Ok my question is the Flex is used for derating right
Flex and Derate are two different things, have different purpose and in the Airbus cannot be used together. I believe that option is available in Boeing (Derate + Assumed temp).
1) will the engine control deliver a thrust that is available at 15 deg oat ? ( seems unlikely The physics of a gas turbine wont allow it.)
2) Or will may be try and fail and in the process may be cause damage to the engine ?
3) Figure out i am being stupid and give a warning may be ?
Flex temp cannot be lower than OAT and Tref (corner point). FADEC has ADIRS input. FLEX mode is armed only when temp inserted in MCDU is above OAT. So if you enter Flex temp lower than OAT and set thrust levers to FLX/MCT it will trigger a caution '
ENG THR LEVERS NOT SET and ask you to set TO/GA (If crew does not take any action then FADEC will automatically select TOGA thrust after 8 sec).
New aircrafts with CFM (not sure about IAE) trigger
ENG SAT ABOVE FLEX TEMP if inserted value is lower than SAT/OAT before applying take off thrust.