View Full Version : 737NG FMC Descent Energy Management
Just transferring onto the 737NG. This happened to me a couple of days back during Training, and want to understand the tech details/parameters of the following:-
When in a VNAV descent and leveling off at an intermediate descent level, due to ATC I now go 'high' on FMC profile. At a point somewhere shortly after, the FMC computed commanded speed bug on the PFD drove back towards Flap Up speed. Then when cleared for descent, from above profile, the aircraft accelerated to try and regain profile, and the speed bug incrementally jumped back to FMC selected descent speed.
All fairly simple in airmanship terms in what one might want to achieve in terms of energy management when running high, however I can't find any FMC systems description for this automatically happening. Can any one enlighten me, perhaps with suitable reference, of exactly at what point the FMC reverts into this energy conservation measure, and the parameters it applies?
Many thanks
30W
Not sure 'exactly' when it does it but the 'parameters' are what you saw and what an old-fashioned pilot would have done without an FMC. Back to min clean (that's as slow as you should go to 'adjust' total energy) and then increase speed when able to regain the profile. Beware that while the 'bug' may return to FMC speed the aircraft may well not and may dive for the ground and try to exceed Vmo/Mmo
Intervention is often needed and a quick glance at excess height, height/distance to go etc plus some mental arithmetic will determine whether you need speedbrake or not (but often not despite what the CDU tells you - it uses a very crude algorithm) and VS/LVL CHG would almost certainly be your best options.
BOAC,
Yes many thanks, certainly I understand what we are trying to achieve, and indeed how we can achieve via other non VNAV methodology. I've got 15,000hrs on other Boeings, where the FMC did NOT drive energy conservation management automatically once constrained above profile.
What I DO want to understand and have knowledge of however is the exact parameters the FMC uses to initiate and conduct this. If it does it, there SHOULD be a system description of it somewhere out there - I just can't find it.
I never like accepting 'thats what it does', without understanding the parameters etc that trigger it, so it's a quest for further in depth info if anyone can provide some?
Thanks in advance....
OK - sorry! I did say I do not know exactly, but maybe CaptainS&L will pass by with more knowledge. I have just accepted it from the -300 onwards and never felt 'the need to know':)
Beware that while the 'bug' may return to FMC speed the aircraft may well not and may dive for the ground and try to exceed Vmo/Mmo
In order to avoid it select vnav speed instead of vnav path on your descent page each time you are above your flight path.
Denti
15th Dec 2011, 11:42
Which would simply descent on the preprogrammed descent speed, usually parallel and above the descent path. Easier to use speed intervention or level change.
Thanks for the input folks. Found this:-
The FMC uses a special program called “Energy Compensation” at certain times during an ACT PATH DES. This program goes into effect when the MCP has been temporarily set to an altitude above the planned descent path. The airspeed cursor will slowly move toward a slower airspeed while the “TARGET” speed on the FMC remains constant. The airspeed reduction improves the capability of recapturing the planned descent path. When the airplane is cleared to resume the descent, the airspeed will slowly build up to the FMC target speed as the airplane recaptures the planned descent path.
Understand the principle, and indeed how one might achieve desired goals through differnt modes etc.
After a guru who understands exactly WHAT slower speed? Flap Up speed (not displayed above 20,000' of course!) or something different?
At what point does the logic 'trigger' because it didn't happen straight away, but at some point afterwards?
Cheers
Regarding 'displayed' speeds, just because it is not displayed it 'knows' UP is Vref 40 +70.