GE-90 Lock and Lapse-what is it?

[JammedStab]

I think it is some sort of EEC software for the -115BL engine.

[tdracer]

Jammed
I'm going to do this pretty top level, let me know if it's not clear (as if you wouldn't :))
Lock and Lapse is pretty much standard on all modern FADEC - Pratt started it first on the initial PW4000, GE adopted it on the GE90-115B (GE also uses it on the GEnx on the 787 and 747-8).
What lock and lapse does is 'lock' the takeoff power setting, then 'lapse' the N1/EPR power set with altitude/airspeed in a prescribed manner. As a result, say you initiate takeoff from sea level, lock and lapse will determine the N1 (or EPR) that you'll see at 2k pressure altitude - which may not be the same N1 that you'd get at that throttle setting if you'd initiated the takeoff from 2k pressure altitude.
While not necessarily obvious if you've not worked it, lock and lapse makes the power setting and limits analysis much easier to define and analyze across the takeoff envelope. It's particularly helpful for analyzing assumed temp derated takeoffs where the throttle set is well below full rated takeoff - without lock and lapse you need to go through all the various altitude/airspeed power setting tables to make sure you get at least as much thrust as you would at the assumed temperature - with lock and lapse the analysis becomes trivial...
Does that help?

[JammedStab]

Thanks,

I appreciate the details which is nice for the technical guys but an alternative dummies explanation of what you posted above would be appreciated as well.

Is there any difference from a pilots point of view between the earlier GE-90's and the -115B?

[tdracer]

I'm afraid I can't dumb it down much more than this statement:
What lock and lapse does is 'lock' the takeoff power setting, then 'lapse' the N1/EPR power set with altitude/airspeed in a prescribed manner. As a result, say you initiate takeoff from sea level, lock and lapse will determine the N1 (or EPR) that you'll see at 2k pressure altitude - which may not be the same N1 that you'd get at that throttle setting if you'd initiated the takeoff from 2k pressure altitude.
As for the original GE90 and -115B, I was involved in the original GE90 and I'm reasonably sure we didn't have lock and lapse - that was implemented on the -115B (no significant involvement). I presume it was to address some of the issues I referenced in the earlier post. Among other things, lock and lapse makes it easier to tailor the thrust lapse to give better limit margin for EGT and the high rotor speed - due to the transient dynamics of a big turbofan engine, EGT and N2 typically peak about 120 seconds after power set then slowly drop, so by lapsing the N1/EPR correctly you can get better limit margins (again, not directly involved so it's conjecture about why they used in on the -115B).
For the GEnx, we copied a lot of the FADEC logic directly from the -115B - including lock and lapse (we copied the -115B so directly that we actually uncovered errors in the -115B logic that had gone previously unnoticed :uhoh:). As I was directly involved in the GEnx-2B on the 747-8 (and lacking significant experience with lock and lapse), I was able to borrow expertise from the 777 group to help out with the lock and lapse implementation.

[JammedStab]

As for the original GE90 and -115B, I was involved in the original GE90 and I'm reasonably sure we didn't have lock and lapse - that was implemented on the -115B (no significant involvement).

Thanks,

There seems to be a GE-90-115B and a -115BL(which I assume is the Lock and Lapse).

This comes from revision highlights to our QRH which mention two Performance Packages (10 and 20) which equate to the respective models in the previous paragraph.

[DaveReidUK]

Your assumption is correct.

[JammedStab]

And is shown to the pilot as being installed by looking at the Ident page of the FMC.