Richard McDonald Woods

Airline assumed temperature v derated thrust takeoff usage

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Which airlines normally propose assumed temperature takeoffs, or derated takeoffs or forbid their use? How do they decide?😃

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Derated takeoff power settings are proven to increase engine longevity and reduce engine maintenance costs, so I would expect that virtually all operators use reduced power takeoffs, except, of course, where performance considerations dictate full power (TOGW limited by engine out climb gradient, runway length, runway contamination etc).

Regards

 

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First up to clarify the terminology for anyone unsure:

Assumed Temperature (ATM): Advance the thrust lever a little bit less than full for takeoff, but no less than 75% of the full thrust available

Derate: Strap on (by means of software) a less powerful engine.

This distinction is very important because there are Vmc implications.

Because de-rating effectively re-sets the maximum takeoff thrust to a different (lower) value, we can apply an assumed temperature reduction to a derated thrust takeoff. As an example, if we were to apply a 20% de-rate we could then, given sufficient runway etc, apply a further 25% assumed temperature reduction on top of that. This is clearly much less than 75% of the full rated thrust (with no derate) but this is OK because we are pretending we have strapped on smaller engines.

The sticking point is that Vmcg/Vmca are calculated based on the maximum thrust output. When we de-rate, we are reducing the maximum possible thrust output and so the Vmc numbers will also change (reduce, because there is less thrust asymmetry for the rudder to overcome).

This means that if we do our 20% derated takeoff with 25% ATM thrust reduction on top, if we have an engine failure on takeoff we are limited to the maximum derated thrust. In other words, we can remove the assumed temperature reduction to give us a bit more thrust if we desire, but we must not firewall the remaining thrust lever as we may well be below Vmca for the full rated thrust and would very swiftly lose control of the aeroplane.

The advantage, on the other hand, is that if the takeoff is Vmc limited (think slippery runway) then using a de-rate may actually give us an improved RTOW by reducing V1.

Full thrust (ie no ATM) takeoffs are required on a periodic basis to verify engine performance and may also be required on a contaminated runway.

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Yep, to be more precise, I should have used the term "reduced thrust" rather than "Derated" above.

Regards

 

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On the Airbus, we call it FLEX, short for flexible temperature.The maximum takeoff thrust is limited by outside air temperature (OAT). By assuming a higher temperature than the actual one, the thrust can be reduced...

Quick Example:

  • An A320 at 77 tons (maximum structural takeoff weight) with an OAT of 4 degrees Celsius.
  • The performance limited weight for the OAT is much higher than 77 tons, but for the sake of example, let’s say 83 tons, but I’m making this number up.
  • By calculating the temperature at which the takeoff can be made with maximum thrust at 77 tons, you can reduce the thrust.
  • Lets say this value happens to be 40 degrees (I am also making this number up). Then you have it: FLEX value is 40.
  • During cockpit preparation, pilots will insert this value on the performance FMS page.
  • During takeoff, they will move the thrust levers to the detent marked “FLEX/MCT”. The FLEX thrust will be delivered, regulated by the engines’ computers, called FADECs

This thrust reduction helps reduce fuel consumption, engine stress and maintenance costs.

There are some limitations for the use of FLEX takeoff, but the above explains the principle.

Hope this is helpful....

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Posted (edited)
On 7/12/2018 at 2:09 AM, busdriver said:


On the Airbus, we call it FLEX, short for flexible temperature.The maximum takeoff thrust is limited by outside air temperature (OAT). By assuming a higher temperature than the actual one, the thrust can be reduced...

Quick Example:

  • An A320 at 77 tons (maximum structural takeoff weight) with an OAT of 4 degrees Celsius.
  • The performance limited weight for the OAT is much higher than 77 tons, but for the sake of example, let’s say 83 tons, but I’m making this number up.
  • By calculating the temperature at which the takeoff can be made with maximum thrust at 77 tons, you can reduce the thrust.
  • Lets say this value happens to be 40 degrees (I am also making this number up). Then you have it: FLEX value is 40.
  • During cockpit preparation, pilots will insert this value on the performance FMS page.
  • During takeoff, they will move the thrust levers to the detent marked “FLEX/MCT”. The FLEX thrust will be delivered, regulated by the engines’ computers, called FADECs

This thrust reduction helps reduce fuel consumption, engine stress and maintenance costs.

There are some limitations for the use of FLEX takeoff, but the above explains the principle.

Hope this is helpful....

Nicely explained, Wing.  i also owe you an apology, in that you wrote some interesting notes after I asked you about the US slam-dunk approach, and I didn't respond to say thanks for the good read as usual.  Thanks, and apologies for not replying in that thread!

Fantastic explanation as usual, Simon...you're a real star in our little community!

Back to FLEX/assumed temp - I've heard some operators knock a few degrees of their possible maximum assumed/FLEX temp for a "safety" margin - is this common out there?

