The APU will not produce enough hydro power to successfully steer the airplane expertly the way Sully did.
My speculation, as a former AF-trained accident investigator, is that the engines possibly retained enough integrity after sucking in geese that they could continue spinning and running the hydro pumps and generators long enough for Sully to find a suitable spot.
And this former AF-trained mishap investigator agrees.
(Norton AFB, Class of ‘93)
On an A320, you are exactly right. the APU is shut down before take off.
the backup systems would have been operational until impact, as it A320 is designed to handle a ‘deadstick landing’. The Pilot would have had all the control he needed to ‘slide it in”.