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Old 14th Sep 2020, 22:35
  #8 (permalink)  
tdracer
 
Join Date: Jul 2013
Location: Everett, WA
Age: 65
Posts: 2,925
Originally Posted by Fursty Ferret View Post
Bowed rotor protection.
Exactly. Not familiar with what Pratt does (although I'd be surprised if it's much different), but on the LEAP engine, if residual EGT is above a threshold (which it generally will be if the engine has been shutdown for less than ~six hours), it will motor the engine at a specific N2 speed range (less than max motoring) for about a minute to allow the high rotor to thermally stabilize. Once that's done, it goes ahead and proceeds with the normal start.
GEnx does much the same thing.
Bowed rotor is caused by the differential cooling after shutdown (hot air rises, so the bottom cools quicker than the top...) which will literally result in a small 'bow' in the rotor. Starting the engine with a bowed rotor will cause high vibes and can rub compressor seals resulting in a permanent loss of performance. Worse case it can even cause compressor blades to crack or fracture.
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