View Full Version : Flex T/O limitation

1st Nov 2008, 07:13
Dear Aviators,

We all know the conditions to perform a Flex T/O: 25%, T ref, what a flat rated engine is, the Thrust vs Temp graphics, etc...

One of these conditions is that Flex T/O thrust should be greater than MCT.
On the other hand, and correct me if I'm wrong, there is no time limitation for Flex T/O, although we always run chronos happily during Flex T/O roll.
Is this not contradictory?
MCT: Max CONTINUOS Thrust, I understand that this thrust can be kept until fuel burns out.

What about Flex T/O if I always have the TOGA 5 or 10 minutes joker?
How can be a greater thrust than MCT unlimited?

It may seem absurd and it is not an important matter, just want to check if there are more crazy guys like me that think about this silly topics.


Alternate Law
1st Nov 2008, 08:00
In our operation, (A340), timing is done during flex T/O as our SOP dictates that TOGA will be selected following an engine failure beyond V1.

This then ensures that a stopwatch is running so that the 10min limitation can be observed.

1st Nov 2008, 08:18
Hi Altn Law (protection lost?):)!

Thanks for your reply.

According to T/O requirements Flex T/O ensures enough obst clearence without any thrust change. Nevertheless, the pilot has always TOGA available for climb improvement. So far, so good.
If the pilot considers the use of TOGA, then a limitation of 10 mints applies. I think the stopwatch should start at that very moment and not before.

But that's not the question, we can put a post about stopwatches and debate this, the discussion is about any limitation about flex and if so why and if not maybe we should consider a definition change between MCT and TOGA.

That's my intention. Thanks anyway.

1st Nov 2008, 08:21
Whilst we do not have an SOP that mandates TOGA thrust in the event of an engine failure, it is always available. Therefore limiting TO thrust (even if an assumed temperature is used) to 10 mins assures that the time limit for operating in excess of MCT is not exceeded.

just BTW, GE-90 110 and 115 engines are certified to use up to 40% de-rate.

1st Nov 2008, 14:28
there is no time limitation for Flex T/O The takeoff time limitation applies. Look in the limitations.

Plus, are you sure that you are limited to 25%? Mr Airbus uses 40% on some aircraft, easy way to find out is look at the TFLEX MAX temp.


FE Hoppy
1st Nov 2008, 16:34
One of these conditions is that Flex T/O thrust should be greater than MCT.

Not sure about this one!!

On the e-jets the lower thrust limit is 25% of rated thrust or CLB 2 + a little bit.

The CLB 2 limit is often the most limiting factor and is lower than CON (MCT)

Chris Scott
2nd Nov 2008, 01:20
Like FE Hoppy, I don't think Flex Thrust has to equal or exceed MCT, but can't quote chapter and verse yet. On the A310-200 with CF6-80A3 engines in 1984, "TLIM" (TMAX-FLEX) was ISA+57. After using it for a normal take-off with A/THR, and subsequently selecting CLB thrust via the TRP, the throttles usually advanced very slightly, reflected by the N1. This always seemed to discredit the benefit of using so high an assumed temperature, and I don't remember it happening on other jet types.

So in that case, the engine was in practice often using slightly less than CLB thrust for take-off. CLB was, of course, lower than MCT at low altitudes, but we still complied with the 10-minute rule. I suppose that it would be impracticable to have one rule for a Flex temp of ISA+57, and another for ISA+20 (say), when the thrust would exceed MCT.

Like mutt, I have always assumed that comparing the TMAX-FLEX of various turbofans would give a fairly good indication of the percentage of maximum thrust-reduction, as most of them seem to be flat-rated up to a similar TREF of about ISA+15 to 20. I now notice that the CFM-56 on our A320s had a TMAX-FLEX of ISA+46, 31 above its TREF of ISA+15; whereas the CF6-80A3 had values of ISA+57 and ISA+18 respectively, a greater difference at 39. This might explain why the CF6-80 at maximum thrust reduction seemed to use slightly less than CLB thrust for take-off, whereas the CFM-56 did not; but my course notes suggest that both engines are supposed to be limiting thrust reduction to 25%. So I must admit to being a bit perplexed on this one.

2nd Nov 2008, 08:00
One of these conditions is that Flex T/O thrust should be greater than MCT.

