Did a short hop LFMN to EGKK.

 

For some reason the auto-throttle refused to set an appropriate thrust for the descent causing excessive speed (not for the first time). On this ocasion it lead to the peculiar situation where the aircraft was simulatneously demanding more drag and complaining that the speed brake was extended (because the engines were not at idle).

 

I have set the option to ignore thrust lever position in the options (NEVER) and in any case they were in the idle position.

 

Thoughts appreciated:

 

Edit: Forgot to say that FSUIPC not used for any axes.

Have you set A/T MANUAL OVERRIDE to NEVER in simulation menu? 

 

EDIT: OK, you have.

OK, so I'm not crazy. This is the exact problem II keep having.

 

I still think it may be me doing something wrong.

 

 

 

You're not crazy, or maybe we're all crazy

 

A few friends and I have noticed strange behaviour in VNAV descent, large oscillations in V/S in situations it should be no problems for the FMC to calculate a smooth descent.

 

Still I am far from testing this in a neutral environment so I am not pointing at anything so far, but it feels a bit weird.

I have similar problems with vnav mode. Once it's enganged, it doesn't follow the "path". The plane decents sometimes slower or faster and the magenta indication of the vertical path goes up or down from the center position.

am I doing something wrong?

Be aware, the behavior of the real aircraft is for the A/T to go in to hold mode during a VNAV descent at which point the throttles need to be manually moved to idle.  If you are using hardware throttles, you will need to move them to idle as well unless you have the "A/T Override" option set to never.

 

It doesn't appear this is the OP's problem, but I suspect there are more than a few people out there who may have this as the cause of their problem.

Howard,

 

Can you give me the exact details of this route please?

- Coded route

- ZFW and fuel load

- Cruise altitude, CI etc

 

I'll try it.

Hey Ryan, wo you are still working?? Thought you are in the caribic together with Robert and the team

 

I had some issues on VNAV descend also, as if I start the descend before TD with "descend now" or just by pressing the ALT knob, it does not descend on a 1000ft/min descend as the NGX does as soon the path changes - sometimes the aircraft goes into a level flight or even tries to climb in order to get to the correct speed.. is this realistic?

According to the fcom the descend now button on the fmc should command a vs of 1250 down but when I tried that it didn't work bit could be user error. Will try again tonight.

According to the fcom the descend now button on the fmc should command a vs of 1250 ...

Ah, ok thanks!

I'm having the same problems. I haven't been able to do a proper VNAV descent, and disconnecting my controllers didn't help. I'm not talking about final approach, but since FL200 or higher.

 

- On my first flight I had very bad weather at descent, and the AT would go 90 N1 at every gust, and it oversped at about 6000 ft. I assumed it was because of the weather. So I reset my AS settings and decided to try later at good weather.

- On the 2nd and 3rd flights I've had the exactly same issue as the OP. So I descended with FLCH and V/S.

- On my 4th flight I started with FLCH from T/D. On the 3rd and 4th flights engine would go from idle to 45 - 60 N1 with FLCH selected even with controllers disconnected! So I had to descend with my hand on F1.

- On the 4th flight the auto throttle got me to 10 knots above selected speed (Vref+15) and slowly rising to 15. Wind was 3 knots head wind and no gusts.

- I've tried two more simple take-off and landing flights to reproduce this last item but it didn't happen again.

 

That's all I had time to fly until now. I didn't save any screenshots from the flights because at first I thought I was doing something wrong, but as soon as I have time I will try to reproduce the problems and share everything I can with PMDG.

Similar experience on my first flight yesterday:

 

KIAD to KJFK at FL210, VNAV and LNAV engaged for most of the flight.

 

15NM before the TOD I preset altitude 11000 in the MCP. Right after that the plane initiated a descend by itself (13NM before TOD), and even though it was shallow and managed to make the 11000 altitude restriction at the next waypoint, there was about 3000 feet deviation from the VNAV path. Further I selected 3000 feet in the MCP and continued descend with LNAV and VNAV. The descend speed was 280 knots (VNAV calculated), but before arriving at 10000 feet, the plane started slow acceleration and busted the restriction at 300+ knots. At that point the FMC was showing DRAG REQUIRED, while the throttles were at about 50%. I had to change mode to FLCH and have some spoilers to get the speed management under control.

 

OPUS real weather was on but nothing major. The PMDG throttle behavior is "NEVER" and FSUIPC is disabled for the throttles input.

 

I thought the above experience is a direct function of not reading the T7 manual and applying too much 737NG tricks while flying the new plane. And yet, the MCP logic is pretty much straight forward. Also I see similar stories above. 

 

There will be more flying, I will post back...

Guys can you answer Ryan's questions specifically so we can attempt to recreate?

I have same problems with VNAV - erratic descent rate and nearly always arriving 3000 to 4000ft above selected waypoint altitudes.

I'm also confused over PMDG set up "AT not in hold/arm mode only/always etc".

I would be really grateful if someone could explain in plain English what these settings mean and how they should be applied.

Apart from this I love the B777 which is a joy to hand fly.

Thanks

Frank

Hi Ryan/George.

 

I made lots of saves along the route so I can upload them via the support site if you prefer. RSVP.