Drift Down is a maximum thrust/minimum rate descent necessitated by an engine failure in a multi-engine aircraft in the latter stages of climb or during cruise when an aircraft cannot maintain its current altitude and terrain clearance or other factors are critical.
читать дальшеThe optimum cruising 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, an aircraft’s optimum cruising altitude will exceed the One Engine Inoperative (OEI) service ceiling. An engine failure occurring above OEI service ceiling will therefore necessitate a descent and in most cases a Drift Down procedure will be followed.
The Drift Down procedure entails setting maximum continuous power/thrust on the operating engine(s) whilst countering any adverse yaw with rudder, and then trimming and disconnecting the autothrottle(AP)/autothrust(AT) system where applicable.
(Note that on some aircraft disconnecting AT may not be required and may actually make the desired profile more difficult to achieve; as always, it is important to know, understand and carry out the manufacturers/ operators approved procedures.)
The appropriate drills and/or checklists for the failure are completed when time and capacity allow. At the appropriate speed, a descent to the Drift Down altitude is initiated while maintaining maximum continuous power/thrust.
By definition, the OEI Service Ceiling is:
the altitude to which, following the failure of an engine above the one engine inoperative service ceiling, an aeroplane will descend to and maintain, while using maximum available power/thrust on the operating engine and maintaining the planned OEI 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 cruising altitude for the conditions. In this procedure, maximum continuous thrust is set and (except in aircraft where AP & AT will maintain the required level of thrust and descent profile) the autothrottle disconnected. The speed target is adjusted to the best engine out 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 speed for the current altitude and the descent is continued until Drift Down altitude is reached. Cruise will then continue at best 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, yaw compensation and disengagement of autothrottle (where appropriate) but the descent is started sooner while at a higher speed, maintaining this speed during the drift down profile. By definition, a higher speed will result in a lower engine inoperative cruising altitude. This strategy may be used to meet ETOPS criteria on certain routes. If so, this should be clearly annotated by the Operator in either their SOPs and/or the flight plan given to the pilots.quated from
@темы:
умная мысля,
крылья