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Thread: VNAV issues.

  1. #1
    300+ Forum Addict David Rogers's Avatar
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    VNAV issues.

    Hi,

    I use the full version of the PM Boeing-type suite. (GC, MCP, CDU).

    I am having some problems with VNAV. (The first may not be a problem but the 2nd certainly is!).

    On the 737NG, on take-off, if VNAV is engaged during the intial (V2+10) climbout, should the FD be commanding V2+10 upto the acceleration alt, then switching to 250kts pitch ?

    I am finding that as soon as I engage VNAV, it goes to 250kts pitch climb.

    Am I getting confused with the 767 and 747, and does the 737NG not provide V2+10 (or any climbout) as part of VNAV ?

    My second issue is that occasionally, I will reach cruise but the VNAV cruise speed set sticks on 286 kts, and not the mach speed I have set in the VNAV CRZ page. (It is as if the VNAV thinks it's starting a descent as soon as cruise is reached!).

    My 3rd issue is that VNAV will usually start to command a decent before the T/D point that is indicated on my ND. (Although the descent can be reasonably accurate so perhaps this is a CDU issue with the placement of the T/D psuedo waypoint?).

    Thanks for any help!
    David R
    Durham, England

    1979 Mooney M20J Cockpit builder ......

  2. #2
    150+ Forum Groupie michelmvd's Avatar
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    Hi David,
    I can't speak for a B737, but this is how VNAV should work for the B744 and probally also for B767/777.

    According SOP, VNAV must be armed on the ground (together with LNAV)

    Command Speed VREF-20 after rotation

    VNAV becomes active at 400ft RA

    At the moment LNAV/VNAV becomes active, the TOGA modes end.

    VNAV will command a pitch down at the preset altitude of the FLAP/ACCEL.HT in the TAKEOFF REF page of the CDU and command a VREF+100 speed.

    (exemple : 10/1500 : aircraft speed will be set to VREF+100 and will pitch at 1500 Ft to start the flap retraction scheme.

    VNAV will command CLB speed (or CLB+1,2) at the moment the preset factor in the THR REDUCTION (TakeOff page) is reached.

    'exemple : FLAPS5 CLB1 : aircraft will come in CLB1 speed mode at the moment FLAPS5 are set. TO thrust mode (or TO1/2) ends at that moment.

    All this must happen with or without A/P on. (F/D mode)

    This is how it works in the real B744, VNAV behavior in PM for B744 is totally unrealistic.

    B. Rgds
    Michel
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    Hi Michel,

    As we have to deal with PM, could you write the same tutorial how you would do this with PM? And how we can change parameters with th cdu?
    As I also have some pr.oblems with the cdu, I would be glad to have a good checklist on how to use it.

  4. #4
    300+ Forum Addict David Rogers's Avatar
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    Hi Michael,

    Thanks, that was interesting, however the 737 differs quite starkly to the 744 in terms of VNAV annunciation and use. For example, you cannot arm VNAV while on the ground in the 737 in the way you can in a 76-, 74- or 77- . Actually Boeing offered an option for this for the 737 once but shortly after they abandoned this facility in the 737 and issues SOP revisions instructing any airlines that had the option on their 737 fleet never to arm VNAV on the ground in any of the 737 series. (You can read all about this at www.b737.org.uk ).

    Similarly I suspect that the VNAV on the 737 does not inlcude the 'climbout' phase as part of the VNAV profile.

    In practice, on the PM 737 suite, I am gettng unrealistic performance on the climb phases of my flights - when I climb out purely on TO/GA to my acceleration altitude, then switch to VNAV the flight director command no climb at all, until the aircraft has accelerated to 250kts.

    I don't believe this is realistic as, having spoken to a real 737NG pilot, the aircraft does not level out, at say 2000', while it accelerates from V2+10 to 250kts! ....I am told that the real aircraft's flight director will command a pitch that maintains a degree of climb, while still allowing the aircraft to accelerate to 250/10000.

    This is also the case on virtually all "VNAV" armed FS add-ons, including PMDG737, 747, LDS 767, 737PIC, etc, etc.

    I have to say that although I am generally quite pleased with PM overall, I find the CDU software the most disappointing from a realistic performance perspective. I definately feel that the PMDG and Level D CDUs are more accurate in terms of performance, with far fewer serious bugs.

    The regular bugs I experience with the PM CDU include:-

    - Loss of waypoints in LEGS and RTE pages, once a arrival, departure or rwy is selected in DEP/ARR page.

    - Commanding a 'Direct to' for the current waypoint (by line selecting and re-entering current waypoint) often fails to work.

    - Annoying 'repeats' of waypoints in the LEGS page; eg... POL, TNT, BUZAD waypoints are entered as part of route once, but will sometimes be repeated again; (ie... EGNT, RAPUM, POL, TNT, BUZAD, WELIN, POL, TNT, BUZAD).

    - Erroneous Top of Descent calculations and waypoints.

    - VOR1 and VOR2 frequencies that are 'hard entered' (manually) (with a 'M' annunciation) are regularly replaced with auto tuned frequencies!

    But as I have said above, the worst of all is the loss of your en-route waypoints as soon as you select a SID, Star, Approach or Runway in the DEP/ARR page.

