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737 Take off Thrust Question

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737 Take off Thrust Question

Old 2nd Oct 2020, 18:03
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737 Take off Thrust Question

Hi all, Type Rating on B738 due next year. Fleet fitted with CFM56-7B26.

I am having trouble deciphering this paragraph:

"The full rated takeoff thrust for the installed engine is available at a thrust level position less than the forward stop. Fixed or assumed temperature derated takeoff thrust ratings are set at thrust level positions less than full rated take off. The maximum rated thrust is available at the forward stop"

I am unsure what 'full rated take off thrust is'

My question is what is full rated take off thrust? It's not the maximum the engine is rated produce, and it's not the derated/ATM maximum, it is somewhere between but what exactly?

Thank you in advance.
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Old 2nd Oct 2020, 18:19
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Full rated thrust of 26K is set at a thrust lever angle, when you apply a derate of 24K or 22K the thrust lever angle is less than 26K. If you use assumed temperature as well (FLEX on Airbus?) the thrust lever angle is even lower for each of these ratings.
The maximum rated thrust is the maximum thrust the engine is capable of delivering with the thrust lever at the forward stop and you would use it only if terrain contact is imminent in the GPWS and Windshear escape maneuvers. And I would guess it's also more than 27K bump.

On the latest models, the full rated thrust is indicated by an amber line in the Primary Engine Indications on the Upper Display Unit. This amber line is also the limit for autothrottle operation.
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Old 2nd Oct 2020, 20:48
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Originally Posted by Banana Joe View Post
The maximum rated thrust is the maximum thrust the engine is capable of delivering with the thrust lever at the forward stop and you would use it only if terrain contact is imminent in the GPWS and Windshear escape maneuvers. And I would guess it's also more than 27K bump.
Not quite. Max rated thrust (for the ambient conditions) is achieved with the thrust lever ~2 degrees short of the forward stop. In those last two degrees of throttle travel, you get what is known as 'headroom' - basically limited overboost. Headroom is to account for various tolerances and to make sure you can always get full rated thrust. It depends on the specific conditions, but headroom is usually between about 0.5% and 1.0% N1 (figure roughly twice that amount of thrust).
The max rating available for the CFM56-7 on the 737-800 is less than the max rated thrust the engine is capable of (used on the -900ER). On the shorter 737-6/7/800 models, the max ratings are lower to account for the ability of the tail to counter the thrust pitch-up (shorter aircraft means less moment arm for the tail surfaces). On the non-FADEC 737-300/500, this thrust limit was via a physical block in the throttle - on the FADEC NG this is done with a rating plug on the FADEC.
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Old 2nd Oct 2020, 21:27
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Thanks guys, but I’m still curious, What is the difference between ‘full rated take off thrust’ and ‘maximum rated thrust’ where FRTT isn’t a derate or ATM value?
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Old 2nd Oct 2020, 22:03
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http://www.b737.org.uk/powerplant.htm#Engine_Ratings

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Old 2nd Oct 2020, 22:07
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Originally Posted by tdracer View Post
Not quite. Max rated thrust (for the ambient conditions) is achieved with the thrust lever ~2 degrees short of the forward stop. In those last two degrees of throttle travel, you get what is known as 'headroom' - basically limited overboost. Headroom is to account for various tolerances and to make sure you can always get full rated thrust. It depends on the specific conditions, but headroom is usually between about 0.5% and 1.0% N1 (figure roughly twice that amount of thrust).
The max rating available for the CFM56-7 on the 737-800 is less than the max rated thrust the engine is capable of (used on the -900ER). On the shorter 737-6/7/800 models, the max ratings are lower to account for the ability of the tail to counter the thrust pitch-up (shorter aircraft means less moment arm for the tail surfaces). On the non-FADEC 737-300/500, this thrust limit was via a physical block in the throttle - on the FADEC NG this is done with a rating plug on the FADEC.
Correct about the Classic, which is what I fly at the moment. My experience on the NG variant is still limited.

And according to the 737 technical site, the forward stop - or just short of it - is actually 27K bump, available as an option.
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Old 2nd Oct 2020, 22:09
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Thanks, I’ve seen this but it doesn’t clear up what is meant by ‘full rated take off thrust’
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Old 2nd Oct 2020, 22:35
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Unfortunately terms can get confusing, also between the FCOM and the FCTM sometimes and I admit I might have had a brainfart.
Hopefully this image will clear your doubts. It's a screenshot from the well known 737 handbook app.
If you derate your engine to 24K the position would be less than 26K.


The FCOM goes on to say that if the EEC operate in alternate mode, you would get 26K thrust before the forward stop (or earlier than 2 degrees before the stop).
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Old 2nd Oct 2020, 22:40
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Originally Posted by course_profile View Post
Thanks, I’ve seen this but it doesn’t clear up what is meant by ‘full rated take off thrust’
'Full rated takeoff thrust' is exactly that - the full rated thrust. Anything above that is technically an overboost. As I noted, you can get a little more than that due to the designed in headroom. You're not supposed to use the headroom except in an emergency.
If you're operating in Alternate mode, you can potentially get a lot of overboost - especially on a cold day. Again, you're not supposed to go there.
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Old 2nd Oct 2020, 22:49
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My FCTM (rev. 18) does not use the term max rated thrust, but only full rated thrust.
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Old 2nd Oct 2020, 23:18
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Firewall an NG - what do you get
B737 NG Certified/Rated/Derated thrust

A couple of old threads that might help.
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Old 3rd Oct 2020, 11:05
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These replies are really appreciated. Thank you.

