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An assumed temperature takeoff intentionally requires a pilot to use more runway than would be normally required. Why would someone want to do that instead of getting off the runway in the minimum distance possible?

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An assumed temperature takeoff (also known as "flex" on Airbus/Fokker aircraft) is where the engine thrust is reduced to match the performance on a higher-temperature day, up to either the maximum thrust reduction allowed or the maximum assumed temperature that meets takeoff distance requirements. Unlike a derate, assumed temperature takeoffs do not reduce the minimum control speed used for calculating takeoff speeds ($V_{MCG}$ for $V_1$, $V_{MCA}$ for $V_2$).

The minimum selectable temperature is the lowest temperature beyond the flat rating temperature range (the range of temperatures where maximum thrust is achievable, limited by an engine control unit), and the highest temperature is the temperature at which the maximum thrust reduction allowed will be met. Except with specific certification and approval (such as 40% flex on the A340), such a procedure must not reduce the thrust beyond 25% of the nominal takeoff thrust (either full rate or a takeoff derate). For example, the range of assumed temperatures permitted is 30-75°C (flat rated to $ISA+15$, thrust at 75°C is 75% of normal rating).

The main reason for performing an assumed temperature takeoff is to conserve engine life, therefore reducing maintenance costs. According to Boeing, engine wear exponentially increases with the takeoff thrust used -- with engines costing about 10 million USD each, this has the potential to save a lot of money for airlines.

If you are taking off using assumed temperature, you may (but are not required to) add thrust to TOGA in the case of an emergency, however if you are using a takeoff derate, you must not increase the thrust beyond the derated N1, as this could cause a loss of control (higher thrust = higher actual $V_{MCG}$).

Not all aircraft allow assumed temperature in conjunction with derates. For example, you can only select a derate or a flex temperature on all Airbus aircraft (if a derate option is offered).

NOTE: do not attempt to "copy the jet jocks" in a piston aircraft and attempt a reduced throttle takeoff: only at takeoff throttle levels is extra fuel injected for cooling and reduction beyond this point could result in CHT levels rising to unacceptable levels, increasing the risk of engine damage.

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Imagine you're doing a takeoff at a high density altitude. You need more runway to get up to speed because your engine is producing less power and you need to move faster to move the same amount of air over your wings as you would at a lower DA.

Aircraft can pretend the DA is higher by using a higher than normal temperature when calculating takeoff information. In jets with automated engine controllers, this setting -in a round about way- allows you to set the engines to produce only the minimum thrust required to get off the ground safely. Since that results in less thrust, there is less wear on the engine and that adds up significantly over time.

Your trade-off for this decreased wear is that you need more runway, which is usually readily available. If you suddenly need full power you just move the throttle to the Take-Off/Go-Around (TOGA) or emergency power setting (or push a TOGA button on the throttle), which overrides the temperature preset.

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  • $\begingroup$ Very good answer, wish we could accept more than one! $\endgroup$ – Lnafziger Dec 30 '13 at 15:39
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Airshow pilots will sometimes use more runway than necessary. They'll get the wheels off the ground but remain in ground effect (sometimes only a foot off the runway, which I count as "still using the runway") while accelerating. The extra speed will permit them to make a dramatic vertical climb that's a lot steeper than Vy or Vx would permit.

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    $\begingroup$ Good point, but my question is about assumed temperature takeoffs! :-) $\endgroup$ – Lnafziger Dec 29 '13 at 18:10
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It's basically a question of economics. Derating your takeoff means using a lower thrust setting for the engine which means less wear and thus less money spent on maintenance.

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