Certainly it is typical on a low weight take-off in a Boeing for the thrust to increase to climb thrust after take-off.

2nd Nov 2008, 21:52
In the early days of reduced thrust takeoffs, Qantas changed its procedures to limit reduced thrust to the relevant climb setting after a crew thought it strange that they had to increase thrust at the climb reduction point during the takeoff. The performance boss at the time (Wal Stack) concurred and the limitation was applied .. this probably goes back to the early 60s .. I can recall Wal's relating the tale when I was a student in the late 60s and he was a department Industry lecturer.

2nd Nov 2008, 22:04
To Mutt.

We should not mistake Derated T/O with Flex T/O.
Both are valid as thrust reduction methods, but they are completely different:

1.- Derated T/O is not available on every Airbus models.(Narrow bodies)
2.- It is true that some models A340-500/600 have DERATE levels up to 40%
3.- Flex T/O is based on a reduction based on an assumed Temp. This Temp can be sent through the FADEC (via MCDU) or TRP, depending on A/C model.
4.- This Assumed Temp should ALWAYS: TFLEX>TREF, TFLEX>actual OAT, TFLEX<Tflex MAX. (ISA+55,57 or whatever the ENG manufacturer says)
5.- This Thrust reduction cannot exceed 25% of max T/O thrust.
Regulations AMJ 25-13/AC 25-13(4): "Is at least 75% of the maximum T/O thrust for the existing ambient conditions"
6.- On Flex T/O, TOGA thrust is always available, on Derated T/O TOGA thrust must never be selected. (This applies for Airbus philosophy) [Getting to Grips with Aircraft Performance 7.2.4 Page 92)
7.- Flex T/O thrust IS NOT considered as a T/O operating LIMIT (GTG Perf. 7.1.1page 87). Derated T/O thrust IS considered as a normal T/O operating LIMIT GTG Perf 7.2.1)
8.- Flex T/O forbidden on a contaminated RWY, no limitation for derated T/O applies. The perf speeds for Derated T/O are specially calculated for that reduction of thrust, decreasing Vmc (JAR/FAR 25.149). Thats why sometimes it permits a higher TOW an short RWY's than if using TOGA, it reduces the ASD.

A short glance at Getting to Grips with Aircraft Performance will clarify a lot of doubts.


2nd Nov 2008, 22:48
Thanks again John Tullamarine!

Wal Stack's legacy from the golden sixties still applies.
Tflex thrust cannot be lower than the Max Climb Thrust (EPR) at the same flight conditions.
That's written on the Introduction of my A320 RTOW (IAE V2500) and as the max 25% reduction, the FADEC takes both limitations into account to determine Flex EPR. That's based on JAR/FAR.

Another amazing line in this intro is that: The Flex T/O thrust cannot be lower than the Max Continuous Thrust used for the final T/O flight path computation (at ISA+40)

Now for those reluctant guys for the MCT/CLB affaire, I wonder where is located the MCT/FLEX detent on the Airbus lever?
Last time I looked at it it was beyond the CLB detent, so according to the more beyond the greater the thrust (basic) rule, it should be clear that FLEX thrust is always greater than CLB if you respect the limitations.
The peculiar detent system on the Airbus levers is such thet MCT detent coincides with FLEX detent, which can be misinterpreted.
Think on what happens in your last sim after cleaning up after and ENG FAIL and climbing at green dot. You get out from FLEX to MCT by advancing or retarding your lever from FLEX and back again to the same detent to get MCT, but if you take a look at your EPR or N1, or if you listen carefully you'll notice a power/thrust reduction.


2nd Nov 2008, 22:52
People get confused by thinking that the 10 (or 5) minute limit for TOGA applies only at absolute TOGA thrust but it's actually anything from TOGA down to MCT/Climb thrust


Anything above climb thrust, which can be considered max continuous during the climb phase (MCT is a variable limit depending on conditions) is limited to whatever the manual says for TOGA.

The Airbus detents confuse the issue slightly because you see a seperate positions for FLEX and CLIMB/MCT when often they achieve the same thrust - no thrust change at acceleration alt etc.

FLEX can never be below climb thust (due to performance requirements) therefore you are always in the (TOGA) limit region during takeoff.....Hence the stop watch start!

