Jump to content

Archived

This topic is now archived and is closed to further replies.

jgoggi

Problem with altitude restrictions during VNAV descent

Recommended Posts

I had already written about what I think is something not working correctly in the PMDG 777.

 

I am in cruise at say FL 360 and start the VNAV descent (early, before T/D, but ATC instructed me to descend) to say FL 200. One of the waypoints ahead has an altitude restriction, say /240A (FL 240 at or above). The airplane seems to be willing to reach that altitude immediately and starts an incredible dive up to 10000 ft/min, it goes into overspeed and there appears "VNAV PTH" crossed  by a yellow bar on the PFD flight mode annunciator. I tried it several times and it always happens. I don't think this is normal... I have SP1c.

James Goggi

Share this post


Link to post
Share on other sites

Probably a problem with your procedures. Assuming that the 240A restriction is in place on the LEGS page, the proper way to fly this would be to start your descent as instruction by ATC and if before TOD the aircraft should descend at about 1000 fpm until intersecting the vertical path. Keep 20000 dialed into the MCP if that is your clearance limit. After the 240A restriction, the aircraft should continue on path until 20000 then FMA will indicate ALT HOLD.  The path after the 240A restriction is independent of the 20000 limit.

 

The agressive descent after the 240A restriction could be due to winds but unlikely, and I've never seen a 10000 fpm descent. The FMS would be providing UNABLE DESCENT AFTER XXXX before such an agressive descent would be commanded. Something is wrong but not with the 777.

 

Please provide flight plan path with altitude restrictions from just before TOD, also provide TOD location with respect to an enroute fix and distance before TOD that ATC requests lower.


Dan Downs KCRP

Share this post


Link to post
Share on other sites

I would like it to be my problem, but unfortunately it's a problem with the 777...

 

I think it can be easily reproduced: you are at fl 360. Before starting your descent, you should have in front of you T/D, WAYP1, WAYP2, etc. Set /240A to WAYP2 on the LEGS page, then start your descent to fl 200 (set 20000 on the MCP and press the knob, or select DES NOW on the FMC VNAV page). 

As soon as the descent to fl 200 starts, it's in THR/LNAV/VNAV SPD mode, then it changes to SPD/LNAV/VNAV PTH and right after to SPD/LNAV/VNAV PTH, while the airplane dives until 10000 fpm. I forgot to say that, absurdely, on the FMC the request "DRAG REQUIRED" appears...

I can reproduce it 100% of the times...


James Goggi

Share this post


Link to post
Share on other sites

 

 


Set /240A to WAYP2 on the LEGS page

 

What is the crossing altitude at WAYP2 before setting it to 240A?  I am trying to rule out that you are not setting up a vertical path that is unrealistic.

 

In my experience, the descent angle at altitude is never more than 250ft/nm and usually less. Check your path angle.


Dan Downs KCRP

Share this post


Link to post
Share on other sites

 

 


What is the crossing altitude at WAYP2 before setting it to 240A?

 

It's about 250, if I remember well, written in small numbers (expected altitude). I can enter 250 or 260 instead of 240, nothing changes. 


James Goggi

Share this post


Link to post
Share on other sites

Okay, you've set an AT-OR-ABOVE that is below the path. So far so good but I cannot reproduce this unless you provide the information I asked for earlier. The flightplan information and CI is a start. You said you can reproduce this so providing this information should be easy.


Dan Downs KCRP

Share this post


Link to post
Share on other sites

Might also be a hardware problem with your throttle. Kyle (scandanavian13) has a post somewhere explaining the technical details, but an early descent puts you in Hold mode -- basically manual thrust. If your throttle is pulled all the way back at the top of descent, you'll go into a dive unless you take over the throttles. Try a VNAV descent without DES NOW or touching the throttles. Pull the throttles all the way back before starting the descent.

