Why can't the flex temperature be lower than Tref? Also why can't we perform a flex takeoff on a contaminated runway?
The engine is flat rated below Tref, and the thrust decreases above this temperature. The point of using a flex takeoff is to run the engine at a lower power setting, where its limit would be at a higher temperature. Using a temperature below Tref would result in the same thrust as Tref. The graph from Airbus below helps to illustrate the relationship between the variables.


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