Wednesday, February 1, 2006
The Lowdown On Descents
There’s a right way and a wrong way to bring your airplane down
Descents are too often regarded as throwaway maneuvers. Pilots place great emphasis on proper techniques for takeoff, approach, landing and cruise, but few are educated in the best techniques for descent. If you’re one of those pilots who loves to fly low and slow—or even low and fast—descent planning may not be much of a concern. Most of the time, Cub and Champ drivers need hardly worry about descents from 1,500 to 2,500 feet AGL.
For the rest of us, however, descents should be more meaningful. General-aviation airplanes that typically cruise in the bottom 25,000 feet of the sky can improve efficiency by narrowly controlling descents. In some instances, proper descent and approach can be a prelude to a successful landing.
|Statistically, most pilots stop training after getting their private pilot licenses. This means most pilots learn only that descent means pulling some power off, or if they’re extremely motivated, predetermining a descent point, or a spot on the chart where they’ll start heading downhill toward landing. There’s really a lot more to it.|
Different airplanes descend in different ways. Military fighters are perhaps the most extreme. As I learned firsthand in 1979, a tactical descent in an F-15 Eagle may involve a roll to inverted followed by a pull-through to near-vertical down with the speed brake fully deployed, often at 35,000 fpm. (That’s 350 knots straight down.) Fuel is critical in an F-15—full tanks are worth only about 1.5 hours—so transitions to and from high altitude are flown expeditiously.
Fuel-conscious airliners almost always descend at near-idle power, averaging 1,500 to 2,000 fpm in their profile letdowns and coasting downhill in the most efficient glide possible. A 747-100 burns about 3,400 gallons of Jet A per hour, and at today’s prices, the sooner the crew can reduce power, the better. Heavily loaded Boeings, Lockheeds and Airbuses often boast glide ratios half of those for general-aviation models, on the order of 15:1 or even 20:1 for the big guys, and that means they’re notably more efficient during power-off glides.
In contrast, descents in general-aviation aircraft can vary significantly, depending on conditions and model. In the summer, what looks like semiclear air below can be uncomfortably choppy, especially over uneven terrain. Similarly, it may be miserably hot in the lower layers of atmosphere. There’s often a perpetual inversion overlying much of the country, especially in the vicinity of major cities. That means it could be the same temperature or hotter from ground level to 5,000 feet. For most of us who fly airplanes without air conditioning, that’s a good reason to maintain big altitude as long as possible.
Conversely, if you’re flying above an undercast and have reason to believe there might be ice lurking below, you might want to minimize your exposure by staying high as long as possible, dropping through the frozen glop at the last possible minute. This will demand coordination with ATC, but controllers in the Far North are usually fairly savvy to operational demands. I’ve used this technique on the North Atlantic dozens of times during all four seasons to avoid becoming a popsicle.
Pilots who like to fly their turbocharged models in the flight levels can expect a different attitude from ATC than if they were letting down from 10,000 feet. By definition, operation in the flight levels demands an IFR clearance, and for that reason, controllers are more demanding with pilots operating in the positive control environment. If you’re cruising at FL240 and request a lower altitude, the controller is likely to expect a minimum of 1,000 to 1,500 fpm down, even if he or she approves descent at the “pilot’s discretion.”
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