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The new LNAV system (after last update) vs the old one

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    The new LNAV system (after last update) vs the old one

    The context is a straight (RNAV) approach with an IAF at 4000 feet and then a FAF at 4000 feet where a 3 degrees slope starts towards the runway. This is how the aircrafts behaves:

    - Previous LNAV system (before update):
    Whatever the position of the aircraft relative to the IAF if I do Direct to the IAF, the new path goes to the IAF waypoint. If the aircraft is positioned after the IAF (relative to the runway) then it goes to the IAF and initiate a "U turn" in a kind of arc (not well depicted on the PFD though but nicely flown) to intercept the course between the IAF and the FAF. All speed/altitude restrictions are met perfectly.

    - New LNAV system (after update):
    The "unexpected" behavior occurs if the aircraft is positioned after the IAF (relative to the runway) when I do the Direct to. In this case the path doesn't go back to the IAF and the aircraft tries to turn as soon as possible (a kind of shortcut) to intercept the course between the IAF and the FAF. Even "worse" if the aircraft is positioned after the FAF (relative to the runway), the path turns directly to intercept the course after the FAF which positions the aircraft too high and too fast for the final course

    Is this a bug/issue with the new LNAV or this is the correct behavior?

    With the new LNAV system, how can I get the aircraft to go to the IAF (what ever the current position is) and avoid shortcuts?

    Sam Haj Houssain

    EDIT: as requested, an example of this behavior is the RNP Z 04L/04R at LFMN. As possible scenario, after GA the aircraft is heading to NERAS and ATC clears you for the approach again before getting to NERAS
    Last edited by Samhaho; 05Aug2022, 10:15.

    #2
    Sam,

    it would help a lot if you could tell us airport and approach so we could take the chart as a reference. It‘s difficult to imagine what‘s going on.
    Ryzen 9 5900X, RX 6900XT, 32GB DDR4 RAM @3600MHz, 4k
    Marc Eland
    GFO Beta

    Comment


      #3
      If you position the aircraft between the IAF and the FAF, or after the FAF, you can't expect it to align itself correctly on the ILS. You're asking too much of it. I don't know what the real aircraft would do but I don't think it could possibly steer the path you expect of it. The turns would be too tight.

      Comment


        #4
        Thanks for replying. I edited the post to mention an approach example where the behavior described occurs: RNP Z 04L/04R at LFMN. However as you can see this is a basic approach configuration, there is nothing special about it. The behavior would occurs for any approach. The key point here is where the aircraft is positioned when the Direct to is requested.

        Sam Haj
        Last edited by Samhaho; 05Aug2022, 09:39.

        Comment


          #5
          Originally posted by Kevin Hall View Post
          If you position the aircraft between the IAF and the FAF, or after the FAF, you can't expect it to align itself correctly on the ILS. You're asking too much of it. I don't know what the real aircraft would do but I don't think it could possibly steer the path you expect of it. The turns would be too tight.
          I am not asking the aircraft to align itself immediately, although this is what it does. It is the opposite, I am asking it to go all the way back to the IAF before doing the "U turn". Again It used to do that very well before the latest LNAV update.

          Sam Haj
          Last edited by Samhaho; 05Aug2022, 09:39.

          Comment


            #6
            Originally posted by Samhaho View Post

            I am not asking the aircraft to align itself immediately, although this is what it does. It is the opposite, I am asking it to go all the way back to the IAF before doing the "U turn". Again It used to do that very well before the latest LNAV update.

            Sam
            Depending on the "U turn" the plane might not have space to do it. In the old LNAV it would draw it but would not being able to fly it correctly. If you can post a screen of the magenta I am sure it would be more helpful.

            Also please sign with your full name first and last as per rules. Thank you.
            Chris Makris (Olympic260)
            PMDG Technical Support
            http://www.pmdg.com

            Comment


              #7
              Thank you for the feedback. Actually With the new LNAV the aircraft is trying even more tighter "U turn" as it initiates th turn to intercept the final course without going all the way back to the IAF which would have make the "U turn" less tight. the question here is rather why it doesn't go all the way back to the IAF. Again with the previous version of LNAV this used to work perfectly

              I'll post a screenshot, however unfortunately no way to show the behavior with the previous version of the LNAV

              Sam Haj
              Last edited by Samhaho; 05Aug2022, 10:17.

