Quote:
Starting the APU early in the accident sequence proved to be critical because it improved the outcome of the ditching by ensuring that electrical power was available to the airplane. Further, if the captain had not started the APU, the airplane would not have remained in normal law mode.
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The NTSB concludes that, despite being unable to complete the Engine Dual Failure checklist, the captain started the APU, which improved the outcome of the ditching by ensuring that a primary source of electrical power was available to the airplane and that the airplane remained in normal law and maintained the flight envelope protections, one of which protects against a stall.
In Flight 1549 there are two aspects. They had damaged Engines with low N1,N2 not failed Engines. They had all hydraulic systems working but generators could could have got disengaged unpowering the AC1 &AC2 getting in ELEC EMER config and alternate law which was avoided by APU start. In PK8303 case it is at least clear that their both generators failed because RAT was deployed and they were in alternate law. If they had the required hydraulic pressures in Green and Yellow systems is not known, left engine the report said needs investigation. If they didn't have then they would have remained in alternate law even with APU on due to G+Y fail. If they had and AC buses were intact APU was able to connect then they would have been in Normal Law. But then they would have activated alpha protection to hit the buildings even harder. It even happened to flight 1549. Sullenburger had dropped his 19kts below VLS which triggered alpha protection . So either way APU or no APU PK8303 was doomed.