Vnav won't allow TOD calculation
  • I thought I'd mastered the VNAV until I followed a flight plan that
    would not allow  me to enter one or show TOD. The plan is YBNA-YSTW(
    route Berni-Y23-Olrip ) is a short flight with an ILS on 30R at YSTW.
    Here is the problem. The transition for the ILS 30R is Bradd at a height
    of 5600'. On the Majestic Q400 FMS the ILS arrival for 30R is entered,
    when added to the plan, Bradd is shown with a height of 5600, this is as
    it's supposed to be, now while some 150nm from YSTW and while trying to
    set up VNAV for Bradd, the VNAV component will not allow me to enter
    any data for this, or transfer a TOD to the plan, even though Bradd is
    shown.


     


    What am I doing wrong - Bradd at 5600' is in the flightplan but won't allow a TOD to be calculated.

  • I tried your flight and was able to engage VNAV for the descent to BRADD in the normal way.

    I was cruising at 20000ft and pressed VNAV soon after cruise was established. I selected BRADD and it showed the usual screen, with 5600 as the target altitude.

    To arm  VNAV, I press once to confirm 'no offset', again to confirm the altitude and finally again to confirm the default descent rate. It then armed and started the countdown to TOD which was shown on the navigation display.

    The only observation I have is that the flightplan shows the little arrow pointing up (ie >5600) whereas the VNAV page shows the arrow pointing down (ie<5600), but I have noticed this before on other flightplans.

    So I can't shed any more light on your problem.

    Have you successfully used VNAV on other occasions or is this a one-off?

    PS in the raw flightplan/approach, there was a "no link" between OLRIP and BRADD. This "no link" needs to be deleted, VNAV won't work across a "no link". 

  • Thanks GraemeO, I appreciate you making the effort to duplicate the flight. What happened on this flight and some others -  the  FMS   follows the route using LNAV, at cruise I select the approach, in this case ILS 30R, with Bradd transition. Now the problem, Bradd is shown in the FMS as 5600, so I want my TOD to get me to Bradd at this level, when I enter VNAV on the keypad to do this Bradd is pre-copied there, but no distance or time to descent is shown in the VNAV window and TOD is not shown on the route as a circle.

    I got around it once but assigning 5600 at waypoint MSO, which is immediately before BRADD, then entering in the ILS approach, after the TOD was calculated, but I shouldn't have to do this.

    I'm aware of the "no link" requirements, but can you tell me how you got the TOD calculation to get to BRADD at 5600 without going the round about way.

    Hope this isn't confusing.
  • I sometimes experienced similar problems. It seems as waypoints that were added through the arrival selection (sometimes) cannot all  be selected for VNAV calculation.
  • Hi Striker - yes, I did actually experience what you describe when I repeated the flight (I must admit that after engaging VNAV on my first trial flight, FSX froze so I didn't actually land).

    On my second flight (after my post), I repeated the attempt to select BRADD in VNAV, but this time, the fields for altitude and descent rate were not populated (as you experienced) - very strange.

    I then tried another waypoint (there aren't many!) and that was ok, so I reselected BRADD again and interestingly, the data was there. Probably needs more trials to see if there is a pattern.

    Good point from ChrJsV, something to remember and probably a feature of the real FMC.

  • FMS will not accept certain navigation types between you and the descend waypoint. Such as intercept, distance to dme, and a few others. VNAV should be used with caution to the point in the STAR or IAP