February 1, 2003
By Alton K. Marsh
You're VFR, not used to flying high or even flying very far, but the best winds are at 9,500 feet above the ground. So you're up there, marveling at being almost two miles into the sky, and wondering when you should descend.
Chances are you weren't taught in flight school how to get down when you're really up there, or if you were, you don't remember it. Most training is conducted below 5,000 feet agl. Sure, there is a standard formula you can apply. You want to descend about 500 feet per minute for the ear comfort of your passengers, and, let's see, you are how many minutes from your destination? That, of course, depends on your speed. And how much altitude do you need to lose to reach pattern altitude? It can be computed, but maybe the mental gymnastics of that exercise aren't worth the increased pilot workload. But if you don't do the math, you could end up over your destination with several thousand feet to lose.
Personally, when I am 30 miles out and high, I start thinking about getting down. I consider high to be anything above 5,000 feet agl. I can then amend the 500-foot-per-minute rule to fit the altitude. That rule takes care of most unpressurized flying at higher altitudes.
Although this article is primarily for the VFR pilot, that rule can help alert the IFR pilot as to when ATC should allow a descent. It may be that the controller is unaware that your aircraft is not pressurized and quite possibly is unaware of its descent capabilities. The controller might be thinking about a slam-dunk approach when you are thinking glide-and-slide.
Most GPS receivers, including the portable ones, have descent-profile planning features called vertical navigation, or VNAV. Just program the altitude you want to reach along with the rate of descent you want, and your GPS will beep when it is time to descend.
The disadvantage is that programming is required. Often it is easier to simply pull the throttle back when the time seems right rather than spend time head-down to program the box. Remember that if you are talking to ATC through the use of VFR flight-following services, you can simply tell the controller that you are descending. You need not wait for permission, and it's not the controller's job to do your vertical navigation planning. Generally the controller simply acknowledges your report of descending, or may instead alert you to traffic, obstacles, or airspace that make descending a bad idea. Safety is the reason you were talking to ATC in the first place, right?
Perhaps you prefer to descend at airspeeds in the yellow arc — or at least at the bottom of it — as long as you are flying in smooth air. (Rough air or abrupt movement of the controls while flying at speeds in the yellow arc could overstress the aircraft.) One chief flight instructor considers it a "return on investment" to descend at higher than cruise speeds. That is, you spent lots of fuel and time to get up to altitude to take advantage of winds or avoid clouds, so you should get some payback in the form of time savings at the end of the trip. But even he suggests going no faster than the bottom of the yellow arc. That way, should you encounter turbulence during the descent, you can quickly slow down into the green arc.
Some pilots have put considerable thought into descent planning. For example, one Bonanza pilot suggests descending no faster than the gear operating speed. First of all, that speed is still in the green arc and near V A — maneuvering speed — thus eliminating the need to slow should you encounter turbulence, as is common at lower altitudes. Second, the gear can be quickly lowered to help with the descent should that be necessary. Third, the power setting for that speed is a known quantity, 18 to 19 inches of manifold pressure and 2,200 rpm. Consequently, the pilot has a plan for descents, even choosing the power setting for all descents in advance.
Remember that manifold pressure increases on the way down. Perhaps it saves on workload to choose a slightly lower manifold pressure setting than needed for descent (but one high enough to prevent rapid engine cooling) while still at higher altitudes, knowing that pressure rises to the correct setting on the way down. Otherwise, know that you need to take back off the inch of manifold pressure gained for every 1,000 feet of altitude lost. For aircraft without a constant-speed prop, choose an initial rpm setting that avoids the rpm redline once descent airspeed has been achieved.
Once the descent is established, use trim to remove any pitch forces. And of course, use a descent checklist to make sure nothing is overlooked. Make sure you know what the manufacturer recommends as far as carb heat, mixture, cowl flaps, and other items are concerned.
Do you want to reach pattern altitude at the airport, or a few miles prior to reaching it? It's a good bet that you would prefer to be level at pattern altitude and looking for other traffic at least a couple of miles before arriving at the airport before entering the pattern. That way traffic flying at pattern altitude is easy to spot, because if it is at your altitude it seems as though it's flying on the horizon. If you are higher, traffic appears below the horizon against ground clutter, making it more difficult to see.
Getting down a little early also allows time to reset power for the pattern, retrim, reset your brain to the slower speeds needed for approach and landing, and clean up the cockpit. A neat cockpit is a happy cockpit. You need time as well to perform the before-landing checklist.
Aren't we through planning yet? After all, getting down is dirt simple. Pull the power back, the houses get bigger, what's the fuss?
Vertical navigation implies that you know how far down you intend to descend, and you can only know that if you research the elevation of the destination airport. You can find that information on charts, in AOPA's Airport Directory, the Airport/Facility Directory, most commercially available flight-planning software, and in the database of your GPS receiver.
There is additional planning you can do as well. Tuning in the airport weather information and figuring out the runway in use, as well as knowing the direction of traffic for that runway, also can affect descent planning. For example, do you want to be west of the airport when reaching pattern altitude? East? It is rare that you would want to be overhead, unless you need to see the windsock or just need to locate the field.
Don't forget to self-announce your position and altitude at nontowered airports. You may be 10 miles out and still have no clue where the airport is, especially if it is hidden amidst city clutter, but other pilots will be able to avoid you. Knowing your distance and direction from the airport and your altitude is helpful to them.
Then there is the temporary-flight-restriction issue. Check the notams to avoid descending into one of the instant TFRs that can pop up these days. A quick telephone call to the destination airport before the trip begins can provide valuable information on special-use airspace and even local procedures. While you're on the phone you might as well see what the locals think is happening weatherwise as well.
Greg French, a chief flight instructor teaching in Frederick, Maryland, suggests pilots keep their descent rates at or below 700 feet per minute to protect the passengers from excessive ear-pressure changes. "You'll want to stay in the ear green zone," he jokes. Pilots might not mind or even notice the effect of a higher rate of descent on their ears, but passengers definitely will. An ideal descent rate is 500 fpm, although IFR pilots realize that a higher rate is needed on nonprecision instrument approaches.
Often pilots traveling at higher altitudes pull the power back only slightly — satisfying the need for speed well into the yellow arc — and point the nose down to descend at rates of 1,000 fpm or greater. Don't do that! That's bad for both the airplane and its passengers. Descent should be a sedate, relaxed phase of the flight.
Perhaps you don't normally fly long cross-countries, and therefore don't select higher altitudes. But you may still encounter them. For example, you might ask for clearance through Class B airspace, and the controller may have only higher altitudes available where traffic conflicts can be avoided. Or you might need to climb to avoid growing cumulus clouds. The point is, you may find yourself higher than you meant to be some day, and a little advance thought about returning to Earth can ease the fear of entering orbit.
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