The Lowdown On Descents

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.

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."

Controllers don't differentiate between turbine and piston airplanes in the flight levels, and that means they sometimes may assume your airplane is pressurized and that high descent rates are no problem. You'll often hear controllers ask you to increase your descent rate, but you'll rarely be asked to slow your letdown. In 30 years of operating turbocharged, turboprop and pure jet aircraft in the flight levels, I've probably been asked to expedite my descent a hundred times, but I've rarely been asked to reduce the rate.

There's a noticeable difference in descent planning from the flight levels and letting down from a mile or three MSL. For one thing, winds are less of a factor down low, so you're likely to see a slight improvement in groundspeed at modest heights and exactly the opposite while descending from high altitude.

When operating at a comparatively low level and flying VFR in a typical 160-knot, single-engine retractable, calculating the vertical descent point can be a simple matter of dividing the altitude to be lost by your chosen descent rate, comparing that to your expected average descent airspeed and, assuming there's no topography in the way, starting down at the appropriate time. For example, if you're cruising at 9,500 feet and planning a 500 fpm descent to a pattern altitude of 2,000 feet, five miles from the airport, that's 7,500 feet to lose (15 minutes' worth at 500 fpm). If you realize a 20-knot cruise improvement (consistent with Vno limits, of course), you'll want to start down about 50 miles out.

In the VFR world, most general-aviation pilots like to be down to pattern altitude at least five miles before the airport, so it's necessary to add that to your distance calculation. This gives you time to stabilize and prepare for the approach, even if it's just a standard 45 entry to a downwind, base and final. Another benefit of getting low early is that it's far easier to spot traffic at the same altitude or even a little above you against a featureless blue sky than trying to pick it out of a mishmash of houses, freeways and shopping centers below.

As mentioned above, descents from the flight levels can often result in a slower groundspeed. If that seems contrary to common sense, remember that jets, turboprops and turbocharged piston airplanes are much more efficient in the thin air above 18,000 feet. Similarly, a fringe benefit of high-altitude flight can be tailwinds. For these reasons, pilots of turbine equipment prefer to fly high as long as possible. Controllers oblige with their own "keep 'em high" policy, typically assuming pilots will oblige with descent rates of at least 1,500 fpm.

When you're flying VFR at 17,500 feet or less, there are a couple of methods you can use to figure your descents. One is to simply choose a descent rate (500 fpm in the example above) and back in to the descent point.

Another is to assume a given ratio of altitude to forward distance. The airlines like to assume a 3:1 ratio---three miles of horizontal travel for every 1,000 feet of altitude loss. More than coincidentally, that's roughly a three-degree glide-slope and will dovetail nicely with a typical ILS approach.

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?