As previously mentioned the aircraft are cooled on the ground using either pre conditioned air (PCA) which will come in the form of a mobile cart or a system connected to the air bridge. The other option is the aircraft based APU.

I don’t know anything about temperature control of PCA but I’d assume the ground agents can set it, those of us on the aircraft have no control over it. If the APU is used then we can control the temperature (effectivity of the APU does vary by aircraft and environmental conditions). Which one gets used basically comes down to what the contract for ground handling/local airport regulations state. There are various airports where it’s deemed to be more economical to use the APU or that the PCA has been noted as not suitable for use...maybe unsurprisingly that’s the case at T5! The PCA units that were installed at T5 are useless and last summer a decision was made to dispense with them and use the APU for all ground cooling. This is meant to be an interim measure while the state of the PCA units is looked into. There are a couple of mobile PCA carts scattered around the apron which are meant to be a trial and in the case of a broken APU I’d like to think that one of these would be connected...