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Old 20th December 2010
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CaptainD CaptainD is offline
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Re: A tearful ( plus rain,sleet and snow! ) farewell to a retiring aeroplane

Dear Garrie,

Thanks for very kind comments, you are more than welcome to ask some questions. Pprune certainly is mine of information which can make a day evaporate before you know it.

The 757 was powered from its APU that is tucked away in the tail cone. The engineers were very kind to do that instead of using external power. Due to the strong wind you are spot on that the engine fan blades were turning as a result of the wind. The Rolls Royce RB211 is unique in that it has 3 independent fan rotors. It may surprise you that the large blades on the front rotor of the engine will turn even in light wind, windmilling as it is known.

As each individual fan blade of the rotor is mounted in a slot. there is a little free play as a result they make a clattering noise as they slowly rotate. It really can be quite loud. This is why during periods of longer storage the engines are covered at both ends to stop them rotating and prevent any rubbish getting in the engine.

So to answer you question, the engines are not turning because the APU is running, just the wind does that. However to start the engines it is normal practise to use high pressure bleed air from the APU which is fed to the engine being started to crank over the engine during the start sequence. It may surprise you that the smallest of the 3 rotors is cranked during the start sequence that in turn will cause the other two rotors to turn. If the APU is not available for use, the air supply to start the engine will come from a high pressure ground air start cart. On the 757 a minimum of two ground air carts are needed to start an engine such is the volume of air that is needed.

Sorry Garrie I seem to have gone on a bit!

Cheers

Chris
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Garrie (20th December 2010)