By Ryan Ferguson
(cont'd from page 1)
Weather, terrain, and density altitude
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Look at the muscles on this virga. Virga is common in Colorado; the air is so dry that the precipitation evaporates before it hits the ground. This one was approximately 50nm wide, and though we passed at least 10,000 feet beneath it, were treated to a very rough ride. The picture belies the true size of this monster. |
I'd crossed the Rockies twice prior to attending the ground school. But, I didn't consider myself a 'mountain pilot' because I had always filed IFR and flown well above the mountains. Now that we're down in the rocks, this is a strictly VFR operation. Not just VFR, good VFR. The CPA recommends at least fifteen statute miles visibility. Fifteen? In Florida, we're happy with anything above five! Bill Standerfer, CPA instructor, explained that in Colorado, visibility was frequently measured as high as 100 miles - many of the factors that contribute to haziness elsewhere in the country aren't normally present in Colorado. The air is dry and visibility is often far better than the east coast on the best of days. The reason you don't want to dip below fifteen? Simple… if visibility is low chances are higher it'll deteriorate even further. Once you cross the continental divide, you're on your own; airports are few and far between. Stranded with nowhere to land in the mountains could easily prove fatal. Strong emphasis here: if you want to fly in the mountains, you need good VFR, and it should be forecasted to stay that way.
Bill gave an excellent lecture on mountain flying weather, including winds, discussion on rotors, mountain waves, and the types of clues a pilot can derive from various cloud formations. A friend of mine also attending the course, an atmospheric chemist, remarked on how refreshingly accurate and concise Bill's lecture was.
As the day wore on, the ground school discussion centered on topics including survival gear, how to cross a mountain ridge (always at a 45 degree angle until crossing), and of course, the all important density altitude.
The Double Whammy
Speaking for light, normally aspirated, general aviation aircraft, high altitude operations offer some challenges specific to the takeoff, maneuvering, and landing phases of flight. By far the most critical operation is the high density altitude takeoff. The double whammy: not only must the airplane be at a higher true airspeed to achieve flying speed, but it must do so with an engine (or engines) that's not capable of making sea level horsepower. Or, to put it another way, it's much like taking off at cruise power settings. Of course, the higher you go, the less horsepower your aircraft can make; you may take off at 65% or even lower power settings!
"The most common problem for flatlanders is the tendency to fly the approach below the normal indicated airspeed for landing. Thus, an area of heavy emphasis for mountain flying is to fly by the numbers and approach to land at the normal indicated airspeed."
The CPA teaches that the way to compensate for this is to be light. As a rule of thumb, being 10% under maximum gross weight provides a 20% performance benefit over the POH numbers. Instead of 100-200fpm climb rates, you'll see up to 300-400fpm, as a rough example. In my 3,600 max gross aircraft, that means taking off weighing no more than 3,240 lbs. No full fuel/full pax takeoff here.
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Forward slip into Granby airport. Field elevation 8,203 ft. (Check the altimeter!) |
There's not a lot of new stuff going on in cruise flight unless you are on the ragged edge of your aircraft's performance. You shouldn't be cruising under 1.2 Vs1 indicated airspeed, and if you are doing so with maximum available power, you're pushing it. A lower-powered single engine aircraft, such as a Cherokee or Cessna 172, might have a noticably high deck angle at cruise. You'll always want some power left over, above and beyond what you need to hold altitude. Surprisingly, the smaller and lower-powered airplanes are doing just fine out here - they just have to be flown in the proper configuration (read: weight) and with total respect for the weather.
The challenge for landing at high-elevation airports? You're going to be landing at a higher true airspeed and thus you'll need more runway length. In the mountains, that's not usually a problem - all the runways are quite long. You're not using a lot of power to land, so the only issue is getting the plane on the ground and stopping it. The most common problem for flatlanders is the tendency to fly the approach below the normal indicated airspeed for landing. Thus, an area of heavy emphasis for mountain flying is to fly by the numbers and approach to land at the normal indicated airspeed. Considering that, as a rule of thumb, true airspeed increases 2% for every 1,000 feet of density altitude, your groundspeed may be significantly higher on short final than you're accustomed to. Next: leaning technique.
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