Following another thread on this forum, I read about entering the Flex temperature in the manual (FMS 10.27). I understand how to enter it, and I also understand that this temperature affects the thrust delivered during takeoff. I"m guessing that a higher Flex temperature will mean a lesser thrust during the takeoff run, right?But here's my question ... what actually IS the flex temperature? What does it indicate? I see that the values can be between the current total air temperature (TAT) and 70C but if the air temperature actually WAS 70C, the air conditioning system in the aircraft would have trouble keeping the cabin below the point where the passengers would cook.Is there some calculation that aircrews use that works out a flex temperature to use? or is it just an elaborate way to pull the thrust back a bit, based on judgement and experience? (as in: "Ohh our takeoff this morning was a bit fast, so this afternoon I'd better raise the flex temperature a smidgeon")

Judgement and experience aside, the flex temperature would in real world operations be obtained from a takeoff table specific for the runway based on TOGW and OAT. Flex, I think, simply refers to telling the FADEC to use this temperature instead of that temperature, it is a flexible input. Yeah, 70C would be a warm day at Scotty's Castle (Death Valley) but remember we adjusting thrust with this variable. My guess is the thrust generated at 70C Flex on a standard day might be thrust available on at 70C day but the engineer in me suspects that it is just a point on a control response curve.

Is there some calculation that aircrews use that works out a flex temperature to use?

Yes, there is. This subject has been discussed extensively here. PMDG's wiki pages have some concrete examples with 747 flex tables and how to use them but the principle applies to any heavy turbine aircraft. This all has to do with calculating so called balanced field takeoff length (for aircraft that must do it by FAR). You can also search very recent posts from Spin737 (hopefully I got his name right) who explained it again.

Very simply - flex temperature is a derated takeoff. Rather then the jet using full thrust on the takeoff the 'system' is fooled into thinking the air temperature is higher than it really is.... an assumed temperature.That is then input to reduce the thrust to preserve the life of the engine when full thrust is not required for a safe takeoff.All dependant on runway length and condition, weather, weight and obstacle clearance.Some systems use flex, others use derate but it means the same thing essentially.John Ellison

Subtle difference between a FLEX takeoff and a derated takeoff. When you derate, you set a new limit for the engine, and may not increase thrust beyond that setting in case of an engine failure.Paul

Subtle difference between a FLEX takeoff and a derated takeoff. When you derate, you set a new limit for the engine, and may not increase thrust beyond that setting in case of an engine failure.Paul

Very true, so you can have both a Derate and Flex takeoff. A good example is PMDG's very own 747.

Please remember that while preserving the life of the engines is an important reason for derated takeoffs it is NOT the primary reason. Flight safety is of paramount importance. Using the full thrust of 4 RB-211s in a lightly loaded 744 results in unmanagable performance. Should anyone think this an exaggeration, I'd recommend a max performance t/o at a cool sea level airport in an aeroplane weighing 280T......I'd also recommend cinching the 5 point harness a bit tighter than normal! :)Preserving the 50-55 second T/O roll with appropriate time for the high speed transition at the 80kt call, then V1, Vr, V2, pitch for speed, Flaps 10+10 etc, allows for a predictable flight sequence- with time to recognize any arising safety concerns and institute emergency procedures.Also, remember 2 presses of the TOGA switches results in all derates being cancelled and max thrust immediately available.

If you're asking about what it actually means to input an assumed temperature, it has to do with the physics of how engines operate. As temperature increases, air thins out and you get less flow through the engines and as a result less thrust. Feeding the EECs/FADECs a temperature that's higher than what's actually there in reality fools the engines in a way if you will and causes them to operate as if the temperature was the value you entered.Carl is exactly right above - derates and FLEX takeoffs are done to make the performance envelope actually manageable for the crew. If you take off in just about any modern airliner with full thrust while light, you're going to accelerate and climb so fast that it may not even be possible to pitch to V2+10, you could have issues with busting altitude restrictions in the initial climb etc. I've actually been on a 757 that had 15 people on board doing a max thrust takeoff to get altitude fast for noise abatement - it is absolutely ridiculous, you feel like you're in a fighter jet.

For comparison:-757 vertical climb -

747 Just taking off - http://www.youtube.com/watch?v=CXkSQW3xul8And lastly 'We have smirnoff' -

John Ellison

Holy crap, that Il-76 one... Anyone know where that took place? That HAD to be an illegal takeoff.

That one of the vodka burner taking off is one of my favorites. Holy crap is exactly right.

Carl is exactly right above - derates and FLEX takeoffs are done to make the performance envelope actually manageable for the crew. If you take off in just about any modern airliner with full thrust while light, you're going to accelerate and climb so fast that it may not even be possible to pitch to V2+10, you could have issues with busting altitude restrictions in the initial climb etc.

Carl, Ryan,Can't buy that one as a valid reason. Someday the runway will be too short for deration or somewhere they have to make a go-around. Poor crew that's unable to manage those (near full-thrust) scenario's.Regards,Harry

Please remember that while preserving the life of the engines is an important reason for derated takeoffs it is NOT the primary reason. Flight safety is of paramount importance. Using the full thrust of 4 RB-211s in a lightly loaded 744 results in unmanagable performance.

I don't know much about the 747, never having flown it. However, on the 737, you cannot derate if you are in a windshear situation or if the EEC is in alternate mode. I've flown with the EEC/PMC in alternate maybe twice, but windshear is very common where I fly. The first paragraph in my 737 performance manuals on derate/reduce says it's all about extending engine life and reliability.Cheers

HARRY- Yes, there are times where no derates are utilized- and field length and density altitude are obviously factors (especially with a balanced field requirement).MATT- Yes, the 744 AOM also discusses not utilizing derates when windshear is potentially present.That said- my prior commentary can be evaluated by yourselves- just load up an empty 744, add about 12T of Jet A and firewall the throttles. The experience will be one you will remember. Add a couple of altitude restrictions on a complicated SID and you will have the time of your lives. :)

HARRY- Yes, there are times where no derates are utilized- and field length and density altitude are obviously factors (especially with a balanced field requirement).MATT- Yes, the 744 AOM also discusses not utilizing derates when windshear is potentially present.That said- my prior commentary can be evaluated by yourselves- just load up an empty 744, add about 12T of Jet A and firewall the throttles. The experience will be one you will remember. Add a couple of altitude restrictions on a complicated SID and you will have the time of your lives. :)

Thanks Carl,I remember those experiences very well. Not in the 744, but in the MD11. Time of my lives, yes.Passengers comfort? The adrenaline keeps you ahead of the aircraft, so even those procedures can be flown really smooth. No need to adjust my 5-point harness differently.Regards,Harry