per minute and usually occur beneath virga. They are very common in the mountainous regions of the Rockies and southwestern United States during the summer. Although it is never advisable to operate underneath a thunderstorm, virga should be avoided, and special attention should be given to any blowing dust located beneath any cloud producing precipitation.
Finally, visibility minimums in mountains should be higher. In addition to the obvious reason of visual terrain avoidance, proper visibility will also reduce the likelihood of getting lost by keeping checkpoints more identifiable and emergency landing sites more visible.
Planning and Piloting Practices
Flight planning should pay special attention to routing. It may not be best to proceed directly via straight line. It is imperative to receive a full and complete weather briefing prior to the flight in order to get as much information as possible about the route of flight. Pilot reports should be monitored as well as given along the route. A detailed navigation log should be created and filled out along the route of flight to avoid getting lost and to detect any anomaly as soon as possible. All visual checkpoints should be unique and should be visually verifiable by a minimum of two distinct characteristics.
When operating any flight, it is recommended to file a flight plan. In mountainous terrain, it may be a lifesaving choice due to the difficulty of search-and-rescue operations. It will also ensure a timely response if an emergency were to occur. Always have at least one alternate course of action should the weather change. Plan the route of flight to maximize available landing sites, and carry a suitable survival kit.
Taxi and takeoff operations are fairly conventional, with a few exceptions. Mixture should always be leaned per the aircraft’s pilot operating handbook during taxi and takeoff to maximize power output and minimize the risk of fouling the spark plugs. When applying takeoff power, a power check should be conducted prior to starting the takeoff roll in order to verify the engine is actually producing full power, and 70 percent of liftoff speed should be achieved by the halfway point of the runway. If not, it is best to abort the takeoff. It is important to note that normal indicated airspeeds should be used, but also that higher true airspeeds will result. During climb-out, consider changing your profile; rather than using a cruise climb, it may be necessary to maintain a best-angle climb until terrain and obstructions are clear or best rate of climb to reach a minimum cruise altitude. Special attention should be paid to the effect of a prolonged climb on engine temperatures, and the mixture may need to be enriched slightly to help compensate. Another technique that may help reduce engine temperatures during hot and high operations is topping the oil quantity a little closer to the maximum limit.
If crossing mountains, try to avoid going straight across them, instead choosing a 45-degree angle. This will allow a smaller 90-degree turn away from terrain, rather than a 180-degree turn. This is especially useful if an emergency or severe turbulence is encountered while crossing the ridge.
After crossing, a 90-degree angle should be maintained to maximize the distance from the ridgeline as quickly as possible.
Descents should also be planned in advance. It may be necessary to change the descent rate after crossing a ridgeline to avoid impacting terrain, and it may also be necessary to plan an approach into an airport based on avoiding terrain in the vicinity of an airport. Special care should be given to determining where the wind is coming from and any crosswind effects off of the nearby terrain, as well as emergency considerations.
Landing is usually the most difficult part of any flight, and mountain flying brings special challenges for the pilot, including the likelihood of obstacles that must be cleared during the approach and landing phase. Flying a slightly steeper approach at a slightly lower power setting may be preferable. Not only will this aid in obstacle clearance but it will also maintain a slightly greater reserve of power in the event a downdraft is encountered. This could be especially critical in an aircraft that has marginal performance in high-density altitudes. However, strict stabilized approach criteria should still be adhered, and a go-around initiated if necessary.
Rewards of Adventure
Although mountain flying presents many special requirements and challenges for a pilot, with proper consideration of these factors, it can be a fun and rewarding experience. For those who would like to approach mountain flying under the guidance of a skilled pilot as recommended by the FAA, it is a great way to enhance the pilot’s portfolio of experience and skill. In either case, mountain flying is something that offers great adventure for any pilot.
|Last Updated ( Thursday, 10 January 2013 08:30 )|