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.
and -7 are
the air is
the OAT is
will use a
You may at
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.
- It also known as: "Flex" (Airbus & Fokker), "Graduation", "Reduced Take-off
or "Factored Take-off Thrust (FTOT)".
- 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.
- 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.
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
- 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.
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.
- Increases TODR.
Therefore you may come to rest on the stopway after a stop from V1.
- 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.
- Increases fuel burn.
Strange, but true. This is because:
- Assuming an uninterrupted climb, it will
take longer to reach the more economical cruise altitude than a full thrust
- Engines are less efficient when not at full thrust.
- 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
Reduced thrust is permissible for tailwind take-offs, subject to the normal
takeoff flight planning considerations.
- Provable Numbers.
If you really want to, you can calculate the reduced N1 by using the
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 !
The following limitations must be observed:.
- Do not use an assumed temp if:-
- Runway is contaminated or slippery
- Marked temperature inversion or windshear
- Antiskid OFF or inop
- Either thrust reverser is inop
- PMC is OFF or inop
- Any power setting instrument is inop
- 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
||(2C with 10kt HWC)
||(10mb x 88kg)
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