Aviation From My Rocking Chair: Shut ’er Down

Hello, again troopers. Let me ask you a question. When you park your aircraft, do you pull the mixture to idle/cutoff, shut off the mags and master then head for home? If so, let’s take a little time and explore this seemingly simple procedure and look at some tips which may prolong the life of that noisemaker up front. Any engine will benefit from a short cooling cycle at idle before shutdown. Turn the airplane into the wind and idle for one or two minutes with the cowl flaps open. Turbocharged engines require a much more extensive cooling period. They are subject to a problem called, “Turbine Coking.” After landing, the turbine rotor is still very hot (usually 1000 degrees F. or more), and the only cooling for the rotor, bearings, and shaft is circulating engine oil. If the engine is shut down at this point, mineral oil (which carbonizes at 600degf) will turn to coke in the center bearing area which can lead to turbine failure on the next startup. So, be sure to run the engine at idle 4 minutes at least before stopping. Good news, taxi time counts which means 4 minutes ain’t bad. By the way, if you own a turbocharged car, this procedure will greatly extend the life of the turbo.

Now let’s shut this baby down by the numbers:

(1) Turn off the boost pumps (2) Pull mixture(s) to idle/cutoff (3) Shut off mags/ignition switches after prop comes to a complete stop.

These first three items accomplish several functions; they keep excess fuel from accumulating in the cylinders where it can mix with oil and foul the bottom sparkplugs. They reduce the chance of prop strike injuries if the aircraft is moved while touching the propeller, and they eliminate “Hot spot” run-on or Dieseling after shutdown.

(4) Remove ignition key (Put it in your pocket now. It may prevent a prop strike accident sometime.) (5) Turn all electrical accessories off. This will help reduce electrical loads at startup. (6) Turn off master switch.

(7) If a Bendix Pressure Carb (not many left) is installed, return mixture to halfway in position. This prevents internal rubber diaphragms from taking a set and effecting fuel flow on next operation. Everything should now be quiet except for the gyros spooling down, and the metal tinkling coming from up front. Before I take my leave once again, here are a few more tips:

a) If you pull the mixture to idle/cutoff and the engine fails to shutdown quickly or “Diesels” suspect internal carburetor or fuel injector leakage (or maybe you forgot item #1 on the shutdown list?) b) Once in a while shut down the engine with the magnetos. If the engine continues to run, P leads are either disconnected or broken.

c) Consider not shutting off the fuel selector between flights. It is far to easy to forget to turn it back on.

d) If sparkplug fouling has been a problem, try running the engine at 1800rpm for 30 seconds prior to momentary return to idle and then shutdown. Sparkplug cores scavenge well only at temperatures in excess of 800 degrees F.