For some reason I can't get the vnav descent to work properly.  Not too long after starting the descent the speed target drops to my vref speed even with the fmc programmed correctly even though I'm still something like 80nm out.  I have to manually override the speed on descent.  It's almost as if it's skipping the waypoints in between and going directly to the speed that's in the fmc at the IF on the ILS.  Is this a known bug or am I just missing something?  I've triple checked everything and there shouldn't be any issues.  No other plane has this issue for me.  
 

edit: should specify I’m on the beta right now so not sure if that has anything to do with it. 

Edited March 2, 20242 yr by sfgiants13

B787 Horizon or Kuro have VNAV problems for me too, either loosing ALT or SPD most of the time not both together as expected from FMC settings or you have to use SPDBRK permanently wich is not realistic. Since there is no solution I'm using V/S instead, is working fine with the help of "green banana" or ALTdiff/300 = range in NM to destination. Best regards

It's Working Title problem, they are behind the VNAV. I get the same bug too when doing direct-tos (at least sometimes).

3 hours ago, cseu said:

or you have to use SPDBRK permanently wich is not realistic

You’d be surprised 😉

Just out of curiosity, what is happening with the flaps? I know on the 747, as you get close in, when you deploy flaps, VNAV will reduce speed to the minimum flap speed for the flaps you have selected. So VNAV slows the aircraft down further each time you lower more flaps. But the speed won't go below the flap speed, not until you put out more flaps, anyway.

Of course, you're saying VNAV slows the aircraft to Vref 80 nm out. That would stall the aircraft if the flaps are up, wouldn't it? And you wouldn't have any flaps out that far, right?

So just to clarify, 80 nm away from the airport, flaps are up, and VNAV slows to Vref? Again, wouldn't that lead to a stall?

(Caveat: I don't know anything about the 787.)

10 minutes ago, prolixindec said:

 

(Caveat: I don't know anything about the 787.)

Jon b is a man of the hour. He knows everything about 787! If I were you I would bug him with all 787 questions 😉

3 hours ago, prolixindec said:

Just out of curiosity, what is happening with the flaps? I know on the 747, as you get close in, when you deploy flaps, VNAV will reduce speed to the minimum flap speed for the flaps you have selected. So VNAV slows the aircraft down further each time you lower more flaps. But the speed won't go below the flap speed, not until you put out more flaps, anyway.

Of course, you're saying VNAV slows the aircraft to Vref 80 nm out. That would stall the aircraft if the flaps are up, wouldn't it? And you wouldn't have any flaps out that far, right?

So just to clarify, 80 nm away from the airport, flaps are up, and VNAV slows to Vref? Again, wouldn't that lead to a stall?

(Caveat: I don't know anything about the 787.)

Normally yes. But I catch it before it gets to that point. Obviously no flaps yet. 

9 hours ago, Fiorentoni said:

It's Working Title problem, they are behind the VNAV.

Not necessarily here. The performance data for the airframe and the flight model itself must be properly filled in by the airframe developer.

If the various Cd values are not correct in the performance data, or they are indeed matching the FM but the FM is too slippery or the idle thrust is too high, the computer will think it needs to start slowing to the next speed restriction far too far in advance, because technically it does need to.

There's a lot to it, as the computer is running an integral simulation of the whole flight using the real aerodynamic and engine model data to produce a speed trend and path solution.

9 hours ago, sd_flyer said:

Jon b is a man of the hour. He knows everything about 787! If I were you I would bug him with all 787 questions 😉

I really don’t ! 
The 747-400 maybe, I had built up a lot of technical knowledge over the years. I can’t seem to get really interested in the 787 too much.

What I do know though is almost guaranteed every flight around 15 seconds after it starts its carefully calculated VNAV descent it’ll start bleating with a DRAG REQUIRED -unable to maintain VNAV speed use speed brakes as required FMCmessage.

2 hours ago, MattNischan said:

Not necessarily here. The performance data for the airframe and the flight model itself must be properly filled in by the airframe developer.

If the various Cd values are not correct in the performance data, or they are indeed matching the FM but the FM is too slippery or the idle thrust is too high, the computer will think it needs to start slowing to the next speed restriction far too far in advance, because technically it does need to.

There's a lot to it, as the computer is running an integral simulation of the whole flight using the real aerodynamic and engine model data to produce a speed trend and path solution.

You might have not read correctly what was the discussed problem here, because VNAV setting Vref as speed target during initial descent is not a performance data related problem. It's a bug in the code.

If the vertical mode is really VNAV, and if the commanded airspeed will result in a stall (because Vref is lower than the clean wing stall speed), then it's hard to see how that's not a bug.

It would be interesting to see a video. Focus in only on the PFD so everything is clearly legible (forget the outside visuals, they're irrelevant here), fly past the T/D point, and let's watch. Don't speed intervene, let's see if a stall develops.

In 747 logic, you'd be in VNAV SPD or VNAV PATH, depending on what thrust is needed, but in no case would the commanded speed be Vref before the flaps were out. It would typically be your FMC descent speed until 10,000 feet, and then 240 kts until starting to slow to the flaps 1 speed as you get ready to extend flaps.

Which brings up a point... can you check the FMC target speed while this is going on? In the 747 it would be on VNAV DES 3/3. Line 2L will show your FMC target above 10,000 feet, and 3L will show your FMC target speed below 10,000 feet. How do those targets compare to what you're seeing on the PFD? (I assume the 787 is similar, but I have no real idea.)

 

 

 

I’ll mess around with it later to see if I can get a stall to develop and report back. 

58 minutes ago, sfgiants13 said:

I’ll mess around with it later to see if I can get a stall to develop and report back. 

It won't stall, it will decelate down to minimum clean speed and stay there unless you extract flaps.

12 hours ago, Fiorentoni said:

You might have not read correctly what was the discussed problem here, because VNAV setting Vref as speed target during initial descent is not a performance data related problem. It's a bug in the code.

Does it happen with the 787-10 too? If no, you would be wrong. It is thinkable, that a discrepancy between how the avionics calculates the descend and how the fligh model matches "that plan" could cause the target speed reduction.

6 minutes ago, fsiscool said:

Does it happen with the 787-10 too? If no, you would be wrong. It is thinkable, that a discrepancy between how the avionics calculates the descend and how the fligh model matches "that plan" could cause the target speed reduction.

It does happen with the 787-10, too. Actually that's where I experienced it for the first time months ago, back when there was no -8 or -9 available. For me it does happen sometimes when I change the flightplan (direct tos, deleting waypoints etc.) during descent, it's quite obvious the VNAV path calculation bugs out then.