ATM + Derate 737

[airyana]

when using a combination of fixed derate and assumed tepmerature reduced thrust how can I determine the best combination ?

I've always assumed that (B737NG in mind) derate 2 plus ATM would provide the max savings, but shouldn't that be reflected in the takeoff N1 being reduced ?
most of the time a no derate ATM takeoff is very close to a derate 2 + ATM N1 value .... how is that possible ?

so a more concise question: when using ATM (Flex) as a method for reduced takeoff power, is the N1 value that results the only indication of how effective it is ?

[c100driver]

Depending on the engine contract it is usually the derate that has the most advantage in engine life costs to the operator. Often the number of derate takeoffs will give an engine credit on lifed items. The ATM reduction will reduce engine rebuild cost but may not give engine life credits.

[B737-pilot]

Derate implies a lower Vmca is one of the factors to keep in mind when using it.
As far as i know you can get the same thrust reduction in two ways ( i am exagerating a little bit), for example 26K rating with 50 deg and ATM = 24k with no ATM as long as you the resulting N1 has the same value. The thrust delivered by CFM engine is a direct proporcional with N1, 3% N1 reduction = aprox 10% thrust reduction.
Regarding the limited life parts i have no ideea.

[airyana]

just to clarify one point:

Assume the full rated thrust is 26k and derate 1 is 24k.

If I pick derate 1 and the resulting N1 is 90%

that is 90% N1 of 24K engine

and compare it to a no derate ATM N1. this N1 will be a percentage N1 of 26K ...

so if I get 90% N1 using these two different thrust reduction methods, each 90% is going to effect the engine in a different way ? even I am confused reading this

is that how it works ?

[B737-pilot]

Well....
The CFM 56-7B , regardless of the rating has the same core; the only difference is in the software (chip setting ).
As i said before thrust is direct proporcional to N1 ( you read the RPM as a percentage of max RPM).
I used to fly 24K and 26K , as arule o thumb, in the 24K engine had lower N1 even at full rated thrust than the 26K.
So if you use derate or ATM and the resulting N1 is the same, the engines will produce the same thrust, regardlees of how you arrived to that T/O N1. I hope it makes sense.

[barit1]

Actually, the thrust is not strictly proportional to N1. Generally, 1% N1 change at TO means about 2 or 3% thrust change. Thus for a 25% TO thrust reduction (max permitted under ATM rules), about 8 - 12% N1 reduction is seen.

Also remember that the first few % is - by far - the most significant effect on engine life. So once you pass a 5% N1 reduction, any further economy is minimal, and in fact the increased fuel burn (lower ROC, longer ops at low altitude) may eat up the engine life savings.

So don't dwell exclusively on engine life, without considering other factors.

[decurion]

Hi barit1,
Your statement that the first few % is - by far - the most significant effect on engine life is not supported by analysis from GE and SNECMA. Further reduction will significantly decreases maintenance costs, and improves reliability. You can find a good article on this topic by SNECMA and GE which was presented at the Boeing Flight Ops Conference in October. “ Reduced Thrust Takeoff— Proven Benefit, Lowering Engine Maintenance Cost”. You can find it on myboeingfleet.

[mutt]

Your statement that the first few % is - by far - the most significant effect on engine life

Look at that presentation again, the statement was that the first few % is by far the most significant.... Look at slide 8, you will see that the savings become horizontal as the thrust reduction is increased.

Also if you look at slide 12, a 5% thrust reduction will give you a $22,000 saving, whereas a 20% thrust reduction will not give you $88,000 of savings, so the first % is more significant......

Rolls once told us that a .01 EPR reduction would result in a 12% increase in blade life with the associated savings, however NONE of the engine manufacturers can provide us with proper and justifiable savings associated with operating with deep derates.... ie, what is the advantage of trying to average 50% reduction rather than 40%.

Mutt

[barit1]

Since I'm retired, I don't have access to myboeingfleet.boeing.com so I don't have any idea how CFMI supports their position. But in any case it's at odds with the relationship between thermodynamics and metallurgy that I was immersed in for a few decades.

The rule of thumb I recall is that a 10 C drop in EGT doubled HPT parts life, and this was exponential; 20 C was worth 4x parts life.

EDIT: Thanks, Mutt, for the clarification.

[latetonite]

It is indeed the temperatures that are significant at the end of the day. So for the B737 the EGT is the mirror indicator. Not the N1`s.

[decurion]

Not sure if we are looking at the same things. Here is a table from that presentation:

Standard Body Airplane

CFM56-7B26

Average 5% reduction from full thrust
$/Year/Engine

22,000 - 26,000

Average 10% reduction from full thrust
$/Year/Engine

40,000–49,000

Average 15% reduction from full thrust
$/Year/Engine

55,000–68,000

Average 20% reduction from full thrust
$/Year/Engine

69,000–84,000

There is no flattening to be noticed in this table. Slide 9 shows a start of some flattening at around 40% reduced thrust. Please note these are maintenance cost reductions.

[Mad (Flt) Scientist]

Looks like it flattens to me. If you take the average values at 5% and 10%, and extrapolate, it actually ends up higher than the maximum value at 20%.

Hope the graph works:

[decurion]

Flattening (to me) means that there is no real change in maintenance cost savings with increasing thrust reduction. Clearly the numbers don’t show such an effect for the range of thrust reductions considered here. At thrust reductions of 40% the GE/SCNEMA paper indicates the start of flattening of the relation between reduction and cost savings for GE90 engines. Don’t have this information for the CFM56 engines.
The "high" and "low" values are the differences between harsh environment conditions and typical (normal) conditions.

[have another coffee]

It must be flattening as a result of the way it is presented. 99% derate will not give you (almost) zero maintenance cost...

Can't be bothered to write out the maths, but what about the maintenance savings when using ATM in combination with a conveyer belt?