 

Charles Nadeau


Charles Nadeau

Share this post


Link to post
Share on other sites

FYI, video from Kyle explaining what might be going on:

 

 

Charles Nadeau


Charles Nadeau

Share this post


Link to post
Share on other sites

I tried without hardware throttle but nothing changes.

I do confirm that at a certain point, I don't know why, the airplane for NO REASON goes up to 7.5 degrees nose down (following the magenta bar of the flight director) and dives up to -10500 fpm, lthough it's already BELOW the VNAV path (since I have commenced the descent early) and with ANY thrust setting I try to give. It's only when it reaches about 350 kts (the red overspeed band starts at about 330 knots) that it realizes that it's in an absurd sink attitude and in overspeed, and starts recovering. I don't see where I can be wrong and seriously think it's an airplane issue. Unfortunately there is no way to post a video or screenshot here, otherwise it would clearly show the issue.

Dan, I have PMed you. Thanks.   


James Goggi

Share this post


Link to post
Share on other sites

 

 


Unfortunately there is no way to post a video or screenshot here, otherwise it would clearly show the issue.

 

There's a video in the post above yours...

 

If you upload something to youtube, just pasting the link here automatically does it (and if it doesn't, just add

and

around the link).

 

As far as your issue goes, do you have a flaps axis assigned in FSUIPC?


Kyle Rodgers

Share this post


Link to post
Share on other sites

No, Kyle, I don't have any flaps axis assigned in FSUIPC. Should I have it? I am also figuring out how to record, I never did it, I suppose I should install FRAPS, shouldn't I?

Anyway I start thinking that there might be something wrong in my 777, because I have another problem, that it doesn't go around when I push the TOGA switch during approach (see my other post of today), but I really can't understand what could interfere with these features, since the rest is ok. Do you have any idea?

Thanks.


James Goggi

Share this post


Link to post
Share on other sites

 

 


No, Kyle, I don't have any flaps axis assigned in FSUIPC. Should I have it?

 

No. I was asking because doing so improperly can cause problems.

 

 

 


I am also figuring out how to record, I never did it, I suppose I should install FRAPS, shouldn't I?

 

I use OBS. It's free.

 

 

 


Anyway I start thinking that there might be something wrong in my 777, because I have another problem, that it doesn't go around when I push the TOGA switch during approach (see my other post of today), but I really can't understand what could interfere with these features, since the rest is ok. Do you have any idea?

 

The fact that you're having this problem and the TOGA issue (assuming you're doing everything correctly there) there may be something wrong with your installation.

 

Do you have a trim axis assigned on your hardware?


Kyle Rodgers

Share this post


Link to post
Share on other sites

 

 


Do you have a trim axis assigned on your hardware?

 

During troubleshooting I tested without any hardware, I deleted FSX.cfg and let the sim rebuld it and I even removed the FSUIPC4.dll file, but unfortunately nothing changes. I really don't know where the problem may be. 


James Goggi

Share this post


Link to post
Share on other sites

The same thing used to happen to me quite some time ago. I believe it resolved itself when I stopped using a certain panel state at the start. My memory is dreadful, though, so it could have been something else. I'll keep crawling through the spiderwebs in my brain to see if I can remember more accurately what made this stop. In the meantime, at least you can rest assured that you're not the only one with this issue.

 

Best,


Walter Meier

 

Share this post


Link to post
Share on other sites

  • Tom Allensworth,
    Founder of AVSIM Online


  • Flight Simulation's Premier Resource!

    AVSIM is a free service to the flight simulation community. AVSIM is staffed completely by volunteers and all funds donated to AVSIM go directly back to supporting the community. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. Thank you for your support!

    Click here for more information and to see all donations year to date.
  • Donation Goals

    AVSIM's 2020 Fundraising Goal

    Donate to our annual general fundraising goal. This donation keeps our doors open and providing you service 24 x 7 x 365. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. We reset this goal every new year for the following year's goal.


    34%
    $8,560.00 of $25,000.00 Donate Now
×
×
  • Create New...