What is the difference between "apu on" and "apu start"

what is the diffrence beween them, which one do you do first and when do you turn them off?

APU Start is used to start your APU at the start of a flight, after a few seconds it will automatically switch to On, you can turn it off when you have engine power.

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thank you.

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so apu starts the engines then once they are on you can turn them off?

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To start the APU, you have to select start, then when it’s up & running it changes to on

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Pretty much: APU start > Engine Start > Engine is running > APU off.

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U can just turn off the engines at that point. That apu provides that “extra kick” for the engines to start and basic minimal power to the aircraft.

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No, APU just provides the power, hydraulic pressure and air flow needed to start the engines. Without APU you would need a ground unit (which we don’t have in IF).

After you’ve started the engines you don’t need the APU as power and pressure is generated via systems connected to the engines.

Before shutting the engines down you should once again start APU (do this when taxing to parking and it will be up and running by the time you want to switch off the engine), so that you still have power (illuminated screens, lights and so on) after engine shutdown.

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So I’ve got a question for the devs or beta testers if they ever find this:

Is the APU solely an AC electrical power source or is it also supplying bleed air needed to start the engines?

To give background, I wanted to try something one day. I started the Number 1 engine like normal on the A320 and B737 families (MASTER/APU/ENG START), then shut down the APU like we often do to save fuel. I know the APU being on doesn’t increase fuel flow like it would irl, and I tried to start the number 2 engine WITHOUT cross-bleeding and was able to start just fine. Normally, the operating engine would need to be run up to provide sufficient bleed air to start the other engine(s) in case of an APU deferral but was able to start just fine at idle power.

TL;DR: I’m curious to see what extent of realism was built into designing the APU for this update and if it will progress in future updates as more systems start being introduced.

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