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1.Not All Take-off Uses TO/GA Thrust?
If the runway length is more than enough for an aircraft to accelerate to its targeted take-off speed, why do we need to push the aircraft so hard with the engines working at its 100% that wears down the engine and increases maintenance cost? That's why the aviation industry introduces the concept of reduced thrust take-off.
An A330 of Malaysia Airlines lifting off at WMKK runway 14R.
2.Difference between Derated Take-off and Flexible Temperature Take-off
Firstly, both derated take-off and flex temp take-off (aka assumed temperature take-off) are take-off using lower thrust setting. The difference between them is that derated take-off thrust is to set a lower limit that an engine is allowed to deliver while flex temp take-off is to tell the engine to deliver a certain thrust based on the fake temperature, as the FADEC (Full Authority Digital Engine Control) will try to reduce the thrust delivered in order to avoid an overheat condition of the engine thus the higher the flex temp set, the lower the thrust will be delivered.
Flexible Temperature of 40 degrees Celsius (the fake temperature) could be told by the MAN FLX 40 showed at the left column in the Airbus FMA (Flight Mode Annunciator) during take-off.
Derated take-off thrust could be selected with pre-defined mode corresponding to a defined thrust amount. (TO, TO-1 and TO-2).
3.Pros and Cons
Both reduced thrust setting will increase take-off length required thus shorten RTO (Rejected Take-off) available distance as it is accelerating with lesser thrust, creating longer acceleration distance.
However, in terms of safety, the performance of derated take-off thrust is clearly known but the performance of flex temp take-off is only a rough assumption. Consequently, aircrafts are not allowed to use flex temp take-off thrust to take-off from a contaminated runway (snow/slush accumulated) as there're no available V1 correction data for a flex temp take-off on contaminated runway due to the rough assumption of flex temp thrust.
Anyhow, flex temp take-off thrust provides more selection and flexibility for different runway length and take-off weight while derated take-off thrust is relatively restrictive for different scenarios application.
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