737 ng FMC
 

Does anyone know why sometimes the FMC in Path descent commands the flaps speed for no apparent reason.(update 10.5) , nobody ive asked seems to know.
Ive always thought its due to speed or level restrictions in the Legs page but it still doesnt make sense.Sometimes it wants to bring the speed back to 200 odd knots in the descent 100 miles out. Any ideas?

"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."


V2 ch11.31 FMS Operation/VNAV

...so your groundspeed's slower and you cover less ground while high on the path, makes recapturing it a bit easier. ATC don't like it much though :ok:

I know it does that alright but thats not the scenario im talking about.

Im talking about when the airspeed cursor goes straight to the Flaps speed when on profile as commanded by the fmc.
ie on the descent page , the speed ,where would be normally be the econ descent speed or whatever youve asked for goes to "FLAPS"

Perhaps, it's because you're on the gradient path (GP) descent leg, as opposed to an idle thrust descent leg. i.e. from at least the FAF inbound to the runway.

We've spotted this many times. It occurs earlier in the descent, and you suddenly spot the speed cursor under VNAV slowly and inexorably reducing, for absolutely no identifiable reason. The only thing to do is put it in LVL CHNG or VS and take control of speed yourself. I think there is a bug somewhere. No explanation.

Update 10.7, same story.

FB

Great question. I am a trainer myself and would be delighted if anyone can throw any light on this issue. Regards to all.

Yes, we're on Update 10.7 as well. I see it frequently and have never been able to identify any possible cause. It doesn't matter whether you are high or low on the descent profile- it can just suddenly motoring gently downwards for no conceivable reason.

I believe U11 will sort it out. Happy New Year!

It sound like the machine wants to set you up for an approach. Do you get the green decel doughnuts at speed redistricted altitude ie 10.000 on your magenta track.... if you do not that's a sure sign the airplane went into approach mode....

b737ng fmc
 

hello all,

maybe the answer lies in Mr. boeing fcom vol 2. page 11.43.3, april 24, 2009.
descent page.
quote : "Speed restrictions (spd rest) : displays the most restrictive of the following speeds : - destination airport speed minus 10kts. - waypoint speed restriction if greater than minimum flaps up maneuvering speed. - minimum flaps up maneuvering speed. - selected vref + wind correction for landing flap setting. - whenever flaps are extended, the appropriate flap speed shall be displayed as xxx/flaps. this shall supersede any other speed restriction. - display xxx/hold when decelerating to hold speed prior to hold entry fix. dash prompts displayed when there is no active speed restriction. manual crew entries or deletions may be made. HOLD or FLAPS speed may not be deleted or modified." end of quote. merry xmas to all, bm
:ugh:

Part of the explanation could be this:
When you are in VNAV PATH (not VNAV SPD), the A/Ps priority will, of course, then be to follow the PATH. If you for some reason end up above this path, the FMC commands a low speed, e.g. 210 or 235 in order to ensure that the thrust levers remain at idle. Why? The FMC "knows" that you must fly at a higher speed in order to follow/ catch up with your path. Since it is not desirable to use more thrust to achieve this higher speed, the FMC commands a LOWER speed to prevent the A/T from adding thrust. Given that you are in PATH mode, the A/P will then naturally have to dive for the path. As you catch up with your path, the FMC will gradually command a higher speed until once again at you pre-selected descend speed. You could go to the descend page and select SPEED i.s.o. PATH if you really wanna follow a (ATC?) given spd.
Does that explain it? :)

No, it happens on profile, and even happens below profile. I can see no reasonable explanation of it.

Does this occur when going direct to the FAF or the runway?

The FMC extends the GP leg associated with an approach from the FAF all the way up to altitude ... but this only occurs if you loaded an approach and then programmed direct to the FAF.

I've seen this happen when the complete arrival has not been put in the FMC, I believe it can occur under these circumstances when the a/c reached TOD with no arrival placed in and the default then is speed descent mode, with deceleration based on Vref rather than a physical point it works backwards to determine when it should slow down... but I can't recall seeing this at high level. If anyone has the answer, we would like to know.