FLEX and derate are not the same thing...Derate reduces the maximum thrust available from an engine. It derates the engine to a lower maximum output in other words..."FLEX or "Assumed temperature" is the maximum ambient temperature that will provide certificated takeoff performance for the given airport/runway and aircraft gross weight". Thanks to a poster on PPRUNE's forum for this...You may not use assumed temperatures for runways contaminated with standing water, slush, ice, or snow. Normally, FLEX is not used with tailwinds on takeoff either...The assumed temperature reduces the takeoff thrust setting without affecting the max thrust available from the engine on selection of TOGA. The reason for doing it is reduced wear, engine longevity, reduced operating costs, and so on...I use the excel file mentioned here, Bryan Schleipen's..., which has so far worked really well every time... It is a function of TOGW, runway length, runway altitude, QNH, as well as headwind/tailwind component. It reduces the calculated takeoff thrust such that takeoff is still possible and safe within the runway length available. You set too high an assumed temperature and you might not reach Vr before the end of the runway :(Combined with the MD11, I have always seen V1 to be such that an abort would still have me stopped within the remaining pavement available, as long as I have the correct runway lengths and other parameters...PM me with your e-mail address if you wish me to send the file to any of you... The zipped file is <250Kb...The MAX FLEX possible on the MD11 appears to be 70

You're pretty much on the ball there, Dan. I understood it as tricking the system into thinking that it is a lot warmer outside than it actually is, so the FMS reduces the engine thrust. It assumes the temperature is higher and as such the air is less dense than it actually is. Hence the term assumed temperature...It could be that the person who designed the GUI (do you mean TOPCAT?) may have made the wrong assumption that FLEX and derate mean the same thing... I therefore assume (man, a lot of assumption going on here) that he means FLEX temperature where derate is given...As I already mentioned, derating is a temporary or semi-permanent fix to reduce the engine's maximum thrust rating. It can be done on an engine temporarily, you see this on Boeing FMS's as DERATE TO-1, TO-2 for example (at least going from memory of the PMDG FMS in the NG)...The temperature TOPCAT comes up with at that position is the one to be entered into the MD11 FMS... you might wish to crosscheck with the Schleipen table to see what it comes up with as well... apparently, the software used in the real world may be applied by both Cap and FO and crosschecked...Andrew

You're pretty much on the ball there, Dan. I understood it as tricking the system into thinking that it is a lot warmer outside than it actually is, so the FMS reduces the engine thrust. It assumes the temperature is higher and as such the air is less dense than it actually is. Hence the term assumed temperature...It could be that the person who designed the GUI (do you mean TOPCAT?) may have made the wrong assumption that FLEX and derate mean the same thing... I therefore assume (man, a lot of assumption going on here) that he means FLEX temperature where derate is given...As I already mentioned, derating is a temporary or semi-permanent fix to reduce the engine's maximum thrust rating. It can be done on an engine temporarily, you see this on Boeing FMS's as DERATE TO-1, TO-2 for example (at least going from memory of the PMDG FMS in the NG)...The temperature TOPCAT comes up with at that position is the one to be entered into the MD11 FMS... you might wish to crosscheck with the Schleipen table to see what it comes up with as well... apparently, the software used in the real world may be applied by both Cap and FO and crosschecked...Andrew

Cool, thanks again, Andrew.I am only using Utopia for now, but will probably get TOPCAT soon enough. It is surprising that with all the sophistication and intelligence of these systems that they don't don't just allow an option for it to be chosen in simpler fasion. Such as it calculates your weights, temp, RW length, conditions etc... then provides a couple options for takeoff thrust. Something like "Normal T/O (Default)" or "Use Max FLEX" and maybe options for "50% Max Flex" and "Minimal Flex". Just a thought. Of course I do enjoy knowing what's behind as much of the logic as I can also.Thanks again, I look forward to using the systems the way they were ment to be used! Also, as you mentioned earlier, I have many times hit VR the same time I was hitting the end of the tarmac haha.

FLEX in the real world is a really complex calculation, as it depends on so many variables. The safest option is to use no FLEX temp... as long as you are not too heavy for the runway length and weather conditions per se...But it can be fun to play with settings and see how much more of the runway is required to reach Vr, and also the effect on V1 with different FLEX temps...I use FLEX a fair bit getting out of longer runways like EDDF, there I have 4000m of concrete to play with...If you are rotating at the end of the runway, then your calculations are off, way off :(Have fun and see you in the fence!Andrew

I have had good results with utopia, I can't comment on TOPCAT since I don't own it. Before I used utopia I would not enter a flex temp because with all of the parameters involved it's something you can't guess. I have to say that with utopia I have yet to run out of runway and that the pitch bar is much easier to follow when a proper flex temp is used. So far I have only found a GE engine TOD file for utopia but it isn't that big of a deal to use it with the PW engines since they produce more thrust. Utopia also worls well with my PMDG 747(F), LDS-767, and PSS 757-200 (F).