How Do Engines Start?

Q: How do the engines start? On TV and movies you hear the whine or see the prop begin to turn slowly, etc…

At least a few of you are picturing keys, I know.

The compressors get spinning either via electric power (most turboprops) or compressed air (most pure jets). Once a certain RPM is established, fuel and ignition are introduced and the engine is accelerated to idle speed. The movement of levers, switches and buttons required to pull this off vary from plane to plane, and the command to start is preceded by a couple of lengthy checklists.

From there, the power levers — or “throttles” if you’d rather — are adjusted to deliver thrust as needed.

With turboprops, electricity from an outside source — usually an external ground power unit (GPU) — gives the juice. A GPU, towed behind a small tractor, looks like one of those generators used at roadside construction sights. The plane’s batteries aren’t used for routine starts because the power draw is very strong.

With jets, the auxiliary power unit (APU), a small motor typically found at the very rear fuselage, provides the compressed air. (Smaller jet engines can be started via electric power, but you don’t find these on most airliners.) The APU, you might recall from a previous column, also provides cooling and electricity while a plane is at the gate with engines shut down.

The first commercial airliner with an APU as standard equipment was the Boeing 727, which debuted in 1964 with Eastern Air Lines. Until then, high-pressure air was plumbed in for starts via external supply. The old DC-8 freighter I flew needed one of these so-called “air carts” at every departure.

Due to aerodynamic resistance during flight, an engine’s innards, be it turboprop or pure jet, may continue to spin even if combustion ceases. Thus a failed engine, depending on nature of the mishap, can sometimes be restarted aloft from this “natural” rotation free of assistance from the APU, batteries, etc. It’s all about rotation.

This article is part of a collection that originally appeared on Salon.com. Patrick Smith, 38, is an erstwhile airline pilot, retired punk rocker and air travel columnist. His book, Ask the Pilot (Riverhead) was voted “Best Travel Book of 2004″ by Amazon.com. Patrick has traveled to more than 55 countries and always asks for a window seat. He lives near Boston.

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