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*** Updated 05 Aug 2023 ***

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What is an Assumed Temperature Thrust Reduction ?

This is a way of reducing the take-off thrust to the minimum required for a safe take-off, thereby conserving engine life and hence reducing your chances of an engine failure.

 

How Does it Work ?

The CFM56-3 and -7 are flat rated at ISA+15C ie 30C. This means that they are guaranteed to give (at least) the rated thrust at the full throttle position when the OAT is below this temperature. Above this temperature, they will give less thrust because the air is less dense.

On occasions when full thrust would be more than is safely required eg light aircraft, long runway, headwind etc. we can choose a thrust setting below full thrust by telling the engines (via the FMC) that the OAT is much higher than it actually is. This higher temperature is called the assumed temperature.

If we fool the engines into thinking that the temperature is much higher then it actually is, by entering an assumed temperature into the FMC, they will use a correspondingly lower N1 to give the rated thrust for the higher temperature when TOGA is pressed. You may at any stage after TOGA is pressed advance the thrust levers further to give the full rated thrust again.

In practice, we find the assumed temperature by entering the take-off tables (see example at foot of page) with the actual takeoff weight, and then determining the hottest outside air temperature at which the take-off could be performed. This temperature is called the "Assumed Temperature" and is entered into the FMC TAKEOFF REF 1/2 page as "SEL TEMP". The ambient temperature is also entered here (as "OAT") and the reduced thrust take-off N1's will be computed by the FMC and displayed on the CDU. On the NG series, the temperatures are entered in the N1 LIMIT page.

Takeoff Ref Takeoff Ref NG

 

Some Facts About Assumed Temperature Thrust Reduction

  1. It also known as: "Flex" (Airbus & Fokker), "Graduation", "Reduced Take-off Thrust (RTOT)" or "Factored Take-off Thrust (FTOT)".

  2. It is not the same as "De-rate"

    A de-rate is a semi-permanent engine fix, used to reduce the maximum thrust available; for instance down to 20k from 22k on -3/700's. It is also used to equalise the thrust where B2 & C1 engines are mixed on the same airframe. When an engine is de-rated, the full (un-de-rated) thrust is no longer available because this would require changes to the EEC, HMU, fuel pump, engine ID plug and the loadable software; non of which can be done by the pilot in-flight.

    A temporary form of de-rating known as a "T/O de-rate" is accessible through the FMC on TAKEOFF REF 2/2 or N1 LIMIT (NG's) but this is prohibited by some operators. The T/O de-rates (TO-1 & TO-2) can be 10 to 20%. It follows that an engine may be de-rated and also be using reduced thrust in which case you could be taking off at Full power -20% -25% = 60% of the full power of the engine - scary thought! Note that a T/O de-rate can overridden by firewalling the thrust levers; this action will give the thrust rating shown on the IDENT page.

    Takeoff Ref 2 N1 Limit NG

  3. Max & Min Temps.

    The normal range of assumed temperatures is from 30 to 55C. However the QRH shows that temperatures from 16 to 75C may be used. The maximum temperature is set by the maximum amount of thrust reduction allowable i.e. 25%; and the minimum is where the engine becomes "flat rated" and no further performance gain can be achieved.

  4. Vmcg.

    Because the take-off is at a reduced thrust, there will be less asymetric thrust in the engine-out case and therefore a lower Vmcg. Some airlines/aircraft use this (when done as an FMC de-rate) to increase the RTOW on contaminated runways. This must be done by a de-rate rather than an assumed temp, so that the pilot cannot accidentally reapply full power.

  5. Saves Engine life.

    Most engine wear comes from operation at high internal temperatures, even a small reduced thrust (30C) can make a significant difference to engine life. Increasing engine life will not only save the company money but it will also reduce the chance of you, or the crew after you, having to practice their EFATO technique.

  6. EFATO

    It is not necessary (although it may be good practice) to set full power on the remaining engine after an EFATO, the performance figures allow for this. Doing so will increase your climb gradient but also the asymmetric thrust.

  7. Increases TODR.

    Therefore you may come to rest on the stopway after a stop from V1.

  8. Inherently Safe.

    This procedure does have a built in safety factor. Say you used an assumed temp of 50C to enable you to use a lower N1, all the figures are done for 50C but the engine will be operating at ambient (cooler) so will deliver more thrust than the calculations allowed for. Also, if you do lose an engine you can still increase the thrust to the ambient temperature limit.

  9. Increases fuel burn.

    Strange, but true. This is because:

    1. Assuming an uninterrupted climb, it will take longer to reach the more economical cruise altitude than a full thrust climb.
    2. Engines are less efficient when not at full thrust.

     

  10. After a reduced thrust take-off, the climb thrust is also reduced.

    This is then gradually increased until the engine is back to full climb thrust at about 15,000ft. The cut-off between CLB-1 & CLB-2 is at about 45C depending upon the amount of take-off thrust reduction. (Effectively whether TO-1 or TO-2 was used.). CLB-1 or CLB-2 may be either pre-armed before departure or selected manually during the climb after a full thrust take-off if desired.

    Climb ECON                                    N1 Limit

  11. Tailwinds.

    Reduced thrust is permissible for tailwind take-offs, subject to the normal takeoff flight planning considerations.

  12. Provable Numbers.

    If you really want to, you can calculate the reduced N1 by using the following formula.

    Reduced N1 = Full N1 x Square route of (Ambient temp / Assumed temp)

    Where the temps are in deg K.

    This calculation is not an approved airline procedure !

 

Limitations :

The following limitations must be observed:.

  • Do not use an assumed temp if:-
    1. Runway is contaminated or slippery
    2. Marked temperature inversion or windshear
    3. Antiskid OFF or inop
    4. Either thrust reverser is inop
    5. PMC is OFF or inop
    6. Any power setting instrument is inop
    7. An RTOW table is not available.

 

How To find an Assumed Temperature:

Move up the appropriate wind column until you meet the actual TOW, assumed temperature is given on the left. See example page below.

Then correct this for QNH (-1o per 5mb, or part thereof, below 1013), and Anti-ice (-2 o if used).

Example: Rovaniemi R/W03 030/10 CAVOK 2/-4 1003

 

Normal speeds  65,400 (2C with 10kt HWC)
QNH Correction   -880 (10mb x 88kg)
RTOW = 64,520

If actual TOW = 60,000kg then assumed temp = 38C, with speeds of V1=147, Vr=150, V2=156.

If V1 was increased to Vmcg then the assumed temp is that at which V1 = Vmcg (and then corrected for QNH and Anti-ice).

Roveniemi  

 

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