Wednesday, February 1, 2006
The Lowdown On Descents
There’s a right way and a wrong way to bring your airplane down
While three degrees is a standard profile for the last 10 miles to the airport, it may be a little abrupt for unpressurized airplanes with piston engines, descending from greater distances. There’s little question that shock-cooling can be a factor in premature engine failure, as different metals inside the engine expand and contract at different rates. The slower you cool the engine, the better.
A gentler 5:1 ratio seems to work better for VFR descents. All other considerations being equal (which happens about one flight in 100), if you have 9,000 feet to lose between cruising level and pattern altitude, you’ll want to begin descent 50 nm out (45 plus 5), providing there’s nothing to hit in between.
In a typical 120-knot, fixed-gear single, you require 22.5 minutes for an average descent rate of 400 fpm. If you’re traveling at 180 knots, you’ll start down at the same point, but you’ll need to maintain a 600 fpm descent to arrive at the right altitude at the proper time. At the below-10,000-foot speed limit of 250 knots, you’ll need to maintain about 1,350 fpm to make the planned altitude.
Deciding whether to descend at cruise power or with reduced throttle has always been a subject of some debate. If you’re flying high enough, you could leave the left knob/lever full forward until the airplane reaches the max 75% power altitude, then begin reducing power gradually to avoid exceeding the limit as you descend. That could be the best way to go for engine cooling, but on some airplanes, it also might drive the airspeed up into the yellow—not a good idea.
You could alleviate the problem by reducing the descent rate and starting down farther out, or you could choose to reduce power slightly to maintain the same indicated airspeed throughout the descent. Unfortunately, the slight increase in airspeed you’ll realize at higher power down low won’t reduce your time en route by much, as it will only apply for a short distance.
If you need to go down and slow down at the same time, the best method is to use drag devices such as speed brakes—if you’re lucky enough to have them—approach flaps or gear. (Definitely don't use cowl flaps. In fact, if the engine has been running warm enough to justify cowl flaps in trail, the descent might be a good time to close them.) Gear is the last choice because it often imposes unacceptable speed limits, whereas speed brakes and approach flaps are less aerodynamically constrained. Be aware, however, that approach flaps may reduce allowable load limits, so you might want to plan ahead if you’re expecting rough air during the letdown.
Don’t forget the mixture during a descent, probably the most common error pilots make coming downhill. You’ll obviously need to push in the red knob to maintain the ideal 15:1 fuel air ratio. An EGT makes the process simpler, and an engine analyzer makes it easier still, but even without either option, experience should allow you to make a SWAG estimate of mixture positions for the lowering altitudes.
During any descent, it’s especially important to clear the airspace ahead. Too often, pilots initiating descent focus on the narrow slice of sky straight ahead, ignoring possible threats from the side. The better policy is to check the airspace diligently, including the sky to both sides and below. Make occasional clearing S-turns left and right that allow you a view of the world below as well as ahead.
All that remains after a proper descent is a perfect landing, and anyone can do that. Right?
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