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LNAV enabled and sometimes it doesn’t

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    LNAV enabled and sometimes it doesn’t

    Question on Lnav. Most of the time before I taxi I arm AT and enable VNav and Lnav on the 737. Sometimes the Lnav light doesn’t come on. The 777 seems to always come on. So why would the Lnav light not show up until after takeoff. I thought it had to do with the flight path but that doesn’t seem to be the case. I am using a check list so what is a good practice to follow on when to engage Lnav. I am not using ATC.
    Dean Salman

    Originally posted by dsalman1 View Post
    Question on Lnav. Most of the time before I taxi I arm AT and enable VNav and Lnav on the 737. Sometimes the Lnav light doesn’t come on. The 777 seems to always come on. So why would the Lnav light not show up until after takeoff. I thought it had to do with the flight path but that doesn’t seem to be the case. I am using a check list so what is a good practice to follow on when to engage Lnav. I am not using ATC.
    LNAV engagement requires an active FMC route that can be intercepted within 3 nm or if a greater distance then the airplane must be on an intercept course of 90 deg or less and intercept the active route segment before the next active waypoint. On the ground before takeoff, LNAV may not be engaged unless the first waypoint is within a number of degrees of the runway heading. I'm not sure but I think it is 60 deg.

    Check your PMDG Aircraft Equipment options to verify the takeoff roll mode, normally this wings level until about 400 ft AAL where then an AFDS roll mode can be selected. The options are wings level and HDG mode.

    I will often seen LNAV not engage before takeoff without knowing why, but it's a simple matter of using HDG or handflying until reaching V2+20 then engage it. My personal preference is to only use LNAV on departure when using a RNAV SID or a non-RNAV SID with a straight forward departure path. Otherwise I use HDG.
    Dan Downs KCRP
    i7-10700K 32GB 3600MHz 3080Ti


      Hey, Dean-

      Dan has given you some good guidance above. I am not a 737 driver, and someone else on the team codes that part of the airplane- but I do know that the 737 has slightly different arm/engagement limitations than the big boeings- so perhaps you are seeing one of those limitations.

      You didn't offer any info on a specific runway/departure configuration- so I am largely guessing- but when out flying real-world, we will generally use LNAV if using a SID that has a defined departure path. If we do not have a programmed departure SID, or if the SID is a vector-to-fix, we'll then choose either TO/GA or HDG. The choice between them really depends upon what we expect to happen in the initial climb. For example, if we get "climb runway heading" we'll use HDG mode. If we get "fly heading XXX when able" then we'll set the heading bug to that heading and use TO/GA for the initial climb. Pilot flying then calls for HDG during the climb and we commence the turn.

      As is always the case in complex environments: Your mileage may vary.

      If there is a specific SID you want to evaluate- just shout out here. Plenty of us here to help you.

      Robert S. Randazzo
      PMDG Simulations


        Thanks for the Feeback, dan and Robert. When I see this again, I can include the route as well. But it sounds like the first way point being within the required turn. I will look deeper into this, thanks
        Dean Salman


          I’ve had the same issue. Whenever I’m departing from DTW’s 21R, with the PAVYL2 departure, LNAV will not stay armed on the ground. When I select LNAV on the ground, the button briefly lights up then goes out. I’ve had the same issue with another airport with a RNAV SID but I don’t recall where.
          Loren Gogins


            Hey Dean,

            LNAV will not arm of the first turn after departure is greater than 5 degrees in this plane. Also just mentioning it both FD's need to be on for LNAV to arm on the ground. Two of several checks the system does