-- Question: Captain, could you explain the following:
1) Is there a standard altitude requirement for retracting and extending the landing gear during approach and landing, or is it primarily dependent on a certain DME (distance measuring equipment)?
2) When you set the altimeter at the airport you are leaving, does the altimeter react to the varying altidudes automatically without correction to that setting during the trip until you get to the next airport prior to landing?
— submitted by reader dprost
Answer: Landing gear extension and retraction are usually part of the standard operating profiles.
On take-off, once there is an indicated positive rate of climb, the flying pilot will call "positive rate gear up" the monitoring pilot will select the landing gear handle to the up position.
For landing, there are more variables. For a landing in reduced visibility requiring flight solely referring to the instruments, the landing gear is usually extended around the final approach fix (around five miles from the runway). This can vary depending on the type of approach being flown. In good visibility when a visual approach can be flown, the extension can be delayed somewhat. During a visual approach from downwind, the gear is often extended when turning on to the base leg.
Gear extension during visual conditions can also depend on if it is needed for additional drag to help slow to landing speed.
As for altimiter settings, before taxiing for departure, pilots adjust their altimeters (barometric) to correct for barometric pressure. This is usually provided by air traffic control and is referred to as the altimeter setting. This provides the elevation above sea level (a very few places set the altimeter to zero (QFE) but we will not talk about them in this answer). Once the flight is airborne, altimeter settings are regularly provided to the pilots to correct for local conditions. Airplanes that fly above the transition altitude (18,000 feet in the U.S.) use a standard altimeter setting of 29.92 (inches of mercury) and the altitudes are then called flight levels. An example would be flight at 35,000 indicated altitude with a 29.92 altimeter setting would be called Flight Level 350. During descent, local altimeter settings are provided and the landing airport provides the final setting for landing.
John Cox is a retired airline captain with U.S. Airways and runs his own aviation safety consulting company, Safety Operating Systems.