>>2030889>Why doesn't it just automatically kick in when power fails?...An APU start is the highest load that can be placed on the batteries. Now, you are trying to a) simultaneously supply the essential loads and b) to start the APU. The APU start is more challenging for the batteries the higher the altitude. If you kill your batteries, you are dead. That's why most transport category aircraft have RATs as the electrical and hydraulic source of last resort, and that RAT isn't going to give you everything, just enough to make it to the ground. The 737 doesn't need a RAT because it has full manual reversion upon a complete loss of electrical and hydraulic power. The yoke and rudder pedals are directly connected to the control surfaces. The 737 batteries can supply enough power to the essential loads (left side instruments, communications, emergency lighting, fire systems, etc.) to make it to the crash site.
>but muh sully started the APU on Cactus 1549Completely different situation. The A320 has two larger main batteries because it is a fly-by-wire aircraft. The batteries have to bridge the gap between complete loss of AC power to supply energy to the computers and hydraulic systems to maintain aircraft control until the RAT deploys. They were about 2500' MSL so they had relatively dense air for a successful APU start and time. They also had some windmilling source of hydraulic and electric power. However, it was not without risk, and it was out-of-sequence in the QRH. This is the difference between cultures in the west and east.
