Ask the Captain: How is fresh air delivered to the airplane 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.

In most airplanes, air is compressed by the engines, cooled by the air conditioning system and then sent to the cabin. (Photo: Getty Images/iStockphoto)

Question: Is oxygen added to circulated air on long flights?

– John, Helena, Montana

Answer: No. The cabin is pressurized between 6,000 and 8,000 feet on long flights. Adding supplemental oxygen is not necessary, because the percentage of oxygen is the same as being on the ground at those altitudes.

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.

Question: While on a flight in the Northeast corridor, I noticed smoke coming out of a few air registers. What caused it?

– No name given

Answer: Often condensation from the air conditioning system will look like smoke. It is similar to opening the freezer on a very humid day and seeing the wisps of condensation. Passengers sometimes confuse this with smoke.

I suspect that is the cause of your observation.

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


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