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Unable LNAV

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    Unable LNAV

    I am on a flight with my flight mode annunciator showing in green as SPD|HDG SEL|VNAV PTH, but under the HDG SEL I have LNAV in white. I feel this is to show that the aircraft is indeed in heading hold and waiting to engage LNAV once I am on an intercept heading. Changing the HDG value correctly causes the aircraft to turn.
    If I set the HDG to the intercept bearing and press SEL, the FMC displays 'NOT ON INTERCEPT HEADING'.so staying in HDG SEL.
    How do I get the autopilot to accept LNAV mode active again? 🤔
    Cheers, Richard

    Originally posted by RichardMcDonaldWoods View Post
    I am on a flight with my flight mode annunciator showing in green as SPD|HDG SEL|VNAV PTH, but under the HDG SEL I have LNAV in white. I feel this is to show that the aircraft is indeed in heading hold and waiting to engage LNAV once I am on an intercept heading. Changing the HDG value correctly causes the aircraft to turn.
    If I set the HDG to the intercept bearing and press SEL, the FMC displays 'NOT ON INTERCEPT HEADING'.so staying in HDG SEL.
    How do I get the autopilot to accept LNAV mode active again? 🤔
    Well first the LNAV in white is in fact Armed. It will stay Armed until the LNAV Path is captured not on a intercept heading only. There are certain constraints listed in the manual that will depict what is needed to capture the course. You’ll have to dig those up.
    It also sounds like from your description that maybe the FMC didn’t sequence the next leg if it’s only giving you not on intercept heading. There’s a couple ways to fix this. Either proceed direct to the next fix or if you really need to intercept the leg you would take the next fix place is in Legs Page 1 LSK L1 and adjust the intercept course heading to the desired value. All this is in the manual as well. Though or there are tutorials on YouTube. It’s a bit complex for a post on my phone.

    That’s my observation from what I’m reading. If you want a quick and dirty fix to your issue just proceed direct to your next waypoint and LNAV will immediately go active.
    Brian Thibodeaux
    B747-400/8 First Officer
    Typed MD-80, BE1900
    My Liveries


      Brian can type more on his phone than I can! Not on intercept course tells you that if you continue your current heading you will never reach the LNAV path. You didn't provide enough information, specifically your heading and your position relative to the LNAV track, for us to do any more than guess. Either a direct to next waypoint or perhaps an INTC CRS TO would work.... can't tell.
      Dan Downs KCRP



        As Brian and Dan mentioned there are so many variables, a picture with the PFD, ND and FMC will help and narrow down the issue.

        Just a few quick guidelines before studying (I know it's a lot) will help:

        If more than 20 NM either side of magenta line wpt sequence will not occur. You need to do an intercept course or direct to, to redefine the airway or legs sequence.

        If the current track/HDG does not cross the intercept leg will trigger NOT ON INTERCEPT HEADING

        LNAV will activate when within 2.5 NM of on an intercept heading to the active leg.
        Kind regards Alex Nicolov
        "You can't make a difference unless you upset at least one."
        [email protected] GHz, Nvidia RTX 2080 Ti, DDR4 3200 64GB


        • thibodba57
          thibodba57 commented
          Editing a comment
          Just to expand on the 2.5nm from course. It says that in the manual but in practice as I’m sure Alex has seen it doesn’t always capture and make a drastic turn to intercept the course. Especially coming off offsets using HDG Sel. Sometimes you have to drive the plane until your practically on top the Course before LNAV will capture. Other times it’ll Capture and make some aggressive turn to join the course. So I’m sure some of the description of when LNAV will capture has been left out of the Boeing manual or most likely just generalized for us dumb pilots to digest. Point being, if you are sure you have the Box programmed correctly and you are sure of it, just keep driving the aircraft to the course with HDG select until it captures. If it doesn’t, well get on top of the course as accurately as you can (Utilize Prog Page 2) then go direct to the next waypoint. Let’s hope it doesn’t go that far though. It shouldn’t.

        Further to what Alex and Brian have said, provided the aircraft is within 3nms of the active route then it can be on any heading to capture the active route. If it is more than 3nms away from the active route then the intercept course must be 90 degrees or less and, in both cases, the route segment must be intercepted before the active waypoint.

        The nature of the turn during the route capture will depend on several factors, including the intercept angle, wind velocity. speed and altitude as well as the distance from the active waypoint and the cross-track error, etc. (all of which will affect the time and/or rate of closure to a greater or lesser extent), The FMS uses this data and more to compute the transitional distance during the turn and the capture will inevitably be more aggressive the closer the aircraft is to the active route if it is at high speed with a large intercept angle. When establishing on an ILS localiser at a large intercept angle it is sometimes possible to go through it and 'lose' the capture, which is why it is always considered good airmanship to monitor the ILS raw data as well as the FMA modes.

        Michael Codd


          Gentlemen, many thanks for your thoughtful feedbacks.😄
          I had not thought about pressing my next TO waypoint and setting it as a direct to. This works well.
          I had also forgotten about having to be within 2.5 mi of magenta for LNAV to engage.
          As age presses on, I find it so easy to overlook solutions to my problems. 😥
          Cheers, Richard