Results 1 to 3 of 3
  1. #1
    Our new friend needs to reach 10 posts to get to the next flight level
    Join Date
    Oct 2007
    Location
    italy
    Posts
    12
    Contribute If you enjoy reading the
    content here, click the below
    image to support MyCockpit site.
    Click Here To Contribute To Our Site

    OVERSPEED during PM VNAV descent?!

    Anyone could tell me why many times during a VNAV descent the speed cue in the PFD does not match the value higlighted by the FMC for the actual leg?.

    I know that the FMC value is only a "target value" but many times what happens is that the aircraft start descending and accelerates to OVERSPEED and if i don't swithch off VNAV mode....well....wings will be ripped off (maybe before reaching the point to wich speed would have been reduced?......who knows...)
    That's a bit annoying both for pilot and passengers!
    CHEERS!

  2. #2
    75+ Posting Member
    Join Date
    Nov 2005
    Location
    IJmuiden, Netherlands
    Posts
    87
    Contribute If you enjoy reading the
    content here, click the below
    image to support MyCockpit site.
    Click Here To Contribute To Our Site

    I noticed the same, not only with descend but also during climb.
    In my legs page , I can see speed/height settings for each waypoint during climb and descend. Sometimes I see the speedcue on the PFD changing when passing a wp. But the altitude cue (is it called that way? anyway I am talking about the magenta coloured altitude the AP is going to) stays the same. Mostly the altitude cue is 10000, that value is in the MCP window. As long as the altitudes for the waypoints are lower in the cdu as the alt set in the mcp window it should use them I think.

    To be short, I do not use vnav anymore, only LNAV. I look at the speed/alt indications in the CDU and guide the plane to that via other modes.

    If you read this forum you will notice many complaints with VNAV. So better not use it till new releases od the sw.

  3. #3
    Our new friend needs to reach 10 posts to get to the next flight level
    Join Date
    Oct 2007
    Location
    italy
    Posts
    12
    Contribute If you enjoy reading the
    content here, click the below
    image to support MyCockpit site.
    Click Here To Contribute To Our Site

    Quote Originally Posted by npbosch View Post
    I noticed the same, not only with descend but also during climb.
    In my legs page , I can see speed/height settings for each waypoint during climb and descend. Sometimes I see the speedcue on the PFD changing when passing a wp. But the altitude cue (is it called that way? anyway I am talking about the magenta coloured altitude the AP is going to) stays the same. Mostly the altitude cue is 10000, that value is in the MCP window. As long as the altitudes for the waypoints are lower in the cdu as the alt set in the mcp window it should use them I think.

    To be short, I do not use vnav anymore, only LNAV. I look at the speed/alt indications in the CDU and guide the plane to that via other modes.

    If you read this forum you will notice many complaints with VNAV. So better not use it till new releases od the sw.

    I agree with you.
    My only difference is that, apparently , I have your same problem on altitude only if I reset FS and do another flight without switching off the PCs (itís like PM saves some data from the already completed flight wich prevent the CDU/FMC to work properly during the second flight in VNAV mode)

    Having done some more tests in VNAV mode i noticed the following:
    ONLY during VNAV descent (during CLB and CRZ everything works finely) the projected speed over the speed bar of the PFD doesn't match the one visualized on the FMC LEGs page for a specific leg.
    The problem is that the value showed over the speed bar during VNAV descent (the purple one) is EVERYTIME the MACH value reported in the DES page of the FMC at the TGT SPD line AND NOT the speed value inserted in the LEGS page (right side) for that specific leg.
    Being this mach value a more or less FIXED and UNIQUE value, the result is that when I leave a high FL (example FL320) the aircraft will accelerate progressively to overspeed (0.80 at 32000ft is good but 0.80 at 15000 means OVERSPEED for the 737!!!!).
    If I change the TGT SPD manually on the DES page to a lower value and the give EXEC, that lower value will be displayed in purple over the speed cue and the aircraft decelerates (but I have to lower the value continuously as the flight level decreases....a little boring!)
    Strangely under 10000 ft (more or less) the descent projected speed change from mach to KNOTS and are again in agree with FMC LEGs.
    I'm not a 737 pilot but i'm wondering if things work this way in the real one....or maybe it's a sofware bug
    Cheers!
    ALEX

Similar Threads

  1. Descent mode
    By wsixpo in forum PM Boeing FMC/CDU
    Replies: 4
    Last Post: 10-14-2009, 02:49 PM
  2. overspeed
    By renegade in forum General Aviation (GA) Builder Disccusion
    Replies: 0
    Last Post: 09-02-2009, 03:17 AM
  3. Boeing MCP build 420 - overspeed problems
    By Ray Proudfoot in forum PM General Q & A
    Replies: 12
    Last Post: 10-04-2007, 10:24 AM
  4. More data for wrong Vnav Path Descent
    By Eddie Armaos in forum PM General Q & A
    Replies: 0
    Last Post: 11-20-2004, 06:33 PM
  5. Vnav Descent Again.....
    By Eddie Armaos in forum PM General Q & A
    Replies: 7
    Last Post: 11-14-2004, 07:50 PM