The smart way down

The first trick is figuring out when to start down. These days, just about every airplane has a GPS, and most of those constantly update the distance and time to your destination. Some even have a “vertical speed required” (VSR) readout. If so, you simply wait until the VSR reaches you desired rate of descent, say 500 feet per minute, and start downhill. 

If you’ve only got distance and time, prioritize the time. If you’ve got 5,000 feet to lose and want to descend at 500 feet per minute, you know you should start downhill about 10 minutes out. 

If you only know the distance, the “rule of threes” can be a useful mnemonic. Take the amount of altitude you want to lose in thousands. For example, if you’re cruising at 7,500 feet to an airport with an elevation of 1,500 feet, you’ve got 6,000 feet to lose. Multiply six (from 6,000) times three (this is why they call it the rule of threes) and you get 18—the distance in nautical miles at which you should start your descent.  

OK, but how quickly should you descend? 

For that answer, simply divide your groundspeed by two and multiply that number by 10. For example, if your groundspeed is 120 knots, dividing it by two gets you 60—and 60 times 10 is 600. Descend at 600 feet per minute and you’ll be on the money. 

For the descent itself, there are two methods to choose from: constant airspeed and dive bomb. 

Constant airspeed is the thoughtful and graceful way down. Leave the elevator trim where it’s been during level cruising flight, then reduce engine power so that the airplane descends at the desired rate. If the engine in the airplane you’re flying is carbureted, simply applying carburetor heat often results in a descent of about 500 feet per minute all by itself. (This has the added benefit of clearing the carburetor of any potential ice buildup during what might be a long, low-power descent—the most likely time for carburetor ice to form.) You’ll have to continue reducing engine power on the way down to maintain the desired indicated airspeed and rate of descent, but those corrections will be minor, and the airplane’s elevator trim wheel shouldn’t require any adjustment. 

The dive-bomb method requires rolling in a bunch of nose-down trim and leaving engine power high. Don’t do this in rough air because the needle on the airspeed indicator will likely climb well into the yellow arc and any turbulence will feel like a spanking. Also, make sure to get to your target altitude well in advance of the airport traffic pattern because you’ll have lots of airspeed to dissipate before getting there, and if you’re in an airplane with retractable landing gear and/or flaps, expect to be well above their deployment speeds. 

The only argument in favor of the dive-bomb method I’ve ever heard with any sort of logical consistency came from a businessman flying a rented Bonanza. When I asked him why he insisted on slamming through bumpy air so mercilessly at high speed, he smiled and said, “Anything that costs this much needs to fly fast.” Fair enough. 

There’s also a lot of consternation about what to do with the red mixture knob during descents. 

Consult your airplane’s pilot’s operating handbook for a definitive answer, but if you’re flying behind a fuel-injected engine, the best answer is usually to leave the mixture knob alone. Don’t enrich the mixture until you arrive at the traffic pattern altitude and run the pre-landing checklist (in preparation for a go-around).  

If you’ve been running the engine lean of peak (https://www.aopa.org/news-and-media/all-news/2019/may/flight-training-magazine/technique-lean-of-peak) during cruise, you might be surprised to see the exhaust gas temperature (EGT) on the cylinders rise as you reduce engine power. What’s happening is the reduction in fuel flow from less engine power has made the fuel/air ratio richer—and hotter. Cylinder head temperatures are almost sure to remain cool, however, even as EGTs rise, and there’s seldom any need to enrich the mixture as long as engine power remains below 60 percent which is typically the case during descent. 

If you must level off at an intermediate altitude or increase engine power for any other reason, go ahead and enrich the mixture. Doing so will keep CHTs and EGTs sufficiently cool at higher power settings. 

There’s a decades-old argument about “shock cooling” and whether a quick power reduction followed by a long period of cold air rapidly chilling the cylinders causes long-term engine damage. Some mechanics are convinced shock cooling harms engine longevity and reliability, and others say it’s a meaningless measure and that engines cool far faster after they’re shut down than they do in flight and it doesn’t hurt them. I don’t know enough about the metallurgical properties of aircraft engines to have a meaningful opinion, so I assume shock cooling is real and avoid long, steep, power-off descents, particularly during the cold-weather months.&

One way to accomplish this is to begin descents early and reduce power slowly. For example, when cruising at 7,000 feet and full engine power in a normally aspirated (non-turbocharged) airplane at 22 inches of manifold pressure, reducing engine power by five manifold inches (to 17 inches) typically results in a roughly 500-foot-per-minute rate of descent at a constant indicated airspeed. The engine gains one inch of manifold pressure for every thousand feet it descends, so keep pulling the throttle back to maintain the desired 17 inches throughout the descent.

If everything goes as planned, you’ll be below landing gear and flap deployment speeds when you enter the traffic pattern.

You and your passengers will have clear ears from the steady and slow descent, and any bumps on the way down will have been encountered at a relatively safe and sane speed. Your engine temperatures and pressures will have remained in the green bands throughout, and you’ll have plenty of time to configure to land on speed and on target.