View Full Version : APU OFF then ON B737NG
flyby797
13th Feb 2018, 09:33
Hi everyone, was a bit puzzled by a FO practice yesterday and i’m looking for some answer. Parked at the gate, GPU connected, we turned APU OFF. 20’’ after that, GPU just was not delivering anymore for some reason. At that point, FO positionned APU switch to ON and reconnected the busses. I never saw or heard of this procedure. Looked for it in our manuals and found nothing. I understand the logic of being in the automatic cooling period but it still puzzled me. Anyone with some reference on that?
EGPFlyer
13th Feb 2018, 11:07
Been a long time since I flew the 737 but does the APU run for a while after turning it off? If so and it’s selected back on in this period then it can be used again to supply the busses.
In a separate note, if you are the captain and your FO is doing something that you don’t understand, isn’t it a good idea to ask them what they’re up to?
Skyjob
13th Feb 2018, 11:49
Yes, had to do this once as well after putting the switch to OFF then realised we needed the APU for take off. While the APU is in its cooldown cycle (60 seconds), carefully replacing the switch to ON will illuminate the GEN OFF BUS light again and cancels the shutdown.
Sounds similar to the 747-400, but on this aircraft you have to select START (not just ON) during the cooldown phase.
flyby797
17th Feb 2018, 12:54
@EGPflyer there is a cool down period of 60 seconds. And when I asked him he said he is used to do it but he couldn’t not find any reference to it. Hence my question
flyby797
17th Feb 2018, 12:55
@tangoalphad agreed but just looking for reference.
flyby797
17th Feb 2018, 12:56
@skyjob any reference to this please?
victorpapa
17th Feb 2018, 16:33
“The EICAS memo message APU COOLDOWN is displayed. At any time during the cooldown period, the APU may be returned to its running condition by placing the APU selector back to ON. “
flyby797
17th Feb 2018, 20:11
@victorpapa thanks! Wish we had the same on 737
777 did the same, as does the A330.
APU slows to about 82% during the cooldown, switch it back on and up it goes back to 100%.
During the 85 second cooldown you have the opportunity to switch it back on.
Note: if you weren’t using the bleed air there isn’t a cooldown, it shuts down after 15 seconds ( A330 )