Executing a proper aircraft descent
Q: How do pilots know when to descend to land on the assigned runway at the correct speed?
— Byron Pope, Detroit
A: Pilots plan the descent based on the wind and air traffic flow. Working in partnership with air traffic control, the descent is executed allowing adequate distance to descend and line up with the proper runway.
In addition to the radar guidance from air traffic control, pilots program navigation computers and radios to ensure proper lateral and vertical paths.
Q: What do pilots do to descend? Decelerate, use flaps, or point the nose down?
— Alan Greener, Coconut Creek, Fla.
A: Decreasing the power and decreasing the pitch (lowering the nose) are usually the two things done to descend. If a quick descent is needed, the power is reduced to idle, the nose is lowered and flight spoilers (panels on the top of the wing) are extended.
Q: How do pilots know what vertical speed to maintain for the descent, when to start reducing (or increasing ) power,
and when to lower the flaps?
— Michael, California
A: Pilots can calculate the speed of the airplane (miles per minute), the distance to go and determine the amount of time required (distance/speed).
Knowing the amount of altitude to descend and the vertical speed of the descent, the proper time to begin the descent can be determined. Most modern airplanes do these calculations for the pilot and provide him/her with a top of descent point in the flight management computer.
As the airplane approaches the area around the airport, pilots slow the airplane to match surrounding traffic and to configure for landing. The profile varies from airport to airport, but generally, around 5 miles from the runway, the airplane is at landing speed, with slats/flaps in the landing position, vertical descent speed less than 1,000 feet per minute and the engines powered up properly.
Pilot experience goes a long way in getting all of these parameters done efficiently and in a timely manner.