If FLEX temp range is from the present TAT indication to 70C then what is the appropriate entry in the FMS? For a example, i see in the FMS manual an entry of a 38C on a specific airport. If the limit is 70C then why 38 and not an entry of 50C or even 40?

FLEX temp (believe it's called assumed temperature in Airbus/Boeing aircraft), in laymen terms, de-rates the output of an engine by making it think the air is warmer than it actually is. To calculate possible FLEX temp ranges you need to know a variety of environmental and configuration data which is then fed into either performance tables or a separate program which can calculate the optimal FLEX temp.Unfortunately the required documentation to calculate a FLEX temperature is not included with the MD-11 package (think it was a licensing issue with Boeing). I believe someone created set of tables which you can use to determine safe FLEX temps. I can't get to Avsim's main site due to my works web filtering, but I think you can do a search for these tables there.Another great option if you're a little more serious is the TOPCAT performance calculation program.And finally most airlines SOP only allow FLEX temps in good weather (i.e. dry runways and within a certain wind envelope). FLEX temp is not required to perform a safe takeoff. It is there reduce wear on the engines and thus save money for the airline.-Bryant Martin

FLEX temp is McDonnell Douglas and Airbus. Boeing does not use FLEX. They use assumed temperature and just a fixed derate.As said, TOPCAT is pretty much required if you don't want to use the AVSIM MD-11 flex temps (bit confusing and too much work for me...)

I agree it is a very complicated matter but adds alot to realism. I also tried to do some search but I could not find any FLEX temp tables only some discussions in the PPRUNE

Flex temp, assumed temp and derate all increase the engines life (so reducing the cost of servicing it) by having it produce less power, but if you reduce the power too much, you wont take off in the available runway. The 'appropriate' entry is the one that lets you take off safely in the current conditions with the least engine wear. Calculating the appropriate entry requires a tool like topcat. In FSX, where engine wear and service costs are not modeled, the value it is not meaningful.In FSX, unless the runway is wet or seriously short, you are pretty safe using a flex (or assumed) temp 10 or 15 C above actual temp, and/or a derate of 1.