Plane & Pilot
Tuesday, September 3, 2013

The Lowdown On Go-Downs

Descents should be more than simply throwaway methods of losing altitude

Those who fly behind a turbo and like to cruise high in thin air can have special problems. Controllers may assume you're pressurized and assign a descent rate that's not realistic for an uninflatable airplane. For that reason, you might want to specify your aircraft type in your call sign. If you have a turbo out front and you're flying at 17,500 feet, you might want to call yourself "Saratoga" rather than simply "Piper," or "Skylane" rather than "Cessna." That way, controllers who are pilots won't automatically assume that you're flying a pressurized Malibu or Centurion just because you're operating at the highest VFR altitude. That's a sure tip-off that you can't comfortably accept high descent rates.

For those who are instructed to go down and slow down simultaneously (a trick some diabolical controllers seem to enjoy), the best method is to deploy speed brakes if you have them. These are operable all the way to redline and don't put undue aerodynamic stress on the airplane. Second choice is approach flaps, if the limit speed is high enough.

Extending the landing gear is the final option, as most retractables have limit speeds that are well below flap extension velocities. These usually are set to avoid structural overloads on gear doors. (One exception is that some airplanes demand low gear-in-transit speeds then, once the wheels are down and locked, allow you to take the airplane practically to redline if you're stupid enough to go there.)

Whatever you do, don't extend cowl flaps as a drag device during a descent. You'll be increasing cooling in the descent automatically. Shock cooling is a proven factor in premature engine failure, as different metals inside the engine expand and contract at different rates. The more consistent you can keep cylinder head temps, the better.

It's important to remember on the way downhill that mid-air threats are still a risk, above, below and in all horizontal quadrants. The tendency is to look straight ahead to spot what you're about to overrun. A better policy is to continue a normal scan just as you would at cruise. The airspace obviously becomes more congested at low altitude. Most general aviation traffic is concentrated below 6,000 feet AGL, especially around busy airports.

Other major airports allow general aviation to fly directly overhead. At KLAX, gateway to the huge Los Angeles Basin, there's a VFR corridor between 2,500 and 5,000 feet that runs northwest/southeast above the four runways. The corridor is as wide as the runways are long, and it allows transitions from the south basin to the north basin and vice versa, with no chance of conflicts with an airliner.

Intelligent descents aren't really that challenging for pilots who are simply awake. There's no great mystery to bringing off a smooth, professional letdown with minimum discomfort for your passengers. Just be sure to tell everyone to Valsalva during the descent, and watch the weird looks people give you.


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