View Full Version : Difference derated t/o and flex t/o

17th Sep 2010, 16:45
Hi all,

From airbus "safety first" publications "Understanding of take off speeds" there is a part that is a little unclear to me.

The magazine reports:
"“JAR/FAR: AMJ 25-13 / AC 25-13
(4)(b) Derated takeoff thrust, for an aeroplane, is a takeoff thrust less than the
maximum takeoff thrust, for which exists in the AFM a set of separate and independent
takeoff limitations and performance data that complies with all requirements of
Part 25.”

Up to here everything clear but then says:

"The objective of flexible thrust differs from that of derated thrust. Both types of thrust
cannot be used interchangeably.
Flexible Thrust is a thrust reduction, designed to save engine life. This thrust is reduced
to take advantage of the available runway length, when full thrust is not necessary
(from a performance perspective), but takeoff speeds with full thrust still apply."

What does this last phrase mean? do we take off with a flexible thrust (so actually a reduced one in relation to the full thrust available) but we use the take off speeds of a full thrust take off run? :confused:

Any ideas?


17th Sep 2010, 17:00
Think about it this way, if you use Flex Thrust with the speeds based on Flex Thrust:

1: Can you advance the throttles to Full Thrust?
2: What would happen to the VMC speeds if you did advance the Throttles to Full Thrust?


17th Sep 2010, 17:04
Well i suppose:

1) yes i can

2) The VMCG and VMCA would direclty increase their value

but this doesn't help me in understanding that phrase which, i think, says we take off with the SAME speeds of a full thrust take off but using a lower thrust from the engines.. :confused:

17th Sep 2010, 17:07
Hi bio161,
the Derate is a default reduction in thrust expressed in percentage (eg. 5%, 10% ... 25% is the max) and having his peculiar RTOW chart; the Flexible Thrust is, instead, a way of reducing engine thrust (and save life) by setting a temperature (TFLEX) to meet the minimum TO performance on a "long" runway for a weight lower than the maximum = far more accurate than derating thrust.

but takeoff speeds with full thrust still apply."

Reducing thrust on TO also means reducing the minimum control speeds, thus the most penalizing situation is the full thrust TO, which is always considered by the regulations.
Think this way: V1= full thrust (directly related to Vmcg) / VR/V2 = FLEX

Hope that helps,

17th Sep 2010, 17:12
Ok thank you guys..All this is clear already! :ok:

But i try to make my question in another way around.

According to the phrase "..but takeoff speeds with full thrust still apply.." my mind gets something like:

"With a TOGA thrust take off we get 120kts V1, 125kts Vr and 130kts V2" (my invention)..Now we take a flex temperature to improve engine life ecc ecc ecc..Then from the previous phrase "..but takeoff speeds with full thrust still apply..", in my opinion, we keep on taking 120/125/130 speeds BUT using a lower thrust setting!

Is this correct? yes or no?

17th Sep 2010, 17:17
Derate. Engines are restricted to the max power output, stipulated within the AFM and agreed by the lessee/lessor to give greater engine life. Once done you can only operate to these power levels/speeds/weight restrictions.

Flexible Thrust. Say you have a ETOW of 77000kgs. You look at your airport performancd charts for your w/speed & direction, temperature, pressure/altitude etc and it shows you are okay for take off to 82000kgs (say speeds 154-162-164).
Flex thrust allows you to look down the charts finding what temp it would need to be for a max payload of 77000kgs (which is all you have) - say that's 50c - with speeds 152-157-159 - you can put this temp into A/C database as OAT and use these lower speeds. Saves engine life. Why use the higher speeds of 154-162-164 for a TOW of 82000kgs when you only need to lift 77000kgs.

There is more to it, but that's the basics.

17th Sep 2010, 17:17

When one does a FLEX take-off the provision is always there to apply TOGA thrust if the crew so wishes therefore Vmcg considerations must be observed.

If the take-off is Vmcg Limited however, a greater weight can be achieved by using a lower thrust setting which means in turn a lower Vmcg. This is known as a De-Rated Take-off and in this case one is not allowed to use TOGA since the speeds may be too low if TOGA were applied after an engine failure.

Hope this clarifies the two. I have only seen De-rated performance to have been used on the A340 (with high Vmcg) cf. most twins.

17th Sep 2010, 17:19
what that means is you can still use use take-off speeds and full thrust - flex is not MANDATORY but with DERATE you now can only ever use the lower settings as per your AFM.


17th Sep 2010, 17:21
Hi guys again and thanx for your replies! :ok:

just got it now! Thank you for your help! :8

Regards to all,

25th Nov 2010, 15:20
so which is more advantageous, flex or derate?

Mad (Flt) Scientist
25th Nov 2010, 17:19
They have different advantages so it depends what your criteria are.