Well, I know that a comparison with the NGX should not be made, but when it comes to VNAV descent, the NGX acts perfectly, the 777 leaves me quite doubtful and it looks to me a bit strange that the real one behaves like that. Whenever I start the descent in VNAV, the thrust goes to HOLD and the system doesn't care about what vertical speed the airplane achieves. Only when it approaches a constraint altitude, it "remembers" of that constraint and both thrust and altitude become controlled again, thus going into level flight. You end up reaching that altitude miles before the waypoint with that constraint altitude. Instead of a continuous descent, it becomes a descent made by steps.

If I start descending before T/D, I knew from the good old 747-400 that the airplane performs a controlled descent at 1250 fpm until intercepting the descent path. Instead this 777 guess what? Goes into HOLD mode and bye bye...

I need to be reassured that this is a real life behaviour... :unsure:

Yup, don't forget that the 777 predates the 737NG series and the VNAV is different. Early descents may not work as you expect them but there was a technical review team consisting of T7 pilots that vetted the simulation, so I trust that there is fidelity. Your CI does have a major impact on descent performance, try playing with that.

 

 

Instead this 777 guess what? Goes into HOLD mode and bye bye...

How far before the T/D did you execute the early descend? And what method did you use?

If you descend before TOD then you get that about 1250fpm (more like variable 1000-2000fpm in reality) you were talking about.

 

HOLD mode is there so you can easily intervene.

You can add a little thrust, or reduce thrust, to adjust the descend path to your liking.

But constraint are adhered to in any case (the constraint altitude will just be reached earlier.....unless the Vanv path is still intercepted (thrust reduced). Or the vnav Path will be intercepted sooner resulting in an idle descent from there on to you constraint.

 

Note that the pitch mode is Vnav Spd in this phase!! (not Vnav Path)

 

 

On a descend with altitude constraints, only the initial descend (to the first constraint) is calculated as an idle descend.

 

If you dont want to reach this first constraint "miles early" then dont start the descend untill the FMC advices you so (assuming you have the proper winds in the FMC forcast page).

 

After the first constraint a line/slope is drawn from one constraint to the other resulting in continuous descends that might, or might not be, idle descends (depends on distance and altitude between them obviously).

They should not be reached early at all, as the aircraft will be in Vnav Path mode (even if it is not an idle descend) and the AT will be in SPD.

No, the early descent really doesn't seem to work!  I have just tried at least 10 times with the DES NOW feature. Cruising at FL 360, VNAV PTH, 280 knots, 40 Nm to T/D, no weather conditions (wind calm).

Dialed 30000 on the MCP and pressed DES NOW.

Thrust mode goes into THR/VNAV SPD and after a while into HOLD/VNAV SPD, ok, as described in the FCOM. But the vertical speed decreases very slowly and the speed tends to decrease to 270 knots. First anomaly: why doesn't VNAV SPD trim nose down in order to maintain 280 knots, also considering that the descent rate is near 0?

I don't touch the thrust levers.

Suddenly VNAV "realizes" that the speed is too low and starts pushing the nose down, but too much, and the airplane dives into an 8000 ft/min descent and overspeed, that can't be recovered!

Please, someone could try this? I tried at least 10 times. Either I do something wrong or there is a big bug in how the system works!  

No, the early descent really doesn't seem to work!  I have just tried at least 10 times with the DES NOW feature. Cruising at FL 360, VNAV PTH, 280 knots, 40 Nm to T/D, no weather conditions (wind calm).

Dialed 30000 on the MCP and pressed DES NOW.

Thrust mode goes into THR/VNAV SPD and after a while into HOLD/VNAV SPD, ok, as described in the FCOM. But the vertical speed decreases very slowly and the speed tends to decrease to 270 knots. First anomaly: why doesn't VNAV SPD trim nose down in order to maintain 280 knots, also considering that the descent rate is near 0?

I don't touch the thrust levers.

Suddenly VNAV "realizes" that the speed is too low and starts pushing the nose down, but too much, and the airplane dives into an 8000 ft/min descent and overspeed, that can't be recovered!

