APU, does anyone fly with this on?

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1

I am wondering whether in MSFS 24/20 this has been modelled or not? Particularly where folk are practising failures in Airliners/Jets. I did not know this was a facility one could leave on during a flight (IRL) below 35000ft in 2 engine jets.

2

That altitude varies considerably from type to type of aircraft depending on the APU unit installed.
Some aircraft do not allow the use of an APU at all in flight.

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3

Would you know if this is a modelled feature in msfs 2024 in those certain aircraft?

4

Every aircraft that has one IRL has one in the sim.
I have not noted any that don’t.

5

I meant with the ability to cruise with the apu on in 2 engine jets

6

I believe in most jets in MSFS you’ll get an EICAS warning if you leave the APU on above FL180, but I haven’t tracked it specifically. I know you get an alert in the Longitude and 787 if I’m not mistaken.

According to the MSFS Longitude manual

  • Max Start Altitude on Ground = 13,500 Ft
  • Max Start Altitude (2 gens) = 15,000 Ft
  • Max Start Altitude (no gens, battery only) = FL310
  • Max Operating Altitude = FL350

787 Dreamliner (in real life, not sure about MSFS
)

  • APU (Pratt & Whitney APS5000) can start and operate up to the aircraft maximum operating altitude of FL431

Different planes will have different procedures, since the APU is “auxiliary”, it should be off unless needed, to save fuel. But if needed in emergency, most aircraft will have specific limits and procedures.

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7

I’m asking because allegedly simbrief allow you to plan a fuel load to fly with this on to prevent a problem in the case of engine failure. So says AI anyway. I just needed to know if this is a real feature or if it is AI gone barmy?

8

Most twins are not allowed to just cruise with them on IRL, emergency only.
It’s never on just in case.
As to how well the altitude restrictions for that are implemented is another matter.
I have received warning alarms on any I have used when I forgot to turn it off and the altitude exceeded the limit, but I always follow the alarm.
IRL, that can lead to fuel/air mixture problems which can be extremely serious.

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9

OK, thanks for the advice, I will not try this then! Beware A.I.!

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10

Only example I can think of for flying with the APU on is Innsbruck. When in the valley, the APU must be on to in case of an engine failure during takeoff/climbout or approach, as the additional bleed air allows a bit more power from the engines

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11

So it appears it is just a function of restarting at a safe altitude in cases of failure rather than a function of safety during cruise - (as suggested by AI). :+1:

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12

Yes, I read that in the AI advice too. That is a crucial point actually - safety in take off in risky areas.

It, APU, basically does supply power to other functions like A/C to allow the aircraft to use all of it’s engine power for T/O, or landing. (rather than sharing engine power for other functions)

Thanks everyone.

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15

From the Boeing Flight crew Training Manual for 737NG chapter 4-13

Before entering the ETOPS phase of flight, the APU must be operating

16

Did it say what ETOPS time rating it has? This is going to be tricky in Sim because the above pilots are saying the sim alarms above certain heights. Must an altitude be descended to in that case or - just assume this is not modelled in the Sim?

I wonder if this is modelled in any 3rd party 737 Max?

17

Operating - as in working rather than running. You can’t go ETOPS with an INOP APU

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18

I think he means you have to get it working before ETOPS limit begins. In which case you would have to reduce altitude to an acceptable height to start the APU.

Judging by everyone ‘s comments about warnings at altitude if APU is left on, I am doubting this feature is actually modelled.
Does anyone know?

19

You wouldn’t fly through the ETOPS zone with the APU running. You would just need to have it working as part of the minimum equipment list prior to doing a flight with ETOPS requirements. If an engine failed, you would reduce your altitude to below the single engine ceiling and then start the APU to improve performance of the remaining engine by reducing the demand on it from the PACKS

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20

As mentioned by Azlog, the APU has to be functional, not running.
If you do not have an APU that works, you cannot fly ETOPS.

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21

I understand the distinction now, thanks :blush:

22

Please reread my quote, especially the last word.

Before entering the ETOPS phase of flight, the APU must be operating

i.e on and running as different to operational

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