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Drift Down Procedure
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Drift Down is a maximum thrust/minimum rate descent initiated by the flight crew following an engine failure in the latter stages of climb or during cruise.
The optimum crusing altitude for an aircraft with all engines operating normally is primarily dependent upon aircraft mass and the temperature deviation from ISA. In almost all cases, the optimum crusing altitude will exceed the one engine inoperative absolute ceiling of the aircraft concerned. An engine failure occuring at, or in the latter stages of climb to, the optimum cruising altitude will therefore necessitate a descent. In most cases, a Drift Down procedure will be utilized.
The Drift Down procedure entails setting maximum continuous power/thrust on the operating engine(s), disconnecting the autothrottle/autothrust system where applicable, countering any adverse yaw using rudder and rudder trim and completing the appropriate drills and/or checklists for the failure. At the appropriate speed, a descent to the Drift Down altitude is initiated while maintaining maximum continuous power/thrust.
By definition, the maximum Drift Down Altitude is:
- the altitude to which, following the failure of an engine above the one engine inoperative absolute ceiling, an aeroplane will descend to and maintain, while using maximum available power/thrust on the operating engine and maintaining the one engine inoperative best rate of climb speed.
Several possible speed strategies are associated with drift down. These include fixed speed and obstacle clearance strategies.
- Obstacle clearance strategy allows the aircraft to maintain cruising altitude the longest, provides the least possible rate of descent and will result in the highest possible engine out crusing altitude for the conditions. In this procedure, maximum continuous thrust is set and the autothrottle disconnected. The speed target is adjusted to the best engine out climb speed and altitude is maintained while the speed slowly decays from the all engine cruising speed. When the target speed is achieved, a descent at maximum continuous power/thrust and the target airspeed is initiated. During the descent, speed is adjusted to maintain the best climb speed for the current altitude and the descent is continued until Drift Down altitude is reached. Cruise will then continue at best climb speed and maximum continuous thrust and, if required, the aircraft will climb as it becomes lighter. If obstacle clearance is not a factor, the descent can be continued and the power/thrust decreased or speed increased as appropriate.
- Fixed speed strategy involves the same immediate actions of maximum continuous power/thrust, disengagement of autothrottle and yaw compensation but the descent is started sooner while at a higher speed. This speed is maintained during the drift down profile. By definition, a higher speed results in a lower engine inoperative cruising altitude.
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