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too tight lnav path

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    too tight lnav path

    It is a bug or exist also in real aviation? image 1 - lnav path is too tight and aircraft can't carry it out. image 2 - aircraft wants to back to the correct path (by taking too hard turn) and can't do next turn
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    You're welcome.


      Thanks! But why PMDG don't set it in the FMC? Or it is impossible and all what we can do is use "spd intv"?


        I'm flying that departure right now just to see what happened, because it seems rather improbable that you'd be up at 14000 feet already halfway through the initial turn. What ZFW and fuel load did you configure, and which climb thrust setting?

        Edit: never mind. A lightly laden 737 could make that climb easily.

        Last edited by Max Rate; 23Jun2022, 15:45.


          Not exactly like this but in real life LNAV also has issues connecting points when the speed is too high. The real FMC would, in this case, create a discontinuity requiring the pilot to modify the speeds on the different points in the legs page. When this happens, we would enter a lower speed restriction and LNAV would connect again.

          On other types of procedures where you would see a INTC in the legs page, the airplane would simply turn to a certain heading or track (all while in LNAV) until intercepting a certain track inbound or outbound some other pseudo or a fix. This is not fully modelled in the PMDG but probably will once we get the LNAV update. Right now, all the PMDG does is try its best to fly from one point to some other point and then the next drawing somewhat of a bezier curve between the points taking expected groundspeed into account.

          Now, having said that, why the hell are you flying so fast? In a procedure like this one, where you'll spend some time flying not towards your destination, what you're interested in is climbing. You can type 210B/ at LUX and fly VNAV SPD, or you can fly VNAV SPD with SPD INTV 210kts of you can just fly LVL CHG 210kts. After LUX you can consider accelerating. We almost always fly slow during the initial turns on a departure not pointing to our destination. Usually, the faster you climb the sooner you'll get a direct (provided you've been granted unrestricted climb)
          Last edited by Aeromar; 23Jun2022, 15:59.
          Omar Josef
          737 FO
          757/767 rated


            By the way, I can confirm that without some kind of speed intervention (either entered into the FMC or by using the MCP), the aircraft flies that initial turn about 3 nm too wide, and completely botches the turn at LUX as well. The departure (RAPOR 7T at ELLX) doesn't have a speed restriction listed in the procedure, though, so the FMC doesn't know any better than to go to climb speed above 10000 feet. I-am-not-a-pilotâ„¢, but I'd say it's incumbent upon the crew to monitor the departure and to intervene if the aircraft in unable to maintain the published flight path.

            EDIT: yes, what Aeromar said.
            Last edited by Max Rate; 23Jun2022, 16:00.


              Yeah, thanks guys! Now I know more