The APU fire warning horn has got nothing to do with all of this. It is in the MAIN wheel well. The ground crew horn in the nose wheel well is the one you heard. It sounds automatically if the ADIRU's are left on battery power i.e when you had power failure and you were on the ground, the horn comes on after 20 seconds.
Loss of equipment cooling on the ground also brings the warning horn on.
Last edited by J31 MAN; 29th Sep 2010 at 09:58.