Ask the Captain: What is your favorite aircraft to fly? Where does the emergency oxygen come from?

What was your favorite aircraft to fly? 

– Knoxville, Tennessee

My favorite propeller airplane was the Gulfstream I.

As for jets, it was the Boeing 737, followed very closely by the Airbus A320. In fairness, I spent 15 years flying 737s and loved it. I was fortunate enough to  fly the A320 for six years and love it. So it is a very close call. 

What are chemical oxygen generators, how do they work and what is their purpose on a commercial airline? 

– Vero Beach, Florida

For decades, airliners have used oxygen generators as the primary means to provide passenger oxygen in case of an emergency. This technology is proven to be reliable and effective. 

Oxygen generators use a chemical reaction initiated by a percussion cap. Once started, this exothermic process causes heat (around 500 degrees Fahrenheit), and oxygen is produced. The oxygen generators produce oxygen for around 15 minutes, which is more than enough to descend in the event of a pressurization problem at cruise altitude. 

Before takeoff, I often hear a grinding or sound like motors coming from the baggage area. What is that? 10/23

– Curious in Indiana

Your question is difficult due to the lack of specifics. I think it is possible that you may be on anAirbus, and hearing the hydraulic power transfer unit . It is a backup power source for hydraulic systems when engine pumps are not available. This can happen when taxiing on a single engine to save fuel.

The power transfer unit has been described as sounding like a “barking dog” or a pneumatic tire wrench. 

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