Q: If the APU supplies ground power, why do you often see the engines turning while a plane rests at the terminal? Isn’t that a waste of fuel?
Jets almost never run their engines at the gate. What you see is the wind spinning the first stage turbofan. Despite its weight and size, the breeze can rotate that fan like a windmill, sometimes quite rapidly. If that seems impossible when a plane is cornered against a building or facing the wrong direction, that’s because the wind is coming from behind. On a modern jet engine, some 80 percent of intake air is blown around the core of compressors and turbines, providing a clear shot at the fan blades from the rear. The wind blows through this duct between core and cowl and sets the assembly in motion.
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.