[12_throw_away]

Found this part interesting:

> somehow Kam Air can keep all of their APUs operating but European carriers I've flown with will go the entire Summer season hopping about the Greek Islands with it INOP

If anyone knows: is it normal to allow dispatch without the APU? I kind of assumed it would be a required redundancy, especially on an airbus where the computers and electronics are what keeps the thing in the air ...

[jenny91]

The APU is not on during flight, so not a safety thing. It's just for providing power (and bleed air) on the ground (if there's no other power source). E.g. can't start up at airports without external power with the APU INOP.

[sterlind]

if you don't have a RAT you need the APU for emergency power if your engines fail in flight.

[addaon]

APUs have a common mode failure with main engines: lack of fuel. Are there designs out there which require emergency power after engine failure and don’t have a RAT?

[scrlk]

There was a Mentour Pilot video on the Jeju Air crash that touched upon this. With a dual engine failure on a 737, you lose both generators and fall back to the emergency batteries (no RAT). The aircraft remains flyable, but it's extremely challenging - you need an operational APU to make things a lot more manageable: https://youtu.be/9GbmGUk8Y0M?t=2001

[aivisol]

If I remember correctly it came very useful during US Airways Flight 1549 accident?