A20N Engine Start
 

The A320NEO seems to take an age between pushback and engine startup to request for taxi. Would a more learned subscriber venture the reason(s) why? After push and start the other day, we waited for the 'request taxi' call and an inbound to the ramp had to hold short waiting for the outbound. The flight deck commented on R/T "yes, we're in the NEO; it takes forever..." - what's different about the NEO that makes for a longer startup time? Thanks for enlightening me!

The engine cranks for a few seconds longer to stabilise the internal temperatures within the engine but it is on a par with an IAE. It does seem an age in comparison to a CFM.

Bowed rotor protection.

The CFM Leap-1A (the other engine option on the NEO) takes an age as well for the same reason, nice and quick if it is cold after sitting overnight but cranks for ages once it is hot.

From experience, it’ll take about a minute longer than the IAE, so about 2 minutes per engine.

Same story with the PW1500 on an A220 (CSeries) and GENx on the 747-8

And the 737MAX. The whole thing from cranking to stabilized can take up to 5 minutes.... thats progress for you!

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.

Out of interest, why wasn’t this an issue on the previous CFM/IAE generation? Bigger NEO engine squeezed in a tighter space?

For fuel efficiency everything about an engine is being made to operate to the limit e. g. Fan diameter to process more air mass, compression increasing core pressure, N1 and higher EGT. So the tolerances are reduced. Older engines had more margins. Some of them also had to motor to bring residual EGT down for start. In PW the separated fan through gearing so it could operate at it's own efficient speed. It allowed them bigger fan and less stages of compression reducing weight.

What vilas said. The trend for decades has been for bigger fans and small, faster spinning cores (faster spinning cores tend to be more efficient) operating at higher temperatures, with ever tighter clearances to minimize losses. The bowed rotor phenomena is not new, but wasn't a major concern until about 20 years ago. I believe the GE90-115B was the first engine to really suffer from bowed rotor problems - and the autostart incorporated a bowed rotor mitigation (IIRC and extra 17 seconds of motoring before fuel ON). In addition to bowed rotor mitigation, engines with smaller cores and bigger fans just naturally take longer to start so start times keep going up.
GE9X on the 777X takes bowed rotor mitigation one step further - there is a small electric motor on the gearbox that uses aircraft power to slowly rotate the core after shutdown at roughly one rotation per minute - that will even out the cooling of the core and prevent the rotor from bowing. It's commonly referred to as the 'rotisserie' for what should be obvious reasons.

Would have been a nice installation on Garrett turboprops -- where procedure is to turn it through by hand when you get out!

The IAE engines already had a significant motoring cycle prior to fuel input (about 30 seconds), I don't know if the CFMs had something similar. At least the NEO can cool both engines at the same time, something I gather the B737 MAX does not do.

If I recall correctly, Concorde also had a de-bow procedure for warm engines.

There’s been a EGT maximum before introducing fuel on big Engines for a long time on the wide bodies I’ve flown. Generally 100c but 150c for some.....Only an issue for turnarounds, especially when there is no wind blowing the Engine around on transit.
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True. The IAE engines take longer to start compared to CFM due to this longer motoring cycle. However, this happens with a cold engine as well.

Interesting info from everyone. Thanks.

Simultaneous engine starts?

IAE V2500 used to have a 50sec dry crank for shortly after it came into service, reduced to 30sec a few years later.

Only dual cooling/cranking. The actual start is still one by one.
(On the A380, 2 engines are started simultaneously. But the APU is more powerful to be able to do that. No idea about the A340 and B747.)

Grateful for everyone's most learn-ed replies... truly fascinating all this tech stuff to me in the air traffic world. Thank you for taking the time to respond.