3rd Nov 2008, 04:11
I totally understand the differences between Derated and Flex/Assumed...

5.- This Thrust reduction cannot exceed 25% of max T/O thrust.
Regulations AMJ 25-13/AC 25-13(4): "Is at least 75% of the maximum T/O thrust for the existing ambient conditions"

Would you believe me if i told you that the GTG and the AMJ's are WRONG? Airbus do operate some aircraft/engine combinations with FLEX of -40%.


3rd Nov 2008, 08:12
Mutt is right - he always is!

My nice new 319s with CFMs have a Tmax flex of ISA+60 which can give a TO N1 of around 78% which magically slides up to a Clb Thr of about 85% at Acc Alt.

All this changed about 4 years ago and my copy of Grips is older than that. Has it been updated?


4th Nov 2008, 08:37
mcdhu: I got also informed by mutt recently about the 40% flex. Did some homework as promised and look:
http://img395.imageshack.us/img395/4759/25flexng9.th.jpg (http://img395.imageshack.us/my.php?image=25flexng9.jpg)http://img395.imageshack.us/images/thpix.gif (http://g.imageshack.us/thpix.php) FCOM 2.02.14 p1 So there must be more than meets the eye.

4th Nov 2008, 09:08
FD - yes, my FCOM says the same, but I'm searching for where the changes came from. I'll email my Tech Pilot and see what he comes up with.


4th Nov 2008, 11:04
FLEX can never be below climb thust (due to performance requirements) therefore you are always in the (TOGA) limit region during takeoff.....Hence the stop watch start!

I fly CRJs, and the -200s are usually flexed down to 85% N1, and reaching acceleration altitude you ask for flapretraction and climb power, which will be around 90% N1.

After having flown an airbus for 10 years, that struck me as rather strange, but it is an approved and safe procedure...


4th Nov 2008, 14:10

According to Getting to Grips, the appropriate email address for this type of question is [email protected] Can I suggest that you email them.

Remember that i did state that it applied to certain airframe/engine combinations, which may mean it doesnt apply to your aircraft.


4th Nov 2008, 16:59
Mutt, you misunderstand me - I'm with you! I meant what I said! What you describe (40% reduction) is what happens on my aeroplanes and I remember when the Tmaxflex went up from Isa+45 to Isa+60 we received an incomprehensible email from the Company Tech Pilot about it. I guess that AI are a bit slow in amending the guff.

As an aside, we are forbidden from approaching AI direct ever since a pilot tried to get them to say that one of our SOPs was a bad idea!!

Happy days!

4th Nov 2008, 17:55
:) I have contacted Fleet Development since last time we discussed this, based on mutt's valued input. Neither I ever wanted to suggest that it was inaccurate, opposite is true in fact. And need to apologize that my previous post was not phrased properly.

They confirmed that 40% flex is available from AB for A320 and CFM and was on table during the definition process. Allegedly Iberia has them.

As far as I understand the rating we opted for is 27k A320 and 23 for 319. Tmax flex is ISA+53, and ISA +60. All are noted as 25 % max reduction by FCOM.

From what I've learned I conclude that thrust provided by 23k rated unit at ISA+60 is the same as 27k at ISA+60 as it is beyond the flat rating for both versions and the engine is in fact the same. However, if I wanted to operate 27k installation at this setting for take-off, it is impossible because 25% rule is not observed. On the other hand, for 23k unit same thrust matches the book because at ISA +60 it provides 75% of its rated thrust.

So my assumptions are:
That 40% reduction is introduced for the stronger version, so that operator may use it down to setting that is available and certified on lower rated versions of the engine.

It is irrelevant as far engine savings go whether you use 27 at 60% or 23 at 75% because it is still ISA+60 thrust.

Operator with 27 k and 25% max does not benefit from the potential life savings - but may never be light enough to use it.

Operator with 27 k and 40% has the widest power range available, and I do not think it is for free, both cost and maintenance.

Operator with 23 k and 25% runs the coolest hot section and on top benefits from better contaminated performace compared to 27@40 (Vmcg); but the potential of the engine stops at the low mark.

What do you think? The quote from my FCOM is actually a 23k JetA1 heatpipe and is correct, although misleading during the discussion.

Horses for courses, obviously selecting the optimal combination for individual fleet/field of ops must be a hedache.

FD (the un-real)