Now I am confused by the 2 quite opposite instructions. Is one in IRL and the other in 2024?
But there is absolutely no need for it to be on and running during flight with both engines running - itās just a waste of fuel. All these twin engine jets flying across the Atlantic arenāt doing so at a low altitude (an even greater waste of fuel) with their APUs running. Iām taking operating to mean working as in the opposite of inoperative
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It would also mean that before you cross an ETOPS ocean you are restricted as to altitude, because an APU canāt run above certain altitudes. That also burns more fuel.
Found this in a PMDG thread. Conclusion - it depends on the age of the plane, your company rules, the particular plane as to whether it has the up to date equipment, also the FAA ETOPS rules.
Like one guy says - you are in a SIM - fly how you want unless you are practising failures, it wont kill anyone!
What is interesting is how ETOPS rules have changed with the evolution of the aircraft. Distances are getting better and better as reliability increases.
Fleet differences
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Some aircraft are equipped with newer APUs that are quieter/more efficient (like those on the A350 or 787).
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Airlines may standardize APU procedures based on aircraft type.
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Different aircraft have unique APU configurations and operating procedures:
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Boeing 737 & Airbus A320
APU primarily used for ground power, engine start, and air conditioning before departure. Typically shut down before taxiing. -
Regional Jets (E.g. Embraer E-Jets, CRJ Series)
May use the APU throughout taxiing due to smaller onboard power sources. -
Long-Haul Aircraft (E.g. Boeing 777, Airbus A350, A380)
APU can be used in-flight for redundancy, particularly on ETOPS (Extended-range Twin-engine Operations) routes.
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ETOPS - Enhancing Safety On Long Flights - AeroSavvy this one gives clear guidance
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OK, I accept your comments. Perhaps I should have clarfied that the APU is started at the start of the ETOPS part of the sector & shutdown when out of ETOPS. That could be a matter of minutes or the amount of time it is in the ETOPS part of the flight. Air New Zealand used to run their 737 APUās all the time. The cost of the fuel was less then the wear & tear on cycling the APU on each turnaround.
This topic has been so interesting, and your points were so well made so I thought that I better do some more research to see if I could find a definitive answer.
You do indeed have a valid point.
The answer is quite a bit complex, is aircraft type and operator dependent, and can vary from country to country.
Generally, an A320 doesnāt run the APU in ETOPS as a requirement, as it has a RAT.
The 737 does not have a RAT, and depending on the airline, may indeed be required to run the APU in flight.
The 737 does not have to run the APU in ETOPS if the airline is āAPU on demand qualifiedā.
Some only require it to be run as a confirmation that it will start, some do that on descent to prove for the next flight, while others run it constantly.
I do want to thank you for tweaking my interest, I had no idea this was a quite complex situation with so many different requirements.
Very much appreciate your input!
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Sorry, I could not see who your comment was directed at. All of us or one person. If me, you are welcome, if not no harm in thanking you anyway for your contribution too. 
I also found it very interesting, it always amazes me how when an issue is really examined it can become complex and informative. 
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Definitely an interesting topic. I did not know that the 737 doesnāt have a RAT. Although to be fair, it sits so close to the ground that thereās no way there would be space for one.
Iāve always found it funny the way that Boeing operates much like a car manufacturer. I always assumed that the different models of aircraft would be much the same from type to type and just sized according to what you want/need. And this is how Airbus operates with the A320, A330 and A340 all having a basically identical cockpit and features (wonāt count A220, A350 and A380 in this as they are newer and also broadly similar to each other). But if you look at cars, not only do they increase in size as you go up from their cheapest models to their most expensive, but you get more features too. 737 has no RAT, generator switching is manual, engine start is more manual (no auto-start and manual switching off of PACKS), cruise and landing altitude for the cabin pressurisation is manual, no FBW, etc. And then as you go up to the larger aircraft, all those systems become included and automated. But I also suppose that when the 737 lacks what everything else has, it is the cheap, no frills school bus and everything else is the luxury coach
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This is an interesting topic - I think itās clear that each combination of airframe / engine model / apu model will have to pass a very specific set of tests for ETOPS certification, and therefore have a specific āprogramā when almost every scenario has been spelled out with a checklist.
