Here on Earth gravity rules. What goes up will come down, most especially airplanes. It's how we get up and down that determines how safe and skilled we are as pilots. Planning and accomplishing an orderly, controlled takeoff and climb followed at some point by an orderly, controlled approach and landing is Job Number One on every flight. Flying skill doesn't start and end with the takeoff and landing, however. On most flights we climb to a cruising altitude, level off, and remain there for a period of time. What follows next--the bridge between cruising and the approach and landing--is the descent phase.
Descending from cruise to the airport is more than simply a matter of yanking the power back and pushing the nose down. The higher you cruise, the more you put into the planning and execution of the descent. Flying high--my preference for most trips because the airplane flies faster and on less fuel than it will down low--means planning for a longer descent. As in a climb, the best descent usually is a compromise in this case between starting down too early and too late.
On one of those rare good days when you have a strong tailwind at your cruise altitude it can be tough to leave that impressive ground speed up there and begin a descent to the destination airport. The tendency, therefore, is to delay beginning the descent. That creates the potential for having to descend too rapidly near the airport to avoid overshooting it, or arriving too high and probably too fast to make a normal, stable final descent and approach.
The other, more significant problem with rapid descents is the potential for shock-cooling the engine. When it becomes obvious that you won't be able to descend to near pattern altitude within a reasonable distance of the airport, it's tempting to pull the power back to idle and point the nose steeply down to get down. That sudden reduction in power and engine heat, coupled with the increased descent rate and volume and velocity of ram air entering the engine plenum, can prove to be too much for the engine's sensitive metal cylinders. The shock of cool air on hot cylinders can result in cracking of one of more cylinders.
The converse of a late and too-rapid descent--beginning the descent too early--is less problematic, but it may not be the most efficient choice. Long, drawn-out descents from altitude can add to trip time as true airspeed declines in the increasingly denser air. Descending early also can take a toll on overall fuel consumption, and subject pilot and passengers to extended time in hot, turbulent, lower-level air.
So, when's the best time to begin a descent? There are too many variables to consider--wind, turbulence, air traffic control, controlled-access airspace, and even passengers' sensitivity to increasing pressure on the eardrums to name a few--to state definitely when to begin descending. However, a workable rule of thumb for single-engine piston aircraft is to start down at the point where a 500-foot-per-minute descent will put you at pattern altitude a few miles from the airport.
The math to determine the top-of-descent point on your course is pretty easy--a simple time/speed/distance calculation. If you are cruising at, say, 8,000 feet msl and pattern altitude is 1,000 feet msl at the destination airport, you have 7,000 feet to lose in the descent. So, if you descend at 500 fpm you'll need 14 minutes to get down to pattern altitude. At a groundspeed of 120 knots (true airspeed is irrelevant in descent planning), you'll cover 28 miles in 14 minutes. There's your plan--start the descent at about 33 miles from the airport to be at pattern altitude five miles from the field. It's difficult to maintain 500 feet precisely from the moment you begin the descent until reaching pattern altitude, so five miles gives you a workable fudge factor.
Begin the descent by reducing power several hundred rpm or, if the airplane has throttle, propeller rpm, and mixture controls, two to three inches of manifold pressure, without changing trim. The airplane will attempt to maintain the trim airspeed, and the only way it can do that at a lower power setting is to descend. Pay careful attention to your descents and you'll soon identify a power setting that will yield a 500-fpm descent in smooth air. Of course, the air gets denser as you descend, which means the engine will produce more power the lower you descend unless you reduce power about 100 rpm or one inch for each 1,000 feet of altitude you lose.
Not every flight is suited to a 500-fpm descent rate from altitude. If, for example, it is uncomfortably turbulent at lower altitudes, you may want to delay beginning the descent and plan instead for a 750-fpm or even a 1,000-fpm descent rate. Reduce power so airspeed remains in the normal operating range. Just don't bring the throttle back to idle or you risk shock-cooling the engine. If the airplane has separate manifold pressure, propeller rpm, and mixture controls, you can safely reduce manifold pressure to 20 inches and not worry about shock-cooling. The engine will produce enough heat to prevent shock-cooling, even with a rapid descent rate.
Another reason to delay initiating the descent is if strong headwinds ply the lower altitudes. You'll know this from your preflight weather briefing and periodic updates from flight service stations along your journey.
Strong headwinds reduce groundspeed, so you may be able to delay the start of your descent and still keep your descent rate to a comfortable 500 fpm or so.
You should consider airspace when planning your descent. On a frequent trip I make, I have to go through Class C airspace to reach my Class D home field. I've learned that the Class C approach controllers will vector me overhead their airport at 2,500 feet, then hand me off to the tower at home base. So, I now calculate my descent to be at 2,500 feet when reaching the Class C airport. Works like a charm.
Air traffic control in busy metropolitan areas can be something of a wild card in descent planning unless you know from experience how the controlling ATC facility handles arrivals. They may prefer to descend light aircraft to very low altitudes far from the airport to keep them out of the way of high-performance traffic. One way to deal with inefficient (to you) ATC protocol when flying IFR is to cancel your IFR flight plan (assuming, of course, that visual meteorological conditions prevail) but retain your transponder squawk code and continue VFR with radar advisories. In that case you at least participate in setting the agenda.
A well-planned and executed descent is a beautiful thing. The changeover from cruise to descent is almost imperceptible, and a total non-event to passengers. It's all about giving into gravity on our terms, and that's just how we like it.
Mark Twombly is a writer and editor who has been flying since 1968. He is a commercial pilot with instrument and multiengine ratings and co-owner of a Piper Aztec.
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