Drift Down Procedures

What is Drift Down?

Drift down is a controlled descent performed after an engine failure at cruise altitude, typically in multi-engine jets, when the aircraft cannot maintain its current flight level on the remaining engine(s). The goal is to descend to an altitude where the available thrust can balance drag, allowing level flight to resume.

Initiating the Procedure

• Set maximum continuous thrust (MCT) on the operative engine(s).
• Maintain current altitude initially while completing engine failure drills.
• Allow airspeed to decrease to the optimum drift down speed (VMD for jets, often called 'green dot' speed).

Optimum Drift Down Speed

• VMD (minimum drag speed) is used for jets, providing the best glide angle and obstacle clearance.
• For propeller aircraft, 1.1 times VS (stall speed) is typical.
• This speed is maintained throughout the descent.

Obstacle Clearance and Level-Off

• During drift down, regulations require 2000 ft clearance above obstacles within 5 NM of track.
• The aircraft will eventually stabilize at a lower altitude where thrust equals drag (the one-engine-out net level-off altitude).
• If necessary, fuel jettisoning can be started at the beginning of the drift down to reduce weight and improve performance, but minimum fuel reserves must always be maintained.

Drift Down Calculations

• Drift down graphs or tables are used to determine fuel used, time, and distance covered during the descent.
• These tools also help establish the maximum permissible mass at which the aircraft can clear terrain by the required margin.

Influence of Deceleration

• As the aircraft slows to VMD, the descent profile becomes shallower, optimizing obstacle clearance and fuel efficiency.
• Weight loss from fuel burn and improved thrust at lower altitudes further affect the descent angle and level-off altitude.