When do I need to keep APU on?

I understand that APU is powering the plane, but I also know that sometimes pilots switch APU (after takeoff or landing). So when do I need to switch APU?


Once the engines are started you do not need the APU on. You can start it on the ground again for when you arrive if you wish.


Do I need to turn it on when parking at the gate before stopping engines?

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Turn on the APU when you spawn. Once your engines are running, turn it off. After I land, I usually switch on the APU and one engine taxi. In IF, you don’t need to turn on the APU for one engine taxi, but for realism I do.


Yes. Turn it on before you turn off your engines so you don’t lose electrical systems.


I’m kind of confused when it running the engines power up the main battery so that when you landed you will not have to restart the APU when you shut down the engines because the engine would have charged the battery Just like it does on a boat

A plane is different from a boat.

When the engines are running, the plane uses the engine’s electric generators to provide electricity to the aircraft. So, before you turn off your engines, turn on your APU. The APU acts as an “engine” when the engines are off. It has a turbine that generates electricity. When the engines are off, use the APU for electrical power.

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Aircraft batteries are only good for less than 30 minutes of run time before they’re depleted if they do not have a source of charge/power. Inflight and on the ground batteries get their charge from the engine driven generators (which are attached to the engines) and/or the APU if operable. Without either of those generators, you would have an electrical failure in 30 mins or less depending on electrical load. 🙂

Planes are way different than boats and cars where if you turn something on, you can leave it running for a couple hours at most (like a AM/FM Radio)


Do you moderators have fuel burn for the APU?

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No. Fuel burn is not implemented into Infinite Flight right now. But IRL, APU’s burn anywhere from 350-500lbs/hr depending on model. To put that in perspective that’s similar to the TBM930 fuel burn at normal cruise at FL300. 🙂


Yeah they burn hard to keep the plane alive

What does turning on the battery in IF do if the apu is required to turn on instruments and lights?

The Master Switch is pretty much still the on and off button, like a real aircraft has a Master Battery switch and such, but it needs a source for the power and that is the APU, many older aircraft just have the Master Switch only since they aren’t new enough to have the APU.

OmG someone should feature request this! That’d be cool.

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That’s true but most of the airplane’s systems run off of AC electrical power because it is such a large area for DC power to work effectively. The main battery only supplies enough DC power for emergency systems and to power the APU’s ECU which will start/control the APU. Even then, to power other systems would require an inverter which would convert the DC into AC power which is not efficient if the engine/APU generators can and should be powering the main systems in normal operation.

AC external power can be supplemented if the APU is deferred or shut down to save fuel until engine start. This is the big black extension cord that is often attached to the airplane from the jetway.

Theres more to it, but, the main battery is essentially supplying power to the APU to allow it to start. The APU is what’s supplying power to the Avionics and other systems. Engines start pneumatically, and once started, act as generators, and take over supplying power to the aircraft, getting rid of the need for the APU. In a nutshell. Lol

You mean the ground power?

Facts mentioned in this post are based in real life, in IF the APU is obviously for providing power when the engines aren’t alive.

Also, the since the engine supplies everything to the plane (thrust, bleed air, electricals). If the APU is on, it will burn fuel, but it can also supply bleed air and pressurize the cabin when the engine is running hot and high. When on ground this can obviously supply power (if no grnd power available) and it can significantly reduce runway takeoff distance because the engine doesn’t need to supply bleed air or provide electricity, it can concentrate itself on providing thrust. The APU takes care of that. The same use can be used for go arounds/ missed approaches. Also, when in cruise, the anti-ice may be activated and that means that bleed air is mostly having to be redirected from the engine, again situations when the APU could be used. In the event of a generator failure, the APU can replace the failed generators and you can hopefully continue the flight or divert. Also, the APU can save engine life as it can reduce workload on the engine therefore reducing peak temperature.

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What happens if you don’t turn the apu on and turn off the engines.

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You’ll say goodbye to all of your screens.


Oh lol. Thanks @Thunderbolt !