Some interesting considerations I havenāt seen mentioned are:
- Can the APU be started at X altitude
- Can it be started with or without the aide of another engine
- Can it be started after being cold soaked by temps at altitude
- What is the fuel and oil burn - which must be factored in to ETOPS calculations
I think each of those will have very specific tests and answers based on year and model, with a requirement for periodic testing to continue to meet ETOPS certification. The airline would then have specific practices that try to meet requirements but also limit fuel, oil, and wear expenseā¦
It makes sense that aircraft from decades ago may have had less capability for their APUs to start at high altitudes after being cold soaked, and thus the program would be to start the APU at some point prior to entering ETOPS simply as a precaution. But that more modern aircraft have APUs, engines, and logic that allows systems instead to pass reliability testing, and not need to be started.
APUs are started from battery power (electric starter) where there isnāt ground power available so that covers that one. A330 actually has a dedicated APU battery
Interesting comment about the 737. I watched a video about Boeing being taken over by Mcdonnell Douglas. There were many black and white excerpts of film showing how while it was Boeing the many safety aspects and precautions that were being built in in the early days. Once MD took over they cut these procedures to the bone for shareholder purposes. So a sad demise of a very popular original product. Still, that is IRL! Ours are great to fly 
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I read that starting the APU for the 737, as itās considered an older aircraft, depends on altitudes, some aircraft can start higher than others, company policy, whether it has been updated or not, and whether the company itself decides to do it prior to ETOPS, or prior to landing in preparation for the next flight. I was surprised at how regulated it actually is. Each aircraft is specifically rated for an ETOPS. 737ās donāt have a particularly high chance of flying more than an hour of ETOPS.
Thatās completely untrue, at least for US operators. Check out FR24 or FlightAware sometimes and see how many 737 variants (NGs and MAXs) fly every day between CONUS and Hawaii. Alaska even flies there from Anchorage, or did last time I checked.
The post screenshotted above from the PMDG forums was - if I remember correctly - one I either started or bumped to get more info at some point when I started flying the 737 NGs in the sim from California to Hawaii. My takeaway, from actual 737 pilots and guys in US carrier operations, is that the APUs do not run for the entire flight. As noted by others above, the APU must be operable and certified pursuant to FAA and carrier procedures, but it is not run without a specific failure requiring it.
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I should have qualified my statement being about the older 737s. Nonetheless, ETOPS distance still depends on many factors for 2 engine planes. These are stated in my 2 or 3 links above.
I note your discussions with your friends and accept their personal experiences.
ETOPS remains, for me anyway, an issue that needs to be addressed for each individual aircraft to ensure that I am doing it right because there isnāt a one procedure fits all plane scenarios.
It also raises the question as to why Career mode enables missions with single engine craft from say New Zealand to Australia. Which many of us have flown in a PC12.
I just have to assume this is not modelled in 2024.
737ās regularly cross the Atlantic as well, normally about five to six hours over open water.
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Indeed.
Anyway, FWIW, there are currently three Alaska MAX 9s and an -800 en route to Hawaii from the mainland right now. There is (or was) an Alaska 737 guy posting regularly around here a couple years ago - if I can track him down, Iāll ask him what they do on Hawaii legs.
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I know that ETOPS compliant 737s have an extra battery, presumably as a backup to ensure they can definitely get the APU started in the event of an engine failure but could serve other purposes too
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That is indeed the reason for the extra battery!
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Certain MEL items might require APU to be running as well for the duration of flight. Interesting case I once had was an issue with APU inlet door not closing which was a no dispatch item, but with APU running there was no fault indication so we kept APU running to get home 
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OK, wow, scary i suppose 
. At least you guys made it. Reading about APUs has filled in an ton of knowledge gaps for me.