getting to grips with aircraft performance 如何掌握飞机性能 1(41)

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Important: When a derated takeoff is carried out, TOGA thrust must never be selected until the aircraft is airborne and above the minimum flap retraction speed (“F” speed). The reason for this is that performance calculations are made for minimum control speeds, different from the ones of TOGA.

Note that in the Airbus philosophy, a flexible takeoff can not be cumulated with a derated takeoff.
1 Standard after 1998, option before 1998 2 Option soon available on A320 family 3 For A340-500/-600, two supplementary derate levels: 32% and 40%

D. EN ROUTE LIMITATIONS
1. EN ROUTE FAILURE CASES
In flight, engine or pressurization failures are potential problems, which must be carefully studied before operating a new route. Their occurrence seriously impact on flight altitudes and, therefore, become very constraining over mountainous areas.
In case of an engine failure during flight, the remaining thrust is no longer sufficient to balance the drag force and to maintain an adequate cruise speed. The thrust necessary to fly at the initial altitude suddenly becomes greater than the available thrust delivered by the engines pushed at their Maximum Continuous Thrust (MCT) rating. The only solution is to then descend to a more appropriate flight altitude, where the available thrust can equal the required thrust, thus allowing the aircraft to level off.
In case of an in-flight cabin pressurization loss, descent is also necessary. It is not dictated by a performance constraint, but by the oxygen system constraint. Indeed, at the initial cruise altitude, the rate of oxygen in the air is insufficient to allow crewmembers and passengers to breathe normally. This is why the installation of an oxygen system is required. As the necessary oxygen quantity must be quite significant to supply the entire cabin, its flow rate is limited to a maximum duration. So, a new flight altitude, where oxygen is no longer required must be reached, before a certain time limit.
The descent cannot be always operated in the same conditions, since, aircraft are sometimes over-flying mountainous areas. This is why, in these particular cases, a route study is necessary to evaluate whether or not an acceptable escape procedure is possible when a failure occurs at the worst moment during flight. If it is possible, it must be clearly defined and indicated to the pilots. If it is not possible, a new route must be found.
Any route study must be done in accordance with airworthiness requirements, detailed in the following sections.
2. ENGINE FAILURE(S)
2.1. General Definitions
2.1.1. Drift Down procedure
In case of an engine failure over a mountainous area during the climb or cruise phase, the Obstacle Strategy or Drift Down Strategy (Figure D1) should be applied. This procedure consists in:
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Selecting Maximum Continuous Thrust (MCT) on the remaining engine(s).

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Decelerating to green dot speed.

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Climbing or descending at green dot speed until reaching the drift down ceiling1.
　
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