Originally Posted by
mustafagander
As tdracer says Inflight starts which are proceeding normally after light up are often aborted coz the acceleration of the core is very slow at altitude. My old mob used to ensure that it was demonstrated in the sim during the 3 year cyclic training/checking programme.a few times to remind us.
The slow start characteristics is particularly true of the newer generation of engines - big(ger) fan, small(er) core. That trend has caused the in-flight start characteristics to really suffer - particularly windmill relight.
When we did the 747-400 and 767 re-engine, the CF6-80C2 would start pretty much anytime you tried it (the only real limitation being the core windmill speed being high enough to provide sufficient fuel pressure - ~8-10% N2), and the PW4000/94" was nearly as good.
Later programs proved much more difficult to get acceptable in-flight start characteristics. On the 747-8 (GEnx-2B), we actually had to resort to depressurizing the hydraulic pump during in-flight starts in order to get sufficient windmilling N2 to get an acceptable windmill start envelope.