              Comment


                #8
                [QUOTE=Samhaho;n208423]
                Originally posted by cmakris View Post

                Depending on the "U turn" the plane might not have space to do it. In the old LNAV it would draw it but would not being able to fly it correctly. If you can post a screen of the magenta I am sure it would be more helpful.



                Thank you for the feedback. actually With the new LNAV the aircraft is trying even more steeper "U turn" as it initiates th turn to intercept the final course without going all the way back to the IAF which would have make the "U turn" less steep. the question here is rather why it doesn't go all the way back to the IAF. Again with the previous version of LNAV this used to work perfectly

                Sam Haj
                The new LNAV is more accurate and much closer to the real plane. You are the pilot if the plane does something that does not look right then you take command As I have wrote in another post real plane LNAV can draw strange lines and is up to the pilot to correct/fly

                On new LNAV a path is calculated using standrd ICAO turn formulas, non aircraft model specific like before, for a nominal 25 degree bank limit and speed values taken from the vertical path calculator (greater of predicted GS/TAS) on all waypoints. All arcs are constant radius, constant speed turns (repeat as dictated by predictions). Arcs are not affected by active ground speed when the aircraft is turning (one exception DTO's see below). The path is created, permanently stored and post-processed by the FMC.
                Chris Makris (Olympic260)
                PMDG Technical Support
                http://www.pmdg.com

                Comment


                  #9
                  [QUOTE=cmakris;n208425]
                  Originally posted by Samhaho View Post

                  The new LNAV is more accurate and much closer to the real plane. You are the pilot if the plane does something that does not look right then you take command As I have wrote in another post real plane LNAV can draw strange lines and is up to the pilot to correct/fly

                  On new LNAV a path is calculated using standrd ICAO turn formulas, non aircraft model specific like before, for a nominal 25 degree bank limit and speed values taken from the vertical path calculator (greater of predicted GS/TAS) on all waypoints. All arcs are constant radius, constant speed turns (repeat as dictated by predictions). Arcs are not affected by active ground speed when the aircraft is turning (one exception DTO's see below). The path is created, permanently stored and post-processed by the FMC.
                  Many thanks for the clarification, very useful. I have no doubt the new LNAV is more accurate. However I am sorry not making myself clear. The issue here is not why the aircraft is not able to fly a "tight" arc or not. I can perfectly understand the constraints for that. The issue here is rather why the aircraft is not going all the way back to the IAF when I request a Direct To the IAF, and instead tries to turn immediately for a "tight" arc. Going back all the way to the IAF in itself doesn't involve any turn or arc.

                  Sam Haj
                  Last edited by Samhaho; 05Aug2022, 10:16.

                  Comment


                    #10
                    [QUOTE=Samhaho;n208427]
                    Originally posted by cmakris View Post

                    Many thanks for the clarification, very useful. I have no doubt the new LNAV is more accurate. However I am sorry not making myself clear. The issue here is not why the aircraft is not able to fly a "tight" arc or not. I can perfectly understand the constraints for that. The issue here is rather why the aircraft is not going all the way back to the IAF when I request a Direct To the IAF, and instead tries to turn immediately for a "tight" arc. Going back all the way to the IAF in itself doesn't involve any turn or arc.

                    Sam Haj
                    If you post some screenshots it would be more helpful to see what is drawn and what it does
                    Chris Makris (Olympic260)
                    PMDG Technical Support
                    http://www.pmdg.com

                    Comment


                      #11
                      Sam,

                      my assumption is that the airplane wants to cut the corner but then - as mentioned by Kevin - there is too little room to do that with what the path will tell it to do next. I know this behaviour from CNC programming when the radius of the miller is larger than the allowed turn radius, the tool would / will run into the contour. But to find the ideal way to avoid this the control reads the next couple of sentences ahead and plans a good way to avoid this, sometimes, if too little room is available, it will just do strange things or just quit the job with an error and a red light, a plane however can‘t just stop, it has to find a solution that allows the pilot to take control, as Chris says. Below the line you‘re the pilot and have to know how to fly the approach correctly. If LNAV doesn‘t work, use HDG SEL or fly manually.
                      Ryzen 9 5900X, RX 6900XT, 32GB DDR4 RAM @3600MHz, 4k
                      Marc Eland
                      GFO Beta

                      Comment


                        #12
                        I'd need to see an example to know if I'm understanding you correctly, but if I am, this sounds like correct behavior unless the IAF is specifically coded as a flyover waypoint (something built into the procedure that a pilot has no control over.)

