A 320 Engine Relight
 

For CFM 56 engines in the Engine Relight Section of the QRH it states Windmill Quick Relight if N2 >12%. whereas for LEAP engine, it states Windmill Quick Relight t < 20 s. What does T < 20 S mean.
Thanks

Not flying/doing SIMS on the NEOs but I have access to the manuals and my take is the answer could be in the FCOM - PRO - ABN - ENG RELIGHT note at the bottom.

Note:
For an in-flight restart, the FADEC decides whether the rotor speed is sufficient to perform a windmill relight or needs assistance from the starter in view of the current engine parameters and the flight environment parameters. Within the windmill quick relight envelope, if the crew is using the Quick Windmill Relight procedure, the FADEC will restart the engine after the flight crew cycles the ENG MASTER lever from "ON" to "OFF" then back to "ON".

Possibly this cycle has to be within 20 seconds to use the Quick Windmill Relight ?

Not an expert on the LEAP, so this is a best guess...

An engine will restart very quickly following a brief fuel interruption, which I think is what this refers to. It's not documented but it can potentially let the engine recover from a locked-in surge condition or possibly the engine master switch being inadvertently selected off.

In this case switching the engine master back on within 20 seconds will allow very quick recovery (on bigger engines such as the Trent 1000 you get light-off within a second).

May of course be completely wrong so will follow the thread.

That T<20 s makes reference to inadvertently selecting the engine master OFF during flight, and then quickly reselecting ON. With these two consecutive actions, the FADEC, regardless of engine mode selector position, will try a quick relight if within the "Windmill Quick Relight" speed envelope.

You can reach this conclusion after reading "FCOM - DSC - 70 - 80 - 30 - Continuous Ignition" and the [QRH] Engine Relight procedure.

The question is why only on Leap?

My guess is because only the FADEC of the LEAP engines can check by itself if the rotation is enough for an engine restart.

As said above the t<20 seconds refers to the time between the flameout (by switch movement or other cause) and moving the switch to ON or RUN to initiate a relight.

I was not involved in the testing for the A320/Leap, but based on my experience from other programs I'm pretty sure the reason for the 20 second limit on use of the quick relight procedure is the lack of adequate engine compressor stall margin after 20 seconds of rundown due to residual heat in compressor section. Within the first 20 seconds after flameout from high power, there is enough compressor speed (N2) to avoid compressor stall when the engine is relit and fuel to accelerate the engine is applied. After that many seconds of deceleration, the compressor speed is low enough that the heating of the air by the hot metal in the compressor introduces enough expansion of the air to, in combination with the backpressure from lighting the combustor and increasing fuel flow to accelerate the engine, cause the front stages of the compressor to stall and cause the engine to fail to accelerate. I don't know the Leap's specific characteristics, but an engine can potentially exceed the EGT limit in this condition.

After you reach that point in the deceleration of the N2 shaft, the only way to get a successful restart is to wait until the compressor cools a bit. By the time the engine has cooled enough, N2 is fairly stabilized and you would follow the stabilized windmill start procedure or starter assisted start procedure. I'm not familiar with the extent to which Airbus has automated selection of those processes, but motoring the engine or increasing airspeed would speed the cooling of the compressor.

The original Trent 800 relight logic dealt with this issue by keeping fuel flow on to hold the engine in a stalled condition but prevent further deceleration, ramping fuel up and back down every few seconds to see if the engine would accelerate. Holding the speed up kept the airflow higher and cooled the compressor faster. Once acceleration was sensed the engine control would proceed with a normal starting fuel flow schedule.

Tdracer was likely involved in control system changes to optimize the quick windmill relight characteristics of at least few Boeing engine installations. Maybe he'll chime in soon.