Some ques on Boeing-NG

[Robin Jain]

Can anybody clarify me the difference beteween MAx Certified thrust, rated thrust & N1 reference BUG in respect of Boeing -NG/CFM56-7B.

Robin

[Nightrider]

Engines are certified by authorities after passing certain test parameters.
The CFM56-7B is certified for thrust rating of 27,300 lbs (121kN).

The engine can be used in several 737 variants, i.e. 600 / 700 / 800.

Due to the obvious differences in the MTOW the max certified rating is not required on all variants, thus a reduced thrust version is available for them.

The so called "rated thrust" is achieved by simply programming a plug in setting pins in a specific order; this is a maintenance job. The engine frame plate will have to be changed as well to reflect to the actual thrust rating.

The N1 ref bug will show you, if you programmed your FMC correctly, the N1 required to provide the full rated thrust reduced for pressure altitude and ambient temperature.

An interesting feature which is not commonly known is the fact that on some 737 you may be able to get some extra thrust.

Let's assume you have an 800 with a 24k rated engine. The 800 is certified for 26k (the 27k is just a non-standard bump power which is not available to all operators). In case you fire-wall the throttles to the full forward stop, you will get 26k power out of the 24k rated engine, this is called emergency reserve thrust.
However, you cannot get 26k emergency reserve thrust on a 600....the 737 600 is not certified for more than 22k.

Confusion perfect?

Example, ferry flight with an 800, FMC set for derate to 24k and assumpted 50 degrees, after pressing TOGA you are looking at the thrust you have programmed.
Pressing TOGA twice will give you the full 24k thrust.
Firewalling the thrust levers will give you the full 26k......bypassing the rating plug.
This works also with engines which are rated at 24k on a fuselage which is certified for the higher rating of 26k.

The N1 bugs, however, will still show you the computer values for the 24k....and this in both cases!

And this is one of the reasons why some companies do not allow the de-rating of the thrust for take-off, only the assumed temperature procedure is used. This way you will never be confused with the actual N1 output being considerably higher than the N1 ref bug show as limit.
But I guess they did not tell this reason to anyone....

[Robin Jain]

Dear Nightrider.
Thanks for excellent information. But still i ve some confusion left.
In the example you gave, you wrote Engine derate to 24 K by assumed temp method. I understand the engine installed on that aircarft is of rated thrust of 26 K. Then when you will firewall the throttle, why it will bypass the ID Plug/plug rating. and then you are talking about the N1 bug, is it same N1 reference bug. Also you wrote N1 bug will show computer values for 24 K in both cases, which another case are you talking about?


Also give me some info on Bump Rating and if possible give me name of some reference book/notes to go through all this knowledge. I once again thanks you for giving me such wonderful info.

Regards
Robin Jain

[NSEU]

In case you fire-wall the throttles to the full forward stop, you will get 26k power out of the 24k rated engine, this is called emergency reserve thrust.


I'm surprised to hear this also. From what I've read on the pilot forums, this would lead to control issues in engine-out situations :bored:

Why doesn't a rating plug have the same effect on different EEC's? Are the EEC's different?

Thanks.

[Nightrider]

Robin, derate and assumed temp are two different issues.
An airline can order aircraft (let's assume the -800) with 27k, 26k and 24k power rating.
As said, the 27k is just a bump power, not used for every take-off and it has to be selected by the crew with the FMC.
The 26k rating is standard. You will find in the FMC a page with the thrust rating, the crew can opt to use only 24k for take-off; this option is only advisable for the crew if their respective airline allow for it.
The 24k rating is a perfect alternate solution for airlines with very light take-off weights, i.e. minimum catering, no holiday charter, low fuel loads. It applies to almost all low-cost carriers.

The assumed temp does not change your thrust rating, it just tells the computer that your environment is that of a hot summer day and thus reduces the fuel input. Basically that's it.

There has been a discussion recently about combination of de-rating and assumed temp procedures, again, this depends on your airline if they allow it, it is not recommended by most.

Back to the thrust rating. The -800 is certified for 26k thrust rating (that includes the 27k bump power). The engine has a rating plug with pins and the engineers set these pins for the rating your company has opted for. Since it is the very same engine for 24k as it is for 26k, this plug decides about the available standard thrust for this engine.
However, this is just the standard as the autothrottle will use it commanded by the computers. In case of fire-walling the throttles the pins for the standard setting in the plug are bypassed and gives you the fully certified thrust for this airframe, the mentioned emergency reserve thrust.

NSEU

as far as I understand the plug has two settings. One setting is the ordered thrust rating, as per the airline, the other setting is the airframe max rating.
The emergency reserve thrust allows for the airframe max rating, whatever this may be.
This feature makes the engines available to several different airframe versions and just needs the correct plug setting for an airframe change.
There should not be any safety concern as you will never get more thrust as has been certified for this particular airframe as long as engineers use the correct plug setting. Why should there be any concern at all, if an airline opts for 24k on the 800 and you need the 26k for evasive maneuver, there are other airlines which operate the aircraft with exactly this power in standard configuration?

Hope this helps