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Airlines like to avoid a full thrust takeoff as it increases the wear on the engine. There are two methods to achieve this: Assumed Temperature and Derate.

In the Assumed Temperature method, the takeoff performance is calculated by commanding the engine to mimic the thrust produced if the outside temperature is higher. Derated takeoff thrust basically tells the engine to pretend it is a weaker engine.

Both methods result in a takeoff thrust setting that is lower than 100% power. What are the differences between these methods, and how do the flight crew choose between the two?

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  • $\begingroup$ I suspect the answer is it depends on which performance tables your FCOM/QRH/EFB has. But I am not a pilot and can't say for sure. $\endgroup$ – Jan Hudec Oct 11 '16 at 20:20
  • $\begingroup$ FWIW, in the 1980s and 1990s on 727s and 747s at the two carriers I flew for, we used the assumed temperature method. The flight engineer looked it up on paper tables, entered the info on a card, and put the card upright on the center console forward of the thrust levers, leaning it against the instrument pane. $\endgroup$ – Terry Sep 1 '17 at 3:40
  • $\begingroup$ On some aircrafts if you start by selecting an assumed temperature and then you change your mind and decide to select a % derated takeoff thrust, the final derated thrust percentage will be with respect to the assumed temperature thrust and not with respect to the 100% thrust. That will produce a cumulative effect with much lower thrust. I have experienced this on the simulator; finally I asked a type pilot who confirmed that it is like that on this aircraft, and that for him and for his colleagues it has been recommended to use only assumed temperature. $\endgroup$ – user40476 Jun 6 at 8:41
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The difference is in the minimum control speeds ($V_{mcg}$ and $V_{mca}$).

In Assumed Temperature takeoff thrust, the pilots are allowed to advance the thrust levers to full rated takeoff thrust. However, in Derated takeoff thrust, the levers should not be increased beyond the fixed decate limit, otherwise a engine failure may result in loss of directional control.

From Boeing 737 FCTM (my emphasis):

The fixed derate is considered a limitation for takeoff. Takeoff speeds consider ground and in-air minimum control speeds (VMCG and VMCA) at the fixed derate level of thrust. Thrust levers should not be advanced beyond the fixed derate limit unless conditions are encountered during the takeoff where additional thrust is needed on both engines, such as windshear. A thrust increase, following an engine failure could result in loss of directional control.

The minimum control speeds are calculated based on the full rated takeoff thrust. The rated thrust is lowered in Derated takeoff but not in Assumed Temperature takeoff. Therefore, by calculating the minimum speeds based on a lower engine rating, the pilots can be ensured that sufficient directional control can be maintained on a slippery runway, whereas there is no such guarantee for Assumed Temperature takeoff.

On a dry and long runway, there is no operational difference between Assumed Temperature and Derated takeoff.

From the Boeing 777 FCTM (my emphasis):

Reduced takeoff thrust (ATM) may be used for takeoff on a wet runway if approved takeoff performance data for a wet runway is used. However, reduced takeoff thrust (ATM) is not permitted for takeoff on a runway contaminated with standing water, slush, snow, or ice.

And:

Derated takeoff thrust (fixed derate) may be used for takeoff on a wet runway and on a runway contaminated with standing water, slush, snow, or ice.

The Boeing 737 FCTM has a nearly identical description.

The A330/A340 FCTM explains that the slower $V_{mcg}$ / $V_1$ can also allow increased MTOW on short runways:

When taking off from short or contaminated runways where ASDA [Accelerate-Stop Distance Available] is the limiting factor, a reduction in the minimum control speeds may generate a take-off performance benefit and a higher MTOW.

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  • $\begingroup$ I didn't mean to answer my own question. I was waiting for some airline pilots to shine some light on the information I have, but since (apparently) no one has... $\endgroup$ – kevin Oct 11 '16 at 20:41
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    $\begingroup$ The statement that you can't do an assumed temperature takeoff with standing water, slush, snow, or ice MAY be specific to the 777 rather than universal. Clearly, with stuff on the runway, lots of power is a good thing, but I think there are some operations that can do assumed temperature with at least some of those, if all other parameters are met. $\endgroup$ – Ralph J Oct 11 '16 at 20:43
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    $\begingroup$ @RalphJ I believe the concern is the reduced friction on the runway, which lowers the tire's traction to counter an asymmetrical thrust condition until sufficient airspeed is achieved. $\endgroup$ – kevin Oct 11 '16 at 20:46
  • $\begingroup$ @RalphJ FWIW, in the 1980s and 1990s on the 727 and the 747 at the two carriers I flew for, we were not allowed to do reduced thrust takeoffs on contaminated runways. As I remember, when the contamination was just rain, it was the captain's call as to whether it constituted standing water. $\endgroup$ – Terry Sep 1 '17 at 3:35
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Both methods will give you an N1 setting that's less than what you'd have if you used actual temperature & no derate (i.e. max takeoff thrust). The computations to get to that N1 setting are different, but at the end of the day, the pilots (or the autothrottle) sets a particular N1, and you go fly. Which method, derate or assumed temperature, or both, that you used to get to that N1, isn't really all that important.

For the pilots, the company will probably tell them which method to use. Or the performance computations will consider whatever permutations are out there, and spit out an answer -- derate to XXk, assume temp of YY degrees, or maybe both. And the pilots put that into the FMC, and you go fly, knowing that the N1 that you get at the end gives you enough performance to do all the things you need to do: 35' at the end of the runway if you lose one at V1, meet obstacle clearance criteria, etc.

The derate method is typically less granular than assumed temperature -- you have two or maybe three derate setting you can select, and that's that. But those steps may be pretty big steps. The software can step an assumed temperature up one degree at a time to find exactly the minimum power that's needed to meet all the criteria. But there is some limit to what temperature you can assume, and if you have the really light airplane on a really long runway, even assuming that max temperature may give you lots of performance to spare, and so in order to bring the thrust down to being only what you really need and no more, you need the bigger "bite" of a derate.

There is a human factor to consider, in that if the crew is selecting a derate and entering an assumed temperature, you want to be sure that they did that process right, and (particularly if conditions or runway change) you don't end up with the wrong combination of derate + assumed temperature. If Plan A was to derate two steps down & use the actual temperature, then Plan B was to derate one step down & assume 45 degrees C, the hybrid of not changing the derate (leaving it at two steps down) AND assuming 45 degrees C will give you less thrust than either Plan A or Plan B, and you want to make sure that an error like that can be reliably trapped.

But again, that sort of decision of what method(s) are used is made more by the company and the performance software than by the individual crew. As a pilot, put what you're given into the box, verify that it's right, and go fly. I'm not overly concerned which computational path the software used along the way to telling me to set 88.3% N1.

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