B737NG blank N1 reference reading

[Levelmind]

Hi,

For a long time I've only every learn about the 737 with
Double Derate, however recently I started looking in to the version without Double Derate, at which it came to my attention that this version does not have any N1 reference readings on the engine display. Also, I've noticed that if assumed temp is used (R-TO + assumed temp), the actual t/o thrust on takeoff, for example, will be set below "the green arrow". And nor does it have the "orange/yellow" limit indication.

After a thorough search I've not found any answer to why this is, only, as stated in the FCOM (2, Chapter 7), that this is this case, but not why!
For some reason I seem to tell myself that it has something to do with un-/aspirated TAT, however I might have completely mixed that up.


Simply, put, what is the reason that 737's without Double Derate do not ever show a N1 reference reading, and why R-TO + assumed temp, applies a thrust setting less than the green reference arrow?



Thanks in advance!

[FlyingStone]

Double derate is a customer option on the B737NG. Without it, the engine indication looks very similar to the one on the Classic (no target N1 displayed, green needle showing full derate).

You'd probably have to ask Boeing for the exact reason why, but here is my understanding. If you take off with derate + assumed temperature on a Double Derate aircraft, your takeoff thrust setting will be marked with a green needle and on start of take off roll, the engine thrust should match it. On the non-Double Derate aircraft, the actual takeoff thrust setting will be below the green needle, as green needle will indicate the full derate in this case (let's say 22K).

If you have an engine failure on takeoff and even though it is normally not required, you would like additional thrust, you can always go to the full derate thrust, as the Vmcg/Vmca (and thus V1/V2) is based on that. On a non-double derate aircraft, that simply means pushing the thrust levers to the green needle (showing full 22K), but on a double derate aircraft you don't see directly where the full 22K rating is. You only see the derate + assumed (green needle) or the full rated thrust (yellow line). Therefore, Boeing requires (ref FCTM) that operators with double derate aircraft recommends against increasing the thrust in case of engine failure, unless operator has developed a procedure which makes the full derate thrust setting immediately available to the crew.

Hope that helps.