I know airlines where they switch off the airconditioning system when they enter the runway, after take off they switch it back on. They do that to safe the system.
That means that from the moment we line up there is no fresh air flowing into the cabin and air is just recirculated. But all the passengers are using the oxigen and are breathing out the co2 in a small space...
Also during taxi they use only one airco system instead of two, but during flight we use the full airco system.
Also they tend not to use the APU on the ground when the engines are switched off until just before push back. Therefore during boarding you may find there is not that much fresh air in the cabin. Something that I personaly find very unconfortable when I travel as a passenger.
As far as I know this is all common practice in many airlines.
Last edited by bobwi; 8th Aug 2010 at 15:04.