                        If the IAF isn't flyover (and there's no reason it has to be), then the FMC builds a smooth path that will pass inside the IAF so the aircraft can join the final approach path without overshooting.

                        I'm picturing the circumstance you describe to be one where you're on something like a downwind or base, closer to the runway than the IAF, and wanting to go out to the IAF and then back in. The box will shortcut the IAF so as not to overshoot. ATC would not clear you to the IAF from that position though.
                        Andrew Crowley

                        Comment


                          #13
                          Originally posted by Stearmandriver View Post
                          I'd need to see an example to know if I'm understanding you correctly, but if I am, this sounds like correct behavior unless the IAF is specifically coded as a flyover waypoint (something built into the procedure that a pilot has no control over.)

                          If the IAF isn't flyover (and there's no reason it has to be), then the FMC builds a smooth path that will pass inside the IAF so the aircraft can join the final approach path without overshooting.

                          I'm picturing the circumstance you describe to be one where you're on something like a downwind or base, closer to the runway than the IAF, and wanting to go out to the IAF and then back in. The box will shortcut the IAF so as not to overshoot. ATC would not clear you to the IAF from that position though.
                          Many thanks Andrew for your reply. You got the picture completely right. However let me try to give more precise description and ask a couple of questions

                          The approach I am using to train is the LFMN RNP 4L. I simulate a go around that it takes me to NERAS (almost 90 degrees to the runway) and then on the way to NERAS (under LNAV) the ATC clears me to the same approach (like you said exactly the aircraft may be closer to the runway than the IAF. So I insert the IAF (LEMPU) as the next way point (a direct to), which makes the aircraft fly a kind of downwind.

                          In the before update LNAV version, the aircraft goes to the IAF, a kind of flies-over the IAF (LEMPU) and turns (very smoothly) to intercept the course between the IAF (LEMPU) and the FAF. while in the new version of LNAV, as soon as I insert the IAF (LEMPU) as the next waypoint then a path is created to join the final course without going to the IAF

                          I have a couple of comments and questions:

                          - Going to all the way back to the IAF (LEMPU) doesn't make the turn less smooth than doing it immediately upon the direct to. However, as you put it rightly, in the previous version of the LNAV the aircraft somehow "overshoots " around the IAF (LEMPU) to intercept the course between the IAF and the FAF. So staying "inside" , as you put it, between the IAF and the FAF makes sens, but why turning immediately (too early) instead of doing that turn close to the IAF?
                          - You said the ATC would not clear me from a position before getting to NERAS, why?
                          - I do not know if the IAF is a fly-over waypoint or not. However in the FMC I can see the mention "bypass" next to the IAF. What does that mean? How do I know from the chart if a waypoint (in this case the IAF) is a fly-over or fly-by? Is there anyway make a waypoint a fly-over waypoint in the FMC?
                          - How can I force the aircraft (in LNAV) to go all the way to the IAF so the approach can be flown correctly? If I let the aircraft to do what it wants, it will turn and intercept the course immediately upon the direct to the IAF which is not good at all, since the aircraft will be too high and too fast.

                          Many thanks again for your help. I appreciate the time you're taking to look carefully to the topic.

                          Sam Haj
                          Last edited by Samhaho; 05Aug2022, 11:56.

                          Comment


                            #14
                            Originally posted by Ephedrin View Post
                            Sam,

                            my assumption is that the airplane wants to cut the corner but then - as mentioned by Kevin - there is too little room to do that with what the path will tell it to do next. I know this behaviour from CNC programming when the radius of the miller is larger than the allowed turn radius, the tool would / will run into the contour. But to find the ideal way to avoid this the control reads the next couple of sentences ahead and plans a good way to avoid this, sometimes, if too little room is available, it will just do strange things or just quit the job with an error and a red light, a plane however can‘t just stop, it has to find a solution that allows the pilot to take control, as Chris says. Below the line you‘re the pilot and have to know how to fly the approach correctly. If LNAV doesn‘t work, use HDG SEL or fly manually.
                            Thanks Marc for your help. Got what you're saying but this doesn't answer the question: what was wrong with the way the previous version of LNAV behaved?

                            Sam Haj

                            Comment


                              #15
                              Originally posted by Samhaho View Post
                              - I do not know if the IAF is a fly-over waypoint or not. However in the FMC I can see the mention "bypass" next to the IAF. What does that mean?
                              From the FCOM:

                              bypass.png

                              How do I know from the chart if a waypoint (in this case the IAF) is a fly-over or fly-by?
                              Fly-by waypoints are depicted as four-pointed stars. Fly-over waypoints as four-pointed stars with a circle around them.

