What is the difference between Assumed Temperature and Derate takeoff thrust?

Question

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?

Answer 1

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 derate limit, otherwise an 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.

Answer 2

General

The actual takeoff weight of the aircraft is often lower than the maximum regulatory takeoff weight. In this case, it may be possible to takeoff at a thrust less than the maximum takeoff thrust. This allows to increase the engine life, improve the engine reliability and reduce the maintenance costs.

Two categories of takeoff at reduced thrust exist:

• The use of flexible temperature concept referred to as "flexible takeoff"
• The use of a fixed derated thrust level referred to as "derated takeoff"

Flexible Takeoff Definition

When the actual takeoff weight is lower than the maximum performance limited takeoff weight, the aircraft may comply with the regulatory requirements with a reduced thrust, called "flexible takeoff thrust".

This takeoff operation is the "FLEX takeoff".

Flexible Takeoff Principle

The FLEX takeoff principle is based on the change in maximum available thrust with OAT.

The maximum performance limited takeoff weight depends on the maximum available takeoff thrust, therefore it is possible to determine a temperature at which the actual takeoff weight would be limited by performance.

This temperature is referred to as "TFLEX (Flex Temperature)".

Flexible Takeoff Limitations

Takeoff at reduced thrust, so-called as "FLEX takeoff", is allowed only if the airplane meets all performance requirements at the takeoff weight, with the operating engines at the thrust available for the flexible temperature (TFLEX).

TFLEX cannot be:

• Higher than TMAXFLEX
• Lower than the flat rating temperature (TREF)
• Lower than the actual OAT

FLEX takeoff is not permitted on contaminated runways.

Some items listed in the MEL and CDL do not permit a flexible takeoff.

The operator should check the maximum thrust (TOGA) by:

• Performing full-rated takeoffs at regular intervals, in order to detect a reduced EGT margin, or
• Maintaining an adequate engine monitoring program, in order to follow-up on the engine parameters

Derated Takeoff Priniciple

The derated takeoff enables to improve the takeoff performance if the TOW is limited by VMCG.

VMCG limitation usually occurs on short and/or contaminated runways. The principle is to impose a lower engine rating to benefit from lower minimum control speeds.

In the case of derated takeoff, the minimum control speeds (VMCG and VMCA) are decreased because:

• The derated thrust is lower than the maximum takeoff thrust
• The effect of temperature on maximum available thrust is taken into account

VMCG Limiting

VMCG only has an impact on the ASD that includes an acceleration and a deceleration phase.

The decrease in VMCG allows to use lower V1 values in the speed optimization, which shortens both the acceleration and the deceleration distances.

The weight decrement linked to the derated thrust is largely compensated by the benefit of shorter ASD. As a consequence, if takeoff performance is limited by the VMCG limited weight, an overall improvement can be achieved by derating.

VMCA Limiting

The penalizing effect on climb gradient of the decrease in thrust outweighs the advantage of a lower VMCA.

Therefore derated takeoff would not improve the takeoff weight if VMCA limited.

It is not permitted to combine a derated takeoff and a FLEX takeoff.

When a derated takeoff is performed, the selection of full takeoff thrust, by setting thrust levers to TOGA, is not permitted below the speeds specified in the "Engine Failure After V1 - Continued Takeoff Operating Technique". The only exception is the Recovery Technique at takeoff after the penetration of unforecasted windshear.

Answer 3

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.