Flex T/O vs Derated T/O
To Mutt.
We should not mistake Derated T/O with Flex T/O.
Both are valid as thrust reduction methods, but they are completely different:
1.- Derated T/O is not available on every Airbus models.(Narrow bodies)
2.- It is true that some models A340-500/600 have DERATE levels up to 40%
3.- Flex T/O is based on a reduction based on an assumed Temp. This Temp can be sent through the FADEC (via MCDU) or TRP, depending on A/C model.
4.- This Assumed Temp should ALWAYS: TFLEX>TREF, TFLEX>actual OAT, TFLEX<Tflex MAX. (ISA+55,57 or whatever the ENG manufacturer says)
5.- This Thrust reduction cannot exceed 25% of max T/O thrust.
Regulations AMJ 25-13/AC 25-13(4): "Is at least 75% of the maximum T/O thrust for the existing ambient conditions"
6.- On Flex T/O, TOGA thrust is always available, on Derated T/O TOGA thrust must never be selected. (This applies for Airbus philosophy) [Getting to Grips with Aircraft Performance 7.2.4 Page 92)
7.- Flex T/O thrust IS NOT considered as a T/O operating LIMIT (GTG Perf. 7.1.1page 87). Derated T/O thrust IS considered as a normal T/O operating LIMIT GTG Perf 7.2.1)
8.- Flex T/O forbidden on a contaminated RWY, no limitation for derated T/O applies. The perf speeds for Derated T/O are specially calculated for that reduction of thrust, decreasing Vmc (JAR/FAR 25.149). Thats why sometimes it permits a higher TOW an short RWY's than if using TOGA, it reduces the ASD.
A short glance at Getting to Grips with Aircraft Performance will clarify a lot of doubts.
Thanks.