It’s pretty ingrained in
almost all of us, apparently, to fire the APU at the hint of a dual (or even a single!) engine hiccup. However, I’m with vilas on this one: it wouldn’t have made any significant difference in the final outcome.
MikeSnow: Notice the report says “normal law”. As vilas pointed out, this means the aircraft always had at least one normal generator working, meaning the APU generator came online before the second generator got offline (if it ever did, as one of the engines had only partial thrust loss). As sufficient hydraulic pressure is generated with very low N2 values, losing only one hydraulic system (I.e. the one supported by the completely dead engine) doesn’t get the aircraft out of normal law, and most likely if the generator wasn’t kicked offline, then the partially failed engine was still supporting its hydraulic system.
It seems the PIA case is quite different, as the photos of the airplane with the gear up show the RAT deployed, meaning it did lose power to both AC busses (read: generators). If I venture a little analysis of the altitude and speed charts in the preliminary report, we see the airspeed after the go around never got above 250-ish, then it goes down to about 200 and stays there until the end of the plot. The point where the speed starts to decrease from 250 could be where the first engine failed, then they seem to have traded speed for altitude (since that one kept going up), speed stabilized in regions consistent with normal green dot speeds, and it’s just one mile (starting from the runway) since the speed starts to go down to the end of the plot, which is when the second generator got offline (I.e.: second engine failure). You need a whole minute to get the APU running and online in the A320. There was never going to be enough time to get it running. And even then, the second engine still failed, so they would’ve gone into alternate law anyway. They only would’ve had double instruments, the critical bits of the plane would be the same as without APU. And obviously that didn’t help them before...
lomapaseo: I think yours is a very difficult question to answer with the information we have available, at least to guesstimate where this particular aircraft would have (crash)landed. They weren’t at typical values of speed, altitude and configuration to try and estimate the flight path. And I guess it all depends on when did they lower the gear