View Full Version : Adding fuel on B737 Max while engine still motoring?

19th Aug 2023, 17:51
As the titles suggests, is anyone aware of what unfolds if you put the start lever up whilst the N2 is still motoring on the B737 Max 8 Leap 1 B engines?

hung start?

has anyone done it by accident?

I am just curious…

19th Aug 2023, 20:22
Not sure I understand your question - are you talking about putting the fuel switch to RUN early during a start, or suddenly putting fuel back to run while the engine is spooling down after shutdown?
Assuming you mean during start, pretty much nothing unusual will happen. While I don't think it's considered full blown 'autostart', the LEAP has a lot of start protection logic that will protect the engine if you do something stupid like turn on fuel at too low of an N2 during a ground start. Further, during start, if the engine is still warm the LEAP will go into a 'bowed rotor' mitigation where it dry motors N2 prior to fuel ON - so it won't turn on fuel regardless.

19th Aug 2023, 20:27
yes adding fuel during the motoring phase with around 23% N2.

just curious if anyone has done it in the sim, or unintentionally etc.

I can confirm that if the start switch is placed to cut off during shut down and then immediately placed back to idle the engine doesn’t re-ignite, it spools down as if the start lever was in cut off, as this has actually happened to me once.

thanks for the response

21st Aug 2023, 18:35
If the fuel is introduced into the engine and there is ignition at an N2 below where the normal start speed you would likely see a hot start or hung start.
I've never done it but back when we had an APU that was weak and wouldn't motor the CFM56 to the required N2 speed the start EGT were usually so high as to abort the start.

22nd Aug 2023, 08:11
I flipped through the related AMM pages, and just like on the classics or the NG the EEC will allow fuel flow also at lower N2 - however, and this is an assumption, I would bet that the EEC stops the BRM program and lets the starter spool up further. You'd probably still cause a relatively warm start.

22nd Aug 2023, 10:35
Fuel into a hot engine as it spools down is never good. Woof!

29th Aug 2023, 20:11
Yes, you can do this on the MAX. You can move the start switches to GRD and the start levers to IDLE immediately. The LEAP engine will continue to motor as normal. The engine valve opens when the start levers are moved to idle, but the spar valve remains closed until the TCMA and EOS tests are complete. Fuel is only introduced after the motoring indication disappears. It's automatic as in the Airbus (ignition - start, master switch - on).