Edited by VHOJT

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Posted (edited)
11 hours ago, VHOJT said:

 Nicely explained, Wing.  i also owe you an apology, in that you wrote some interesting notes after I asked you about the US slam-dunk approach, and I didn't respond to say thanks for the good read as usual.  Thanks, and apologies for not replying in that thread!

Fantastic explanation as usual, Simon...you're a real star in our little community!

Back to FLEX/assumed temp - I've heard some operators knock a few degrees of their possible maximum assumed/FLEX temp for a "safety" margin - is this common out there?

 

Hi VHOJT,

it wasnt me who posted such a nicely written explanation, the credit should go to BusDriver. 

 

With regards to the question of “Knocking a few degree off”.

 

In the old days, the airline I am working for now would input a margin of ~1500ft TORA to actual TORA in order to generate a more conservative assumed temperature calculation.

 

Basically if the publisher TORA is 10,000ft our system only takes 8500ft into account. (I forgot the exact margin because it has been a while since we moved to the new practice, may be it was 500ft or 1000ft, because 1500ft seems to be quite a lot) 

 

however a few years ago, the management realised such a TORA reduction safety pad harms the payload for most long haul departure to east coast of North America in summer.

Therefore they removed this padding, and simply just give us a wider weight band of a particular take off performance calculation. 

 

A weight band of a take off performance (RTOW we called it within our airline) is a range of aircraft Gross weight within which the take off performance is valid. 

For example for a performance of let say TO-36deg V1 168 Vr 178 V2 184, the range of aircraft gross weight is (345.0t - 347.2t). Then in case we have an extended ground delay of 1hr as long as your weight falls in that range we can still go without obtaining a new calcution. 

 

That’s how the company increase the de rate of TO thrust by “knocking a few degrees out”.

 

On the airbus side (330/350) our company recently introduce an airbus Flysmart iPad app to the airbus A330 and further reduce to weight band to input TOW +200kg to -500kg. 

 

Such action coupled with increased V2 speed for long TORA further enable an even lower assumed temperature of 72c for some of our newer A330.

 

This is all I know. As pilot we do not have control over how much de rate we can get in general. If we are not happy, TOGA / D TO1 / DTO 2 is always available for us to “hard tuned” on the ACARS performance page in order to get the V speeds for the pilot selected thrust rating (or to check for a given weight if we can Take off using DTO1 / DTO2 ). 

 

Airbus system is simpler as BusDriver pointed out. Only Assumed temperature (FLEX) or DTO take off. Airbus cannot do DTO1-40c (DTO plus assumes temperature). 

 

However the B777 system is more flexible given the wide range of weight the 773ER operates in our fleet. I regularly took off with DTO 2 -30c or more for a short 1hr 20min trip to Tai pei (RCTP)  or Manila (RPLL) on a 210tons 773ER. 

 

Thats why on a light weight 330, sometime with a TOW of around 156tons with Flex 70c Take off the airplane still climbs like a rocket. 

 

 

Edited by Driverab330
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Thanks Rudy - glad to be of assistance!

11 hours ago, VHOJT said:

Back to FLEX/assumed temp - I've heard some operators knock a few degrees of their possible maximum assumed/FLEX temp for a "safety" margin - is this common out there?

It's an interesting discussion because the thing about takeoff performance is that it is a two-way street; although everybody tends to focus on stopping margins and V1 being the maximum speed from which you can stop, it is very important to remember that it is also the minimum speed from which you are able to go with an engine failure. As a result, whilst on most runways there will be more than one way to skin the cat, it is important to take both cases in to account. For instance, it is likely that there will be a range of V1 speeds available on a given runway at a given weight and thrust setting (Wing's airline, it seems goes about it the other way around - providing one speed but a range of weights). You could choose the lowest V1 speed in the range which on the face of it means you are stopping from a lower speed and therefore stopping distance will be reduced... but if the engine goes bang at 1 kt above that speed, you are then going to have a long acceleration to Vr on the one remaining engine and probably a lower net flight path. Further, if you had selected the higher V1 speed it would have been a moot point as you would have been stopping instead of taking the failed engine in to the air... but then you would also have had slightly less runway in front of you to stop on.

What I gather is relatively common is to request figures for a slightly higher takeoff weight than planned, mainly to avoid having to recompute in the event of a last-minute payload change etc, or to get the figures for an intersection departure if anticipated for the same reason.

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On 7/11/2018 at 4:37 PM, Richard McDonald Woods said:

Which airlines normally propose assumed temperature takeoffs, or derated takeoffs or forbid their use? How do they decide?😃

Hi Richard, 

if I well remember, there is generally no derate-take off (TO1 TO2) on Emirates. Emirates pilots will use only TO with max thrust or assumed temperature calculated by the EFB class 3 that equipped most of theirs 777 (but not all if If i'm not wrong).

 

Cheers,

Bruno Thimothe

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36 minutes ago, BrunoT said:

Hi Richard, 

if I well remember, there is generally no derate-take off (TO1 TO2) on Emirates. Emirates pilots will use only TO with max thrust or assumed temperature calculated by the EFB class 3 that equipped most of theirs 777 (but not all if If i'm not wrong).