Please, someone could try this? I tried at least 10 times. Either I do something wrong or there is a big bug in how the system works!

 

When I start an early descend all works perfect. I have never had any of those problems, not with the original version nor with SP1.....but I have not tried with SP1b yet.

 

But I dont think SP1b has a Vnav bug because we would have heard more people complain.

 

The only thing I can think of is that it is once again one of those FSUIPC.ini cases.

Do you use FSUIPC?

Yes, I do, registered version. I tried another scenario with a different descent path and things were better, anyway this VNAV gives me the idea of an almost completely manual descent, 90% of the time the mode is HOLD/VNAV SPD and you have to adjust the thrust to let the airplane fly the descent profile.

It should be the other way around.

You should be in Thrust Hold mode and Vnav speed only for a few minutes (2-3).

The rest of the descend should be in Vnav Path with idle thrust.

(as you know.....just confirming).

 

So yeah something is wrong.

 

It does not sound like user error to me, so that leaves an install problem or FSUIPC problem or something else I have not thought of.

 

You said you use the descend now feature.

You mean from the CDU?

It should not matter (because it should do the same thing), but try what happens when you press the altitude selector when within 50nm of TOD.

 

Stupid question, but A/T and A/P are engaged right?

 

Although I am not a big fan of uninstalling/reinstalling, you could try to uninstall the 777 and reinstall it.

 

If that does not work, you could try deleting the FSUIPC.ini file.

It will rebuild next time you run FSX,

 

Bad thing about that - all your joystick/yoke axis and button allocations/calibration will be deleted and have to be setup again :-(

Good thing - this procedure has fixed all sorts of problems which is why it is documented in the PMDG intro manual as something to try.

Check your hardware.  In an early descent VNAV will reduce thrust to descend at approximately 1000 FPM and then go into HOLD and VNAV SPD.  My guess is your throttle axis has noise or something that is causing the thrust lever position to change after it's gone into HOLD.  It sounds like your hardware is causing the thrust levers to come back all the way to idle.

I dont know....could be hardware....but he is saying he got his 777 doing an 8000ft/min unrecoverable descend!

 

That sound more like software problems cause I dont think I could make it do that even if I slowed down to 200lt to then dove down to 330kt. with FLCH.

Let alone reach -8000ft/min from a 270kt Vnav Spd descend with all protection features on (overspeed) and AT and AP engaged!

 

 

I don't touch the thrust levers.

This could be your problem. Unless you have selected NEVER for the A/T MANUAL OVERRIDE option, you do need to touch the thrust levers. If you've left them forward after takeoff this will limit thrust reduction on descent and in HOLD mode they will follow your hardware input. I find VNAV works just fine unless I forget to retard the throttles before descent.

yes, Kevin is right......that could be your problem too!

I did a test starting descent at T/D, therefore I did not touch anything, just dialed a lower altitude on the MCP and let the airplane do the rest, and the descent was PERFECT, with IDLE (then HOLD) and VNAV PTH. It's interesting to also compare the actual vertical speed with the V/S number on the FMC VNAV DES page.

What I had problems with was the early descent, in that situation I could not avoid that the aircraft suddenly started a dive at 8000/10000 ft/min and completely went out of control. 

I've used early descent multiple times and only thing I noticed was that the rate was higher than initially expected based on documentation but definitely not excessive as you experienced. I assume your descent is ECON... throw some numbers at us (M/KIAS, ALT, distance to TOD, CI) and maybe we can figure it out.

Similar thing happened to me recently:

 

I believe I figured out what caused it in my particular situation. I saved the flight at TD with time compression enabled @ 4x and then loaded it the next day to finish it. I turned off time compression atleast 40nm before TD, and then used DES NOW about 10nm from TD but I kept getting into these uncontrolled dives. What solved it was hitting the "reset" button in the time compression options after re-loading the saved flight. After that, DES NOW  worked just fine. The only thing I can imagine causing this is that certain VNAV calculations were still operating at 4x rate until reset.