Hi IFixPlanes,
As you say
The SPU can use batterie (28V DC) OR "AC TRANSFER BUS 1" (115V AC)
was the missing link in my brain...
But not only in my brain
I wonder why they provide us with so miserable manuals.
The above information is available NOWHERE to pilots, and please correct me if I'm missing something.
Hi FlightDetent,
I agree, the 737NG APU can be started with either aircraft battery or AC power, with AC power being the preferred when it is available - you'll know it's using AC power to start when the "BATT DISCHARGE" light doesn't come on during the start.
We have a classic fleet, however some of our airplanes have the NG ammeter panel and some the APS2000 apu.
I don't know if these airplanes are the same as the NG, in this particular case, however I've always seen the BAT DISCHARGE light coming on when starting the APU using the GPU, or while taxying after landing...
That's also why I thought that APU starting
requires exclusively battery power, even if AC is totally available.
But I must add that I'm not sure if those with the NG ammeter have the APS2000, or Garret 129.
Our fleet is completely wixed up, I'll check further...
Thanks for your help.