    No-one can argue on the aesthetic qualities of the PM suites but we should not have to put up with some of these bugs given the price paid.
    David R
    Durham, England

    1979 Mooney M20J Cockpit builder ......

  5. #5
    300+ Forum Addict David Rogers's Avatar
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    Quote Originally Posted by npbosch View Post
    Hi Michel,

    As we have to deal with PM, could you write the same tutorial how you would do this with PM? And how we can change parameters with th cdu?
    As I also have some pr.oblems with the cdu, I would be glad to have a good checklist on how to use it.
    Hi Norbert,

    According to one airline's SOPs, this is how you fly the 737NG on take-off and climbout (and how it should be done with PM):-

    - Manually advance throttles to 55% N1.
    - Allow engines to stabilize.
    - Press TO/GA (N1 button if you have CP Flight MCP).
    - [PM increases to t/o power too slowly, so it is best to advance throttles up to around 80% before hitting TOGA / N1].
    - Light forward pressure on the Yoke.
    - Hand on the throttles until V1 is reached.
    - At VR smoothly rotate to 15 degrees.
    - Select a roll mode, if one was not previously armed.
    - Maintain TO/GA pitch (15 degrees) until obstacle clearance alt reached.
    - Then continue climb at V2+10 (can be flown using FL CH or simply by referring to airspeed indicator).
    - At acceleration altitude (usually between 1500-3000' AGL), reduce pitch to accelerate to 250kts (VNAV or FL CH set to 250 can be used).
    - Continue climb at a pitch that maintains 250kts, until 10,000 is reached.
    - Accelerate to FL climb speed (usually 290-330kts), until Mach transition level is reached.

    .....bit fiddly init ?

    Here's the A320 version:-

    - Rotate at VR.
    - 'NAV' will turn green in the annunciator.
    - Follow the flight director... it will autimatically annunciate all of the above stages

    David R
    Durham, England

    1979 Mooney M20J Cockpit builder ......

  6. #6
    Our new friend needs to reach 10 posts to get to the next flight level
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    @david rogers, 280 knots at cruise is very very fast. About mach .78 or something.

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    150+ Forum Groupie sas550's Avatar
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    Ther are other FD issues aswell and they are connected to the MCP software and not CDU. Ie if you take off only using To/Ga mode the mcp shoul go over to flc mode after passing thrust reduction alt. Doesn't happend wether the option is on or off in the ini file.

    If you choose lnav it is supposed to give lateral guidance as soon as u push the button over 500ft. Doesn't happend until a vertical mode is manually set.

    PM are aware of it but who knows when somethings new will come from them. Nothing new in several months now.

    Starts to get abit annoying
    Regards Anders Eriksson


    www.simcockpit.se

    FS9

  8. #8
    300+ Forum Addict David Rogers's Avatar
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    Quote Originally Posted by telly53 View Post
    @david rogers, 280 knots at cruise is very very fast. About mach .78 or something.
    Yeah absolutely agree, but the point is that it is not what is set up in the VNAV CRZ page..... That's the issue I have (it done it again last evening - CRX should have been .760 but on reaching cruise, the VNAV commanded 282 kts and even re-entering speeds into the CRZ page and the individual LEGS entries would not persuade it to change from 282kts!

    Come on PM - when will you release the next build to correct these nasty problems ?????
    David R
    Durham, England

    1979 Mooney M20J Cockpit builder ......

  9. #9
    300+ Forum Addict David Rogers's Avatar
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    Quote Originally Posted by sas550 View Post
    Ther are other FD issues aswell and they are connected to the MCP software and not CDU. Ie if you take off only using To/Ga mode the mcp shoul go over to flc mode after passing thrust reduction alt. Doesn't happend wether the option is on or off in the ini file.

    If you choose lnav it is supposed to give lateral guidance as soon as u push the button over 500ft. Doesn't happend until a vertical mode is manually set.

    PM are aware of it but who knows when somethings new will come from them. Nothing new in several months now.

    Starts to get abit annoying
    Agree Anders - both of those drive me crazy. Is there even an option to stop the MCP commanding HDG SEL on take-off ? I hate this, I believe most 737s are set up to annunciate Lateral TO/GA (ie... wings levelled, runway heading) on take-off, not heading select.

    Also, about FL CH again; when I engage it it always defaults on 150kts, not the actual speed at the time of pressing, as it should !!!!

    I am annoyed to say that PMDG is far more accurate than PM in these regards. Boo !
    David R
    Durham, England

    1979 Mooney M20J Cockpit builder ......

  10. #10
    2000+ Poster - Never Leaves the Sim Trevor Hale's Avatar
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    Quote Originally Posted by David Rogers View Post
    Yeah absolutely agree, but the point is that it is not what is set up in the VNAV CRZ page..... That's the issue I have (it done it again last evening - CRX should have been .760 but on reaching cruise, the VNAV commanded 282 kts and even re-entering speeds into the CRZ page and the individual LEGS entries would not persuade it to change from 282kts!

    Come on PM - when will you release the next build to correct these nasty problems ?????
    Hi Dave,

    This last thing you mentioned regarding the speed issue, I change all the time. I have no issues setting speeds in the CLB/CRZ/Des Pages, it always changes that on the MCP.

    You should check this, because I know I do not have this issue you mention.

    Trev
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