Ask the Captain: Can I trust the oxygen masks?

Question: How often are passenger air masks tested, and how do you know if air is really flowing?

-- Submitted by reader Terry Miller, Idaho Falls, Idaho

A: Most modern passenger oxygen systems use chemical oxygen generators. When the mask reaches its fully extended position, it releases a spring that strikes a cap and initiates the chemical production of oxygen. This is a highly reliable system.

During heavy maintenance checks, approximately once a year depending on airline and airplane, the generators are inspected. There is also a limit on their total usage, and they are removed and replaced on a predetermined schedule.

Q: If oxygen masks are deployed and you're traveling with an infant, what is the protocol? Are you supposed to "share your air" with the infant? Also curious about the same topic when you are traveling with an animal in the cabin.

-- Ashley Covey, Atlanta

A: Most airplanes have an extra mask in each row for an infant, or a passenger or flight attendant walking up the aisle. As mentioned in the safety briefing, put your mask on first before attempting to help others, including an infant.

The additional mask could be used for an animal if another person was not using it.

If a decompression occurs the pilots will descend very rapidly to an altitude where there is sufficient oxygen. Therefore the need for supplemental oxygen is brief.

Q: I have seen movies where the cabin loses pressure and masks pop down. What would cause this situation, and has it ever happened on one of your flights?

-- Dave, Detroit

A: There are situations where the cabin depressurizes and the emergency oxygen system deploys, but they're rare. The causes for a depressurization are varied -- the failure of an outflow valve, a failure in the skin of the airplane (rare, but it has happened), or loss of pressurized air. I have only had a depressurization once. It was in a corporate airplane in the 1970s, and we made an emergency descent as we were trained to do.

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