Flame-out??!
Q: How can a jet engine operate in snowy and rainy conditions without being affected? Does it melt the snow that enters the engine? Does it
evaporate the water? I have always wondered about this.
A:
I'll tell you what the water in the wall cloud did to six jet engines: as we penetrated the wall cloud and got into heavy water,
the entire engine instrument panel would do violent gyrations with engine rpms and engine exhaust temps fluctuating
wildly-pretty scary! Never could figure out why the water didn't put the fires out, but it didn't. Grampz, flying WB-47s , the
weather version of Boeing's B-47.
A jet flies by catching fast-moving air in its inlets, squeezing the rushing air with rotary fans, squirting fuel into the compressed air,
burning the mixture, and letting the hot, burned gases stream out the back: thereby kicking the plane forward. It's a beautiful application of Newton's
third
law of motion: to every action there is an equal and opposite reaction.
A modern jet airliner, the Boeing
777 of American Airlines. Photo courtesy of Wikipedia
Intense rain or hail, says Aspi Wadia of GE Aircraft Engines, can cause the combustor to burn the air-fuel mixture less efficiently. Usually this effect is negligible
because the intense period of a storm is generally brief. Large aircraft engines exchange roughly the volume of air in a normal size house every second. Normal
rain, hail, or snowstorms don't produce enough water to pose a problem. High-temperatures in the combustion chamber evaporate this amount of moisture.
The water only adds a slight amount of steam to the burning air-fuel mixture and doesn't reduce the engine power appreciably.
Furthermore, the engine is designed to extract excessive water, snow, or hail from the air volume before it reaches the combustion chamber. The clever design
uses the shape of the spinner in front of the fan and the distance between the fan and the core engine (i.e., the compressor, combustor, and turbine) to spin the
heavier water, hail, and snow like a centrifuge into the bypass flow of the engine. Bypass flow never hits the combustor since it goes around the core engine.
This design provides the combustor a sufficient safety margin, says Wadia, that it can function under worst-case conditions.
In summary, jet engines evaporate small amounts of water. Engine design extracts large quantities from the compressed air before burning the air-fuel mixture.
That's how jets continue to function with snow, hail, or rain falling all around them.
(Answered by April Holladay, science correspondent, May 16, 2001)
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