Engines help send fresh air to plane cabin

Question: What does it take to get fresh air delivered into the cabin during flight?

– Jim, Thailand

Answer: In most airliners, air is compressed by the engines, cooled by the air conditioning system and then sent to the cabin. Fresh air can also be routed to the cabin from a small jet engine in the back of the airplane, known as an auxiliary power unit, or via a hose when at the gate.

Question: When your flight is delayed and you have to wait on the runway for hours, why is the air conditioning not used? I was in this predicament at LAX and it was awful. Very hot inside the aircraft.

– Doug, Panglao, Philippines Answer: When waiting for departure it is common to shut down the engines to save fuel. The auxiliary power unit supplies electricity and air for air conditioning. Some APUs do not provide as much air, causing it to be hot and stuffy onboard.

The air conditioning system is being used but is not as effective as when engine air is used. The flight attendants can let the pilots know of any issues in the cabin, so talk to them.

Question: Why can’t airplane vents go into “recirculation” mode like an automobile? It would be nice to avoid the smell of exhaust fumes in the cabin while on the ground. – Ben Kaya, Honolulu

Answer: Airliners recirculate a percentage (it varies according to airplane type) of the air to improve fuel efficiency but it is necessary to refresh the air in the cabin every few minutes to prevent carbon dioxide buildup. In a typical airliner the air is cycled every three to four minutes.