737 - Combining derate and assumed temp?
 

Assuming that your operator allows for the use of an assumed temperature, with a derate. How do you determine which combination to use?

For example:

Let's say that the OAT is 0 ºC and after consulting the RTOW tables, you determine that the following are possible:

Full rated - Assumed temp=44 ºC
24K (TO-1) - Assumed temp=36 ºC
22K (TO-2) - Assumed temp=15 ºC

How do you know which combination will be the most efficient, or will provide the least amount of engine wear?

The one with the lowest N1.

The lowest possible N1 for the given conditions, runway and with the optimal flap setting.

To be honest it is too many possible combinations to do it manually so either use a few selected ones or some EFB performance program.

Doesn't matter to us which is the most efficient s just see if you can get airborne by using, in order:
- 22k TAss
- 22k
- 24k TAss
- 24k

You should find that all of those combinations come to a very similar, if not the same n1 values.

The aeroplane needs x amount of thrust to do the job, any one of those combinations (if correct) should do it. Therefore the fan will be spinning at the same (or very similar) speed.

As to which one actually is the best for costs etc, that would depend on the operators individual maintenance programme etc. I.e Your operation may be penalised for using 26k as opposed to 24k for example.

The restriction may turn out to be a contaminated runway. In that case FLEX (ATM) may NOT be used; So use the lowest possible rating (26, 24, 22...) w/o reducing per FLEX.

From a powerplant engineering POV, ANY reduction in N1 (thus EGT) is beneficial, but the first few degrees EGT are the most benefit.

In TO planning terms, the first five degrees of ATM do the most good for hot section life. So if there are any compromises to be made, go for a TFLEX of ISA+20 if possible, and don't worry too much about a higher assumed temp.:ok:

22K & 15c is not a reduction. There is not thrust reduction until above 30c = ISA +15.
Indeed there are authorities that do not allow derates; everything is FULL Power e.g. 26K and then assumed T. I'd always assumed that the power setting calculated had to satisfy the take off profile parameters. It could be derived in many ways. 26K 44c should be within a gnats cock of 24K 36c. Both should be the minimum required to satisfy the legal requirements. I assume and have the faith.

Indeed,minimum assumed temperature.

EFB Tells us !!
 

Lower engine temp = longer life

Does the 737 fixed TO derate also determine the subsequent climb derate (as it does on my 767)? If it does then it's worth going for the biggest fixed derate as it will also save engine life in the climb.

It does, however it is possible to select a higher climb thrust setting and according to CFM there is nearly no impact on engine life doing that, on the other hand it saves some fuel and depending on maintenance contract it could be better to use full rated climb thrust after fully derated/ATM take off to save more money that way. Of course that only makes sense if an unrestricted climb is likely.