The actual takeoff weight of the aircraft is often lower than the maximum regulatory takeoff weight. In this case, it may be possible to takeoff at a thrust less than the maximum takeoff thrust. This allows to increase the engine life, improve the engine reliability and reduce the maintenance costs.
Two categories of takeoff at reduced thrust exist:
- The use of flexible temperature concept referred to as "flexible takeoff"
- The use of a fixed derated thrust level referred to as "derated takeoff"
Flexible Takeoff Definition
When the actual takeoff weight is lower than the maximum performance limited takeoff weight, the aircraft may comply with the regulatory requirements with a reduced thrust, called "flexible takeoff thrust".
This takeoff operation is the "FLEX takeoff".
Flexible Takeoff Principle
The FLEX takeoff principle is based on the change in maximum available thrust with OAT.
The maximum performance limited takeoff weight depends on the maximum available takeoff thrust, therefore it is possible to determine a temperature at which the actual takeoff weight would be limited by performance.
This temperature is referred to as "TFLEX (Flex Temperature)".
Flexible Takeoff Limitations
Takeoff at reduced thrust, so-called as "FLEX takeoff", is allowed only if the airplane meets all performance requirements at the takeoff weight, with the operating engines at the thrust available for the flexible temperature (TFLEX).
TFLEX cannot be:
- Higher than TMAXFLEX
- Lower than the flat rating temperature (TREF)
- Lower than the actual OAT
FLEX takeoff is not permitted on contaminated runways.
Some items listed in the MEL and CDL do not permit a flexible takeoff.
The operator should check the maximum thrust (TOGA) by:
- Performing full-rated takeoffs at regular intervals, in order to detect a reduced EGT margin, or
- Maintaining an adequate engine monitoring program, in order to follow-up on the engine parameters
Derated Takeoff Priniciple
The derated takeoff enables to improve the takeoff performance if the TOW is limited by VMCG.
VMCG limitation usually occurs on short and/or contaminated runways. The principle is to impose a lower engine rating to benefit from lower minimum control speeds.
In the case of derated takeoff, the minimum control speeds (VMCG and VMCA) are decreased because:
- The derated thrust is lower than the maximum takeoff thrust
- The effect of temperature on maximum available thrust is taken into account
VMCG only has an impact on the ASD that includes an acceleration and a deceleration phase.
The decrease in VMCG allows to use lower V1 values in the speed optimization, which shortens both the acceleration and the deceleration distances.
The weight decrement linked to the derated thrust is largely compensated by the benefit of shorter ASD. As a consequence, if takeoff performance is limited by the VMCG limited weight, an overall improvement can be achieved by derating.
The penalizing effect on climb gradient of the decrease in thrust outweighs the advantage of a lower VMCA.
Therefore derated takeoff would not improve the takeoff weight if VMCA limited.
It is not permitted to combine a derated takeoff and a FLEX takeoff.
When a derated takeoff is performed, the selection of full takeoff thrust, by setting thrust levers to TOGA, is not permitted below the speeds specified in the "Engine Failure After V1 - Continued Takeoff Operating Technique". The only exception is the Recovery Technique at takeoff after the penetration of unforecasted windshear.