I know that commercial aircraft continuously freshen the cabin air when aloft by compressing and cooling the air.
Is that true when the aircraft is on the ground, at the gate loading or taxiing?
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asked Oct 22, 2017 at 8:47
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2$\begingroup$ Related: 1: What is this tube connected to a 757 for?, 2: What is this device they bring on board from the jet bridge when we park?, 3: What is the APU actually used for? $\endgroup$– minsCommented Oct 22, 2017 at 18:45
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2 Answers
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The aircraft when on ground and starts to board passengers is switched to ground power or auxiliary power which runs the air conditioning system. When disconnected, the engines supplies the power and bleed air for the cabin pressurization.
"On the ground, the airplane is unpressurized and the outflow valve is wide open. During preflight, the pilot sets the cruise altitude on a cabin pressure controller. As soon as the weight is off the main wheels at takeoff, the outflow valve begins to close and the cabin starts to pressurize."
Source - https://www.airspacemag.com/flight-today/how-things-work-cabin-pressure-2870604/
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$\begingroup$ The air conditioning system requires compressed air to operate (called bleed air), the ground power unit alone cannot provide a/c $\endgroup$– Radu094Commented Oct 22, 2017 at 11:51
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$\begingroup$ when on ground the pressurization does not work only temperature control. these two sub-system are part of enviornment control system in aircraft. Once the pilot sets the pessurization on only after the engines are started and the plane usually is not heavily pressurised upto 10k feet... $\endgroup$ Commented Jan 20, 2018 at 11:50
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$\begingroup$ with packs off there is no new air entering the cabin and thus temperature control is not possible (except perhaps the 787 which has no engine bleed and is all electrical allegedly). $\endgroup$– Radu094Commented Jan 20, 2018 at 17:24
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If the engines are off, ie. durring boarding or while waiting at the gate, the APU can supply the necessary bleed air and electricity to operate the PACKs that supply air conditioning to the cabin. It is up to the crew to decide wether to use it or not, depending on ambient conditions: on a nice 21deg sunny day vs. a cold and snowing -4 deg.
Now that APU running in the tail, for each aircraft parked on the stand, sometimes for dozens of minutes can add up to quite a lot of noise on th apron, so most airports will have regulations as to how much time you can use the APU. Some will even cordially (for a fee) provide air conditioning through a specially mounted tube that connectes to the airplane systems, allowing cool air to enter the cabin while the APU is switched off
