Ask the Captain: Learning about landing gear

Question: On a recent flight from Minneapolis to Las Vegas, the landing gear made a horrible throbbing noise when extended. I have flown for years and never heard that sound. Should I have been concerned?

-- Submitted by reader Donald, Las Vegas

Answer: It could be a slightly out of balance tire. When the weight of the airplane is removed from a tire that is slightly out of balance, it can vibrate. This usually lasts only a few seconds until it stops rotating. It is normal and nothing to be concerned about.

Q: I've noticed that prior to landing, there is a rumbling noise for a short time, and then it goes away and the noise level returns to normal. What is that?

-- Sam, Los Angeles

A: What you are probably hearing is the landing gear being extended. When the doors covering the landing gear open, it causes a rumbling noise. Once the gear is fully extended, some of those doors close, resulting in a lower noise level.

Q: What procedures are followed if the landing gear fails to lower? Can a 777 do a sharp turn at very low altitude to force the gear down?

-- Alex, Qatar

A: It varies from airplane to airplane. Most airplanes use hydraulic power to extend the landing gear normally, and a gravity backup for the alternate method. There are usually only two ways to extend the gear — if both fail, then a landing is made with the gear retracted (a very, very rare situation).

All airplanes, including a Boeing 777, can make sharp turns at low altitudes.

Q: I have been on flights out of Denver where the pilot announced they were going to lower and raise the landing gear just to make sure any snow/ice that might have gotten in there will not cause problems when they land.

-- Ken, Denver

A: This is done on some airplanes by some operators, but is not widely done.

Q: Watching the Airbus A350's first test flight, I noticed they did not retract their landing gear during the full flight. Is this normal, or did they just forget?

-- Jon Masterson, Ventura, Calif.

A: An initial test flight has many tests being done to ensure the new airplane is performing as expected. Stability, handling and system evaluations are done prior to retracting the landing gear.

Captain Chandler did not forget to retract the gear. He, and the entire crew, were following a carefully written test plan.

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