De-rated Thrust vs Flex Thrust (Assumed Temperature) Take Off

De-rated thrust and Flex Thrust (Assumed Temperature) Take Off are two methods of thrust reduction for take-off. Takeoff with reduced thrust helps in increasing engine life and reliability while reducing the maintenance and operating costs.

Since aircraft does not takeoff with maximum takeoff weight it is possible to reduce the takeoff thrust and yet meet up performance requirements. As stated above two methods of thrust reduction can be independently or collectively used depending on aircraft type and system availability.

For example:

Boeing 737 offers both de-rated take off and assumed temperature takeoff

Airbus A320 offers option of only assumed temperature (flex temperature) takeoff

Methods of Reduced Thrust Takeoff:

  • De-rated Thrust Takeoff
  • Flex Takeoff (Flexible Temperature or Assumed Temperature)

De-rated Thrust Takeoff

It is takeoff with thrust less than maximum takeoff thrust for which separate and independent takeoff limitations and performance data exist. For de-rated takeoff the limitations, procedures and performance data must be included in Aircraft Flight Manual.

Note: Difference between De-rated Thrust Takeoff and Flex Thrust Takeoff is given in the footer of this content.

Unlike flex takeoff it is assumed in de-rated thrust that in case of engine failure, the maximum value of thrust available on live engine is the selected value of de-rated thrust. Which is not so in Flex Takeoff.

Flex Takeoff (Flexible Temperature or Assumed Temperature)

Flex takeoff is also called as assumed temperature take off. Thrust produced by engine is modified with respect to temperature. Higher temperature is entered in FMS so that it considers that value of temperature and produce lesser thrust accordingly. This value of entered temperature is called flex temperature or assumed temperature.

  • as the entered temperature value is increased, FMS forces engine to produce lesser thrust
  • maximum reduction of 25-40% (depending on aircraft) is allowed for thrust
  • performance speeds like Vmcg & Vmca doesn’t get affected
  • full thrust can be recovered any time

Difference between de-rated takeoff and flex takeoff

De-reated thrust level corresponds to basic maximum thrust reduction by given percentage. This new value of maximum thrust helps in reducing Vmcg and Vmca. This reduction in Vmcg and Vmca help in increasing MTOW(takeoff performance benefit).

In case of any engine failure during de-rated thrust take off FULL THRUST MUST NOT ME USED AS IT CAN RENDER AIRCRAFT UNCONTROLLABLE (since lower Vmcg and Vmca were assumed with less thrust).

No performance benefits are taken for flex take so full thrust is available at any stage of engine failure.


Aircraft Performance

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