 

Cheers,

Bruno Thimothe

Yes, seems to be correct until few years ago 

I saw some video somewhere that they started opting for it on new 777 (I think I saw it back in 16) cause maintenance discovered that high soaring sand particles were increasing wear and required more maintenances which then made the idea of not having de-rate options kinda unnecessary..

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As derated thrust means you pretend a smaller engine, let‘s say 24 klbs instead of 26, is this valid until after takeoff like the assumed temp or does this mean you‘re derating your engine for the whole flight? On a Boeing the derate/assumed temp option - when going through the CDU preflight - comes just before the TO-page but not as page2 or sth like the TO PERF page including the FLEX option on Airbuses. So I wonder if a derate actually takes influence on the WHOLE flight? (You can‘t just physically change an engine in the air). 

So a derate would actually decrease a lot more than just your takeoff power, if that would be the case. I have never really tried this out so please forgive my ignorance 😄 I‘ve always used TO and assumed temps.

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25 minutes ago, Ephedrin said:

As derated thrust means you pretend a smaller engine, let‘s say 24 klbs instead of 26, is this valid until after takeoff like the assumed temp or does this mean you‘re derating your engine for the whole flight? On a Boeing the derate/assumed temp option - when going through the CDU preflight - comes just before the TO-page but not as page2 or sth like the TO PERF page including the FLEX option on Airbuses. So I wonder if a derate actually takes influence on the WHOLE flight? (You can‘t just physically change an engine in the air). 

So a derate would actually decrease a lot more than just your takeoff power, if that would be the case. I have never really tried this out so please forgive my ignorance 😄 I‘ve always used TO and assumed temps.

Hi Marc, the derate only affects takeoff power.  Climb and cruise thrust limits are not part of the derate/flex equation.  Same for the B737 TO-B bump, the increase thrust is only available for takeoff, not climb and cruise.

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49 minutes ago, downscc said:

Hi Marc, the derate only affects takeoff power.  Climb and cruise thrust limits are not part of the derate/flex equation.  Same for the B737 TO-B bump, the increase thrust is only available for takeoff, not climb and cruise.

alright, thanks for the clarification. 😃

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Many thanks for your thoughts so far.

I have a feeling that BA used not to use any form of thrust reduction on their original B747s, but I have no evidence for this. Currently, I do not use any thrust reduction either, but I would like to know what are current practices amongst the main widebody airlines if anyone has knowledge.

 

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4 minutes ago, Richard McDonald Woods said:

Many thanks for your thoughts so far.

I have a feeling that BA used not to use any form of thrust reduction on their original B747s, but I have no evidence for this. Currently, I do not use any thrust reduction either, but I would like to know what are current practices amongst the main widebody airlines if anyone has knowledge.

 

BA use assumed temperature thrust reduction on the B744, but not (usually) de-rate.

ATM is not permitted on contaminated runways, runways with degraded braking action, if windshear is ancticipated or if the MEL precludes it. However, in such cases a de-rate may be considered and may offer a higher TOPL. Other situations where a de-rate may be considered could be narrow runways or on three-engine ferry flights.

The B777 (and indeed B767) uses the same centralised performance calculation system so I am fairly sure the same will apply there.

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1 hour ago, Richard McDonald Woods said:

Many thanks for your thoughts so far.

I have a feeling that BA used not to use any form of thrust reduction on their original B747s, but I have no evidence for this. Currently, I do not use any thrust reduction either, but I would like to know what are current practices amongst the main widebody airlines if anyone has knowledge.

 

If you are any where below 320tons on the 773ER, I would recommend using de rate of any form. 

 

The use of De rate in normal circumstance makes your rotation and pitch attitude after airborne fairly consistance for lighter weights. 

 

It is almost “dangerous” to fly a 773ER at 200-250tons with full TOGA thrust with a low initial level off altitude of 3000 to 5000ft, because you will be doing 18+ deg pitch up after airborne and climbing at 3500-4000fpm. And you will see ALT capture at 1000ft. (It’s good fun if you have the airspace to yourself until at least 7000-8000ft). 

 

I used to regularly fly the 773ER at regional weight so we always use DTO2-30c or something like that.

 

the interesting thing I have noticed is that DTO2 -56c has less thrust than CLB2. I did a ferry flight once with an empty airplane at 185tons including around 10tons of fuel. We zoomed up at 3500fpm after airborne even with DTO-56c and when clb 2 is set we actually climbed faster. 

 

 

 

 

 

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10 hours ago, Driverab330 said:

the interesting thing I have noticed is that DTO2 -56c has less thrust than CLB2. I did a ferry flight once with an empty airplane at 185tons including around 10tons of fuel. We zoomed up at 3500fpm after airborne even with DTO-56c and when clb 2 is set we actually climbed faster

Wow

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