Assumed Temperature Method is a procedure we use to minimise engine wear and conserve valuable maintenance dollars. The financial difference for a company in terms of extended engine life can be very important to profitability and so is a daily consideration for all commercial pilots. Our job is to operate a commercial service as profitably as we can. So where we have plenty of runway and no obstacles to clear and our weight is such that we do not require a Full Thrust T/O we will use less than full thrust.Now we can't just take a stab in the dark at it so we have a procedure called Assumed Temperature Thrust Reduction. In each aircraft there is a Runway Analysis Book that lists all the airports and runways you would normally operate into. This analysis is a tabulated page listing all the potential variations that may occur on any particular day ie: headwind and tailwind, temperature, wet runway, different flap settings and so on. Note there are also generic figures listed in the QRH but these are for use only as a last resort or if you are on a ferry flight etc as they do not account for obstacles and so on.So we select the appropriate page for the conditions on the day and find out what is the maximum temp we could lift the expected load out of the duty runway and then put that temp into LSK L1 beside the OAT. This will cause the Autothrust to only set enough thrust to lift the load you have told it you will have on board when you entered the ZFW.Our real world limit is a maximum of 60C assumed temp and we often use 48 or 44 but it cannot be less than 30 degrees nor can the Assumed Temp be less than the OAT.Is this not too muddy an explanation? I hope not.Rgds,

Hey everyone, I'm reviving this great thread by Brad beceause I have had some toruble grasping the concept of assumed OAT/de-rated TOs.I've just read about every thread out there regarding this issue and so far I've understod a lot. A special thanks to Brad and Kris for writing so much useful stuff for us simmers. For instance I now know that selecting higher temperatures will trick the FMC into protecting the engines from a very high EGT and thus give us less TO thrust. What I don't understand is the following: I've always heard that hot air is bad for you in TOs - the hotter it is, the less dense it is and thus the less lift you have. Thefore you need longer TO runs to get more speed and be able to lift off. What confuses me is the concept that you always end up entering a higher assumed temperature than the present OAT when you consult the Runway analysis books (or whatever they're called!). If you're entering a higher temperature into the FMC, it will produce less thrust in order to protect the engines - ok - but won't it assume as well that the aircraft is in a situation where more thrust is needed to achieve higher speeds to generate more lift during the run because of the higher temperature (albeit a fictiicious higher temp selected by the pilot)? The other day I was playing with the FMC (this in the PSS B777) to try and get the lowest thrust possible for my takeoff since I was light and the runway was huge and I noticed that lower assumed temperatures (say 3 degrees celcius) also gave me a low computed TO thrust - even lower than if I told the FMC it was 50 degrees out. I'm guessing this is either due to bad programming or the FMC behaves in this fashion because since the air is cooler, it's more dense and thus easier to get airborne.And now the last questions: How does taking off with packs off or bleeds off help? Does it generate more thrust to not have so much bleed air "escaping" from the engine? In the 737 is it normal to always takeoff with bleeds off? (BTW I know that the 777 uses packs off instead of bleeds off but I'm not sure about the 737s). And finnally, in real life, how is the decision to takeoff with or without the packs/bleed made? Does the captain say simply: "boy it's too heavy/hot/high better save on the bleed air", or does he read it off somewhere prior to takeoff?Sorry for the confusing post, but I'm so confused myself that I couldn;t arrange my thoughts in a more orderly fashion!Ohh last thing: Nobody would happen to have some runway analysis tables lying around to share with rest of us?cheers, Victor

Victor,I'll take a shot at this pretty advanced topic. ;-)First read this link. It is really helpful to help you understand the basics of reduced takeoff thrust:http://www.b737.org.uk/assumedtemp.htmAs you can see the engines will give you less than rated thrust above 30