                              Comment


                                #16
                                [QUOTE=Max Rate;n208454]


                                Very useful. Thanks a lot.

                                Sam Haj

                                Comment


                                  #17
                                  Hi Sam,

                                  your observed behavior matches with the expected behavior, since the IF (not IAF) LEMPU is threatened as fly-by waypoint by the LNAV.
                                  A shortcut like this requires pilot intervention and needs to be flown manually with HDG mode until you are close to reach a 90° abeam position to LEMPU.
                                  At that point you may select LEMPU as DCT in order to have the LNAV constructing a realistic turn.

                                  Originally posted by rsrandazzo View Post
                                  (You do have to manage the airplane properly tho!)
                                  You do not have permission to view this gallery.
                                  This gallery has 1 photos.
                                  Last edited by jpschuchna; 05Aug2022, 14:57.
                                  Jan-Paul Schuchna

                                  Comment


                                    #18
                                    You could also insert a custom waypoint another 1-2NM further out from LEMPU, like LEMPU222/-2
                                    Mike Murphy
                                    Commercial, Instrument, Rotorcraft-Helicopter

                                    Comment


                                      #19
                                      according to what he explains, if a bypass is not published on the chart, the fmc would calculate a bypass if the turn is very pronounced?
                                      Jose Zubillaga

                                      Comment


                                        #20
                                        Originally posted by jpschuchna View Post
                                        Hi Sam,

                                        your observed behavior matches with the expected behavior, since the IF (not IAF) LEMPU is threatened as fly-by waypoint by the LNAV.
                                        A shortcut like this requires pilot intervention and needs to be flown manually with HDG mode until you are close to reach a 90° abeam position to LEMPU.
                                        At that point you may select LEMPU as DCT in order to have the LNAV constructing a realistic turn.
                                        Thank you for the interesting input Jean-Paul. Got it now. As Stearmandriver put it " the FMC builds a smooth path that will pass inside the IAF so the aircraft can join the final approach path without overshooting". So this is the priority, not necessarily passing by the IF with the risk of "overshooting" the turn, which was the way the previous version of LNAV worked.

                                        So I understand now the proper way is "manually with HDG mode until you are close to reach a 90° abeam position to LEMPU". Which makes perfect sens.

                                        Helibrewer thanks for the suggestion. I tried it. But still, you cannot predict how the turn is constructed, it may or may not pass by LEMPU. It depends on the inbound course angle and how far the manual fix LEMPUXX/YY is positioned. Which is consistent with the above explanation form Andrew and Jean-Paul.

                                        Thanks all.

                                        Sam Haj

                                        Comment


                                          #21
                                          Originally posted by Samhaho View Post


                                          So I understand now the proper way is "manually with HDG mode until you are close to reach a 90° abeam position to LEMPU". Which makes perfect sens.
                                          The problem with this is that if ATC clears you direct to a point, you can't technically wait - you're expected to turn for that point immediately.

                                          That's what I meant when I said that ATC would not clear you to a point that's farther from the runway than you are, unless they're clearing you to fly a course reversal like a procedure turn. Otherwise you'll never be given more than a ninety degree intercept to a final approach course.

                                          If you're turned in when you're closer to the runway than the IF, you'll either be on vectors or sent direct to the FAF. The best way to handle it if you're on vectors is to create a course intercept to the FAF on the published inbound course. Then when you turn in on base (in heading select), once you're within 90 degrees of intercept, arm LNAV and it'll perform an intercept of the final approach course without overshooting.

                                          Andrew Crowley

                                          Comment


                                            #22
                                            Thank you Andrew ( Stearmandriver ) for the additional detail. When Jean-Paul said that the the proper way is manually with HDG mode until you are close to reach a 90° abeam position to LEMPU, He probably meant that I would not be cleared by ATC to do a direct to the IF but rather I would be vectored at least until to be abeam the IF. Which is totally consistent with what you're saying. At this point the new LNAV system constructs a nice turn without overshooting. I've tested it.

                                            Thanks to your and Jean-Paul ( jpschuchna ) explanations. I understand now the logic behind the correct behavior with the new LNAV system. With the old one it used to take me to LEMPU but at the expense of possible creating an overshooting turn. With the new LNAV it prioritize staying "inside" the IF even if it constructs the turn far closer to the runway. I hope I am summarizing this correctly.

                                            Comment

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