View Full Version : What is flex?

feet dry
8th Jul 2004, 10:13
Questions from a light aircraft chap to any heavy jet chap.

Interesting discussion recently regarding the Emirates emergency landing at Jo'burg and a term kept on popping up.

What is flex as applied to heavy jet considerations?

Is it literally flexing of the fuselage or wings, or is it a theoretical concept applied as an additional factor to maintain safe operation?

What are the consequences of mis-calculation?

Answers on a postcard (if you are so inclined) to the usual address please.

Thanks very much

8th Jul 2004, 10:40
I didn't follow the Emirates emergency landing at Jo'burg discussion, but the term "flex" is used for reduced takeoff-power on Jet engines. It is a flexible temperatur methode which "fools" the engine concerning outside temperature. This results in less thrust than max and therefore less stress for the engine (lifetime).


feet dry
8th Jul 2004, 13:24

Thanks for that, all makes the use application of a flex take off obvious really. i.e. high altitude take offs


Old Smokey
8th Jul 2004, 14:07
Feet dry,

As temperature increases, Takeoff thrust decreases, resulting in lower Maximum Takeoff Weights.

If the Actual Takeoff Weight (ATOW) is less than maximum, thrust may be reduced to the point where the ATOW would be limiting at a higher temperature. This temperature (Flex for Airbus, or Assumed Temperature for Boeing) is applied to the engine computer, and the lower thrust produced meets all of the performance requirements, as though the aircraft was truly at Maximum Takeoff Weight. (It does slightly better, but that's another story).

The 2 big advantages are that (1) Engine stress is significantly reduced, thus increasing engine life and reducing the risk of engine failure, and (2) Maximum rated thrust for the true environmental conditions is still available if needed.