Before you get to the practical (flight) test, you should make a simulated high-altitude takeoff. The instructor will limit your power for takeoff, and you can see how the airplane would do (or not do) at some high altitude.
The biggest problem you’ll have is the very strong ten- dency to rush the airplane off. (This is also the ten- dency the first time you fly the airplane at maximum certificated weight from a comparatively short field.) The airplane seems to be dragging its feet, and you’ll want to pull it off before it’s ready. After you become airborne, don’t pull the nose up “to get extra climb.”
You might put yourself in position 4 or 5 back in Figure 12-13, and performance will suffer even more.
Talking a little more about the density-altitude idea, every 15° (81⁄2°C) above the standard temperature for your pressure altitude bumps the density-altitude up another 1,000 feet. Suppose you are going to take off at an airport at an elevation of 2,200 feet. The altimeter setting is 30.12 inches of mercury, which makes your pressure altitude right about 2,000 feet (1 inch of mer- cury equals approximately 1,000 feet of altitude). The pressure corrected to sea level is 0.20 inches of mer- cury higher than standard, so the pressure altitude is (0.20 × 1,000) = 200 feet lower than the elevation. The pressure altitude is 2,000 feet (which could also have been found by rolling 29.92 into the setting window).
The temperature is 82°F, which certainly is not unusual in the summer. The standard temperature for this pressure altitude is 52°F (59°F – [2 × 31⁄2°F] = 52°F), since the normal lapse rate is 31⁄2°F per thou- sand feet. The temperature (82°F) is 30°F above the standard. Since each added 15°F makes an additional 1,000 feet of density-altitude, your real density-altitude is the pressure altitude (2,000 feet) plus 2,000 feet, or 4,000 feet. You think you’re sitting at 2,000 feet, but the airplane knows that it is at 4,000 feet — and acts accordingly. If the temperature had been 97°F at that airport (entirely possible), another 1,000 feet would have been tacked on and the density-altitude would have been 5,000 feet.
At an airport in the summer, it’s not at all unusual for the temperature to be 15°F (or much more) above its
“standard.” The temperature, rather than atmospheric pressure changes, is the big factor. The not unusual temperature of 82°F in the example added 2,000 feet of density-altitude. To get that effect by pressure decrease (assuming a standard temperature) would mean that the pressure would have to be 2 inches of mercury below normal, or the sea level pressure would be 27.92 instead
17-10 Part Three / Postsolo Maneuvers
Figure 17-13. Takeoff distances for a two-place trainer for climb and cruise propellers. (Courtesty of Grumman American Aviation Corp.)
Figure 17-12. A graphical presentation of takeoff performance.
Chapter 17 / Special Takeoff and Landing Procedures 17-11
Figure 17-14. A crosswind–headwind component chart.
(FAA)
of 29.92 inches of mercury. This is unusual, and such pressure conditions could only be found in the eye of a very strong hurricane.
Talking in terms of Celsius for the earlier example problem, the standard sea level temperature is 15°C, and the normal lapse rate is 2°C per thousand feet, so the following would apply: Standard temperature for the 2,000 feet is 11°C; the actual temperature is 28°C, or 17°C above standard. Since each 81⁄2°C adds another 1,000 feet of density-altitude, the answer is again 4,000 feet.
One thing that you may not have considered is that the moist air is less dense than dry air (all other factors equal), so the airplane will not perform as well in take- offs and climbs when the air is moist. (You might figure on up to about 10 percent less performance under wet conditions.)
One last thing about density-altitude: You may not know what it is at a particular time, but the airplane always does.
17-12
18-1 Figure 18-1. The instructor will discuss your errors on the forced landing.
The high-altitude emergency gives you more time and a greater choice of fields. In earlier stages, however, this is a handicap, so practice is usually reserved for the postsolo period. The average student, when given a first high-altitude emergency, is as busy looking around as a one-eyed man watching a beauty contest. There are too many places to land. You may get confused and change your mind on the choice of a field several times and finally end up too low to do anything but land in the worst field within a 15-mile radius (Figure 18-1).
The high-altitude emergency requires that you do the following:
1. Set up the max distance glide. Start easing the nose up immediately to conserve altitude as well as to get to the best glide speed. At the same time:
2. Pull the carburetor heat ON (use it before the resid- ual engine heat is lost).
3. Pick a field and head for it.
4. Find the wind direction.
5. Try to find the cause of the problem — carburetor ice, fuel tank run dry (switch tanks), etc. Set up a glide pattern so that you will hit a “key position” at
about the same point and at a slightly higher altitude above the ground than the point at which you closed the throttle to make a 180° power-off approach (the windmilling prop will hurt the glide).
This key position is chosen because by this time you’ve shot power-off approaches from this posi- tion. It’s literally impossible to glide for a distant field from 2,000 or 3,000 feet above the ground and consistently hit it. It’s been found to be easier to judge the glide angle if the approach involves a turn.
(Review power-off approaches in Chapter 13.) Let’s take the emergency point by point as dis- cussed below.