Fired up over air conditioning

Question: I was on a flight from Houston to LAX that was delayed on the tarmac for what the pilot called a “long line of planes ahead of us,” then for weather, about an hour in all. In the meantime it got very hot and stuffy in the cabin because the pilot turned off the engines while we waited to “conserve fuel.” Was it really necessary to conserve fuel or was that a cost-saving measure? – Dan, Pasadena, California Answer: When an extended delay is expected, it is normal to shut down the engines and use the Auxiliary Power Unit (APU) for electricity and cooling. This saves fuel, reduces emissions and saves money. If you burn extra fuel waiting for takeoff clearance, it may be necessary to return to the gate for refueling. Flights are planned with the fuel required for the flight, diversion to an alternate airport, plus a reserve. Pilots conserve fuel where they can.

Usually an APU can provide adequate air for cooling. It sounds like on your airplane it may not have produced quite enough to keep everyone comfortable.

Q: Why do the cabins on passenger jets get hot when flights are delayed on the ground after leaving the gate? Does the air conditioning system rely on power or airflow levels only available in flight?

– John, West Lafayette, Indiana A: When the engines are at idle, they do not produce much compressed air used for cooling. This can result in a warm cabin. The issue is the amount of air available for cooling when not in flight, where the flow is robust. Some airplanes can supplement the cooling air with the APU, which can help.

Pilots do all we can to keep the interior cool, as we are in a flight deck that is full of glass and often the warmest part of the airplane.

John Cox is a retired airline captain with US Airways and runs his own aviation safety consulting company, Safety Operating Systems.