In New York, search crews are trying again today to find the missing left engine from US Airways Flight 1549, still resting somewhere at the bottom of the Hudson River. This morning's AP story on the search includes reports of a "compressor stall" in one engine of the plane that glided into the river on Thursday, two days before the accident, and a safety bulletin issued on all similar engines. More detail on the engine problems -- which appear unrelated to the bird strikes that are still considered the cause of the shutdown of both engines on Flight 1549 -- were in Newsday this morning.
Discussion of engine compressor stalls made me think it was a good idea to add a little information about different types of stalls in flying an airplane. The compressor stall reported here is closer to your car's engine stalling for lack of proper combustion; it can shut down an engine in flight.
A stall of an airplane in flight means something different. A question came up from one of my editors over the weekend about what the captain of the US Airways flight was trying to prevent -- a stall -- as he guided the plane into the river. Here's how I explained it:
Stalling refers to what can happen when a plane reduces or loses power. To prevent it requires keeping the wings at the right angle during a descent, when the air speed is falling, so the wings continue to provide lift and the plane acts like a glider. Stalling occurs when the plane's angle of descent falls below the level that keeps it in glider mode. What the Flight 1549 crew managed to do was keep the plane at the correct angle so that descended into the water with the nose up, assuring that the plane didn't drop too quickly, and then let the tail drag in the water first, slowing it down.