Quote from Natstrackalpha:
I am looking at why [ if that is indeed the case] the RAT cannot give enough juice in the Hyd genny to power APU start attempts
- am trying to balance this off against "a successful APU start from the batt is unlikely" . . back to the books, I guess. . .
No manuals or QRH to hand, but if and when you have a chance try selecting the ELEC page on ECAM during a routine APU start on AC EXT PWR. Naturally, the 28V DC has to be produced by the TRs. (I emphasise the plural because, in the early days, TR1 had to do it alone.)
You'll see that both TR1 and TR2 quickly achieve 300 amps, which is top-of-scale on the read-out. Additionally, the two BATS register considerable current draws. That situation lasts for some seconds until the generator effect kicks in as the APU starter motor (and the APU itself) accelerate.
Having said that, the BATs always achieved reliable APU starts at sea-level in my experience, even on a cold morning.
Last edited by Chris Scott; 21st Oct 2013 at 17:07.