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I've noticed on some commercial flights (most recently today in an A319) that as the aircraft descends in preparation for landing, the air in the cabin gets hotter. I also noticed that this seems to be caused by the air conditioning being switched off entirely or to a lower flow rate.

Is there a reason for the cabin/flight crew to switch the air conditioning lower/off for landing?

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  • $\begingroup$ See also aviation.stackexchange.com/questions/22285/… $\endgroup$
    – MJeffryes
    Commented Oct 25, 2017 at 17:07
  • $\begingroup$ @MJeffryes Also possible duplicate: aviation.stackexchange.com/q/8408/65 $\endgroup$
    – Farhan
    Commented Oct 25, 2017 at 19:00
  • $\begingroup$ @Farhan That question is about takeoff. I'd expect the answers could be similar, but it is definitely a different question. $\endgroup$
    – MJeffryes
    Commented Oct 25, 2017 at 19:02
  • $\begingroup$ It could be they are configuring for max thrust in case of a go around. It might also be the a/c packs are running less efficiently due to engines being at flight idle during descent. $\endgroup$
    – TomMcW
    Commented Oct 25, 2017 at 19:32
  • $\begingroup$ The cabin must be re-pressurized when descending, pressurization is done with the packs, so packs should be ON, at least one. In addition, temperature would rise if packs were OFF (due to passengers heat and outside temperature increasing too). $\endgroup$
    – mins
    Commented Oct 25, 2017 at 19:54

2 Answers 2

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Systems vary.

Transport aircraft and larger biz aircraft have airpacks handling the AC and at flight idle power settings there is less bleed air, and pressurization is a priority. The packs simply do not have full power when in a flight idle descent on many aircraft.

The aircraft I routinely fly have separate radial compressor driven refrigerant systems, and have large heat loads from onboard electronics and laser systems. They have to remain cool, and normally both AC systems are left on until just prior to engine shutoff after taxi. If it is a hot environment, we use the APU and keep the AC on after engine shutdown.

Once again, the situation is different in different aircraft, and is dependent upon the cooling system, the power plants, the installations and the design and operational goals.

Addendum #1: Some aircraft APUs can provide bleed air for cabin pressurization, but this is not universal. Also, some aircraft, such as the 787 do not use engine bleed air for pressurization, and rather use electrical compressors. The flow of pressurized air is rather constant in flight.

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  • $\begingroup$ Do you know if the latest incarnations of commercial aircraft (A380, B787, etc.) have a way of driving the AC and maintaining cabin pressure without engine thrust? $\endgroup$
    – Sean
    Commented Oct 26, 2017 at 9:08
  • $\begingroup$ B787 is bleed-less, so no air from the engine is used for AC. All others, especially the B7478 (nearly same engines, GEnx-series from the B787) and A350 (the GEnx is an option there, but Airbus probably decided not to invest so much effort to redesign everything). Of course during flight the default energy-source are the engines, as long as the APU is not used. $\endgroup$
    – Peter
    Commented Oct 26, 2017 at 13:59
  • $\begingroup$ My answer is purely speculation, and someone with knowledge should chime in, but my guess is that the APU can power AC, but not pressurization. However, I do know that the 787 uses electrically powered compressors, so I would therefore expect (another assumption) that pressurization is possible with the APU running. So after the fans quit, the descent may be with a pressurized cabin (and electricity to power the control surfaces). $\endgroup$
    – mongo
    Commented Oct 26, 2017 at 14:05
  • $\begingroup$ @Peter we were typing at the same time, but it appears that the 787 relies on electrical power for compressors to pressurize and also to run refrigerant based environmental control. I wonder how cabin heat is managed...through electrical resistive heat? $\endgroup$
    – mongo
    Commented Oct 26, 2017 at 14:09
  • $\begingroup$ There's a misunderstanding here. A jet pressurized aircraft does not have a separate pressurization and air conditioning system. The cabin pressure controller only controls the outflow valve. Basically, the cabin is pressurized not by letting more air in, but preventing air from leaving. $\endgroup$
    – user71659
    Commented Oct 26, 2017 at 16:00
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When the aircraft descends for landing, the engines are running at near-idle thrust. So there is not much Bleed Air available to be sent through the packs into the Cabin.

So the effect in the Cabin can be similar to "they switched the air-con off".

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