The quoted paragraph is from the company Part B technical manual - not the FCTM, so it may confusion with terminology. It suggests there is a position between max rated and the derated position, but having read these replies I think that might be a mistake.
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Old 3rd Oct 2020, 11:21
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My FCOM (Part B) has the same wording as yours, but the FCTM does not use the term "max rated thrust".

I can see the confusion.
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Old 4th Oct 2020, 05:45
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“Rated thrust” is what you paid for, 24k or 26k etc and occurs at a thrust lever angle less than full forward.

“Maximum thrust” is with the levers at the full forward stop, 27.2k
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Old 4th Oct 2020, 07:11
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Originally Posted by Switchbait View Post
“Rated thrust” is what you paid for, 24k or 26k etc and occurs at a thrust lever angle less than full forward.

“Maximum thrust” is with the levers at the full forward stop, 27.2k
No, No, No.
There is a max rating based on what rating plug is installed on the FADEC. You can only get 27.2k if that's the rating plug on the FADEC, no mater how far forward you push the throttle (assuming Normal mode - all bets off in Alternate mode).
Read the thread...
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Old 4th Oct 2020, 07:48
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Just to be sure on something, since I am mostly on the Classic.
The amber line on the NG with double derate option moves to indicate whatever derate is selected in the FMC. That is, on a 26K engine if 22K is selected - with or without ATM - the amber line indicates the N1 value to get 22K of thrust and not 26K.

It's been too long I've been in a NG sim. Is it correct?
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Old 4th Oct 2020, 17:33
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This is from the Boeing website

"Depending on the airplane-engine model combination, extra performance-reserve thrust may be available for emergency use during takeoff and go-around. For example, performance-reserve thrust is available for a 737-700 with -7B22 engines, since the -700 airplane can accept the higher -7B24 thrust. The engine control allows takeoff/go-around thrust up to this rating when the thrust lever is pushed full forward. If the installed engine has the highest rating offered for that 737 model (for instance, a 737-600 with the -7B22 rating), there is no performance-reserve capability. Like the "overboost" thrust of the 737-100/-200/-300/-400/-500, performance-reserve thrust is for emergency use only."

https://www.boeing.com/commercial/ae...y/sy01txt.html


Having read this I believe 'Max Rated Thrust' means the maximum thrust available on the model of 737NG you are flying. So 27,300lb for every single -800 at the full stop, 24,200lb for a -700 and so on.

'Full Rated Takeoff Thrust' means the highest rated thrust for the model of CFM-56 your individual aircraft is fitted with, which should be the value shown on the IDENT page. This may be 27,300lb for a -800/-900 but may well be lower. For an aircraft with -7B26s it should be 26,400lb - and you will get that value at a TLA short of the full stop.

Obviously, from a planning perspective the extra thrust available is no good to you as you can only plan for 'full rated take off thrust' and the A/T won't give you any more - however I think I have a better idea of what the engines will actually do for me if I ask them.

Any thoughts?
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Old 4th Oct 2020, 17:41
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This exactly what I had intended to say in my first post.. Shorter models have lower allowable thrust due to pitch-thrust couple.

A 737-800 with CFM56-7B26 engines at firewall will actually give you 27,3K of thrust. I stand to be corrected, but the BBJ is limited to 26K.

Last edited by Banana Joe; 4th Oct 2020 at 18:02.
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Old 5th Oct 2020, 03:30
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Originally Posted by tdracer View Post
No, No, No.
There is a max rating based on what rating plug is installed on the FADEC. You can only get 27.2k if that's the rating plug on the FADEC, no mater how far forward you push the throttle (assuming Normal mode - all bets off in Alternate mode).
Read the thread...
TD: The 737NG is different from the Everett airplanes. It has a thrust lever position (can't remember if it's detented, but I think it is) at which the purchased thrust rating is commanded. Several degrees above that thrust lever position is another position just short of the lever stop at which the maximum thrust approved for that airplane minor model (-600, -700, -800, etc.) based on Vmc testing will be commanded. This extra thrust, when available, is referred to as performance-reserve thrust. If the operator has purchased the maximum thrust approved for the airplane model, that region of lever travel will result in no additional thrust (unless you are in alternate mode, in which case you can overthrust the engine). Boeing designed it that way to provide operators with additional thrust for use in an emergency if the crew decides it is needed on airplanes on which the higher thrust rating is approved, but was not purchased. All of this is covered (except my inability to recollect if a detent exists at the purchased thrust position) in the engine control system description document prepared by your former counterparts in Renton, and it's covered in the FCOM.
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Old 12th Oct 2020, 07:39
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Originally Posted by tdracer View Post
No, No, No.
There is a max rating based on what rating plug is installed on the FADEC. You can only get 27.2k if that's the rating plug on the FADEC, no mater how far forward you push the throttle (assuming Normal mode - all bets off in Alternate mode).
Read the thread...
my information is correct for all CFM56-7B24, 7B26, 7B27 fitted to B737-800. Only I incorrectly quoted 27.2k, when I should have said 27.3k....

27.3k thrust will occur when the thrust levers are moved to the maximum TRA of 82 degrees.(the full forward position)
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