PMDG 737 When to Apu When not to APU?

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Here is a million pound question for you RL/hardcore wannabe RL ‘coldanddark’ pilots out there for all aircraft, be it big or small, Asobo or aerosoft…
When do you use the APU and what is the general rule of thumb…

I used to turn it on as the first thing I did when I was on board and skipped the ground power… Not until I watched one of the 737PMDG tutorials, the unnecessary buring of fuel siting at the gate and global warming made me rethink life a wee bit and the virtual ice packs and the skyrocketing fuel prices…

Sitting here at Glasgow airport and waiting for my Ryanair flight to take me to Brussels ‘south’ airport… The ‘other’ Brussels airport 80 km south of Brussles… .. There are alot of planes sitting at gates with the heat exhaust pooring out of the apu units…

Do you keep them running between flights, do you turn them off during turnaround or is it one of those ‘company/airport sop’ things :thinking:?

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APU on prior to engine start, turn off GPU when running, turn APU off when engines started and power on engine generators, turn on APU when taxiing to the gate after landing.

Happy to be corrected if I’m wrong but that’s my understanding of it.

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I use it as follows:

  • Active if there´s no external power (to avoid battery draining)
  • Active before pushback (once disconnected from external power), as you normally need it to start engines
  • Inactive after engines started
  • Active while taxi to parking after landing (specially if you are one of those who turns off one engine to save fuel during taxi).
  • Active anyway short after arrival to parking and until external power is connected if you want to keep air conditioning or anti ice running after engines are switched off for instance and also if you need to operate any hydraulic actuators to open cargo doors. There are several systems which are usually fed or indirectly fed from the APU bleed air and APU generator.
  • Inactive after connecting to external power at the parking

I never use it during flight unless you have any failure, because APU bleed air and APU generator are replaced by the engines ones, so having APU active is useless.

Cheers

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Bit better and more extensive than mine haha.

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I tend to start it on final, so its ready for when i exit the runway.

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Interesting. Based on discussions with pilots from Ryan, Alaska and American, I use the APU when the doors close before pushback until engines are running. Again, after landing, the APU comes on when entering the ramp at a large airport, after leaving runway at smaller airport. Finally, during an ETOPS flight, the APU has to be on during the ETOPS segment for electrical backup only (no bleed is used). An exception is when flying a BBJ into very small airports I may fire up the APU on short final to prepare for the short taxi to the ramp. This applies to the 737/777. The 747 may require additional APU runtime on the ground before pushback to provide adequate cooling not provided by ground conditioned air, but this is normally restricted to 20 min by local authorities.

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There was a specific thread a couple months ago over on the PMDG forums regarding ETOPS usage. Current 737 practice in the US - if the carrier has an FAA-approved APU reliability program - is that APU usage is NOT required during ETOPS segments. But there are caveats and nuances. I think the bottom line is, some carriers still start up the APU once they’re more than 120 - 180 minutes “wet” , while others only start it, run it for a bit to document the reliability stats required for FAA certification, then shut it back down again to save fuel.

The one time I made an ETPS flight in the PMDG 737-700, it was a flight from KOAK to PHNL - I think I ran the APU the whole time just for fun, even though I don’t typically fly with failures enabled.

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Yup, I probably participated in that thread. Not all operators have the FAA approved APU reliability program, I know ASA does and assume UAL with all their island hopping Pacific routes would have. I kept my answer short instead of getting into the exceptions, but you’re right.

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APU is not required for A320 EDTO flights for what it’s worth, it does have a lot of electrical redundancy though.

The rule for APU use is that you need it for air conditioning on the ground, an air supply to start the engines, and as a source of electrical power when there’s no ground power (never just leave it on battery power).

I agree with @geloxo’s policy. Turn it on when you get on the flight deck and off after engine starts. Turn it on again after landing. If the turn around is long or you’re leaving the aircraft, turn the ground power on and the APU off once the passengers are nearly all off (keep them air conditioned while they’re waiting to get off).

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Hmmm, if you take time to read the “introduction” contained in the root folder, you will find some information on the APU in there. If any inflight failures of electrical system, APU ON, even though it will not provide bleed air unless under 10,000 feet.
I usually start out at parking area, not gate, so I normally do not have ground cart until I have power to request one, so I run APU as one of the first things I get going. I have created a check list “hate the way the sim displays it, in weird order”, and it works just fine in the PMDG 737 all versions. It also works mostly in X-Plane 12 stock 737-800, although some systems are just “on” by default.

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Plenty of airports state in their reference charts that APU’s must be shutdown after arriving at the gate after a certain time and must only be started no sooner than let’s say 10 minutes for departure.

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Thanks for the replys guys, been a lot of great info especially regarding the ETOPS flights and having the apu running while between the ETOPS out of range rings.

Regarding a youtube tutorial about the PMDG 737. The guy mentioned quite passionately about the burning of fuel and the environment and turning it off whenever you can…
I thought that more airlines would only use it before engine start up and just use the ground power units during turnarounds…
Waiting for my flight at Glasgoe and seeing basically every aircraft on the ramp with their APUs running… I’m guessing, unless it’s an airport that has noise complaint restrictions in place… most airlines would have their APUs running during turnarounds?

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It varies. I used to work for a company that were hot on shutting the APU down whenever we could in order to save fuel, but then we’d hand it over to the engineers and they’d leave the APU running for hours. Also if you’re on 30 minute turn-arounds, shutting it down after you get ground power and then starting it again prior to engine start can mean it’s shutdown for such a short time it’s not really worth the hassle.