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Different GS indications on FMS 747II and T7?

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    Different GS indications on FMS 747II and T7?

    This question refers to 747II and T7 as well.
    Just posting it once here.
    Recognized that the glideslope angle indication on
    fms of both aircraft is different for the same approach.
    Same navigation data used and same pre-created route.
    Approach = LEPA from FF06L to RWY06L.
    747-FMS indicates angle of 2.98 and 777-FMS 3.07
    How can this be? Thanks.

    Gerhard Dresch
    Gerhard Dresch EDDS, P3DV4.5, Win7 Ult., i7-4790k @ 4.4 Ghz, GTX 1080ti

    #2
    Do both planes have the same AIRAC data?
    Þröstur Reynisson

    Comment


      #3
      Good question; although the variance is -0.67% / +2.3%, which ain't much but still an interesting observation. My guess (all I got here) is that this has to do with possible corrections the FMS calculation of slope makes that takes into account the different geometries of the aircraft (location of GS antenna verses location of rear main gear).

      As is often said, aviation is not an exact science. This is still a 3.00 deg glideslope.
      Dan Downs KCRP

      Comment


        #4
        Originally posted by Þröstur Reynisson View Post
        Do both planes have the same AIRAC data?
        Yes, they have.

        Gerhard Dresch
        Last edited by GerdD; 16Jul2019, 18:01.
        Gerhard Dresch EDDS, P3DV4.5, Win7 Ult., i7-4790k @ 4.4 Ghz, GTX 1080ti

        Comment


          #5
          Originally posted by DDowns View Post
          Good question; although the variance is -0.67% / +2.3%, which ain't much but still an interesting observation. My guess (all I got here) is that this has to do with possible corrections the FMS calculation of slope makes that takes into account the different geometries of the aircraft (location of GS antenna verses location of rear main gear).

          As is often said, aviation is not an exact science. This is still a 3.00 deg glideslope.
          Understand, but should FMS not show this value from the AIRAC data and not calculate something itself?
          Last edited by GerdD; 16Jul2019, 19:05.
          Gerhard Dresch EDDS, P3DV4.5, Win7 Ult., i7-4790k @ 4.4 Ghz, GTX 1080ti

          Comment


            #6
            Originally posted by GerdD View Post

            Understand, but should FMS not show this value from the AIRAC data and not calculate something itself?
            Why?

            You're supposed to follow the raw data anyway.
            Kyle Rodgers
            PMDG Developer Emeritus

            Comment


              #7
              Originally posted by krodgers View Post

              Why?

              You're supposed to follow the raw data anyway.
              What kind of raw data? Is GS angle not a fixed defined value?
              I am talking about the display in FMS where both planes show
              different values and not about the actual follow.
              Last edited by GerdD; 16Jul2019, 19:31.
              Gerhard Dresch EDDS, P3DV4.5, Win7 Ult., i7-4790k @ 4.4 Ghz, GTX 1080ti

              Comment


                #8
                Originally posted by GerdD View Post

                What kind of raw data? Is GS angle not a fixed defined value?
                By raw data Kyle means you stay on the GS that is indicated on your flight instrument, either following the raw GS deviation indicator or the FD cues. Also, the crude navdata we still are using does not define the GS angle, only the runway elevation and final approach fix altitudes are given in the data. This will change in the ARINC 424 data.

                Bottom line you really are spending too much time on this.
                Dan Downs KCRP

                Comment


                  #9
                  Originally posted by GerdD View Post

                  What kind of raw data? Is GS angle not a fixed defined value?
                  I am talking about the display in FMS where both planes show
                  different values and not about the actual follow.
                  What Dan said, but to add detail:

                  Raw data is the diamond indicators on your PFD when shooting an ILS approach. The GS, as defined by the radio signal driving those diamond indicators is a fixed path.

                  What you are referring to is the calculated path between two points that the FMS is defining. This is simply an example or additional value. Ignore it.

                  Example:
                  You are following the Autobahn. I can give you all kinds of calculated course values between Stuttgart and Munchen: drive heading 100 for 40 feet, then 105 for 3 miles, then 090 for 5 miles, and then...(usw.)

                  ...or you could follow the raw data: the motorway.
                  Kyle Rodgers
                  PMDG Developer Emeritus

                  Comment


                    #10
                    Originally posted by krodgers View Post

                    What Dan said, but to add detail:

                    Raw data is the diamond indicators on your PFD when shooting an ILS approach. The GS, as defined by the radio signal driving those diamond indicators is a fixed path.

                    What you are referring to is the calculated path between two points that the FMS is defining. This is simply an example or additional value. Ignore it.

                    Example:
                    You are following the Autobahn. I can give you all kinds of calculated course values between Stuttgart and Munchen: drive heading 100 for 40 feet, then 105 for 3 miles, then 090 for 5 miles, and then...(usw.)

                    ...or you could follow the raw data: the motorway.
                    Thanks for the detailed explanation. Anyway it´s confusing if both planes are calculating different.
                    Gerhard Dresch EDDS, P3DV4.5, Win7 Ult., i7-4790k @ 4.4 Ghz, GTX 1080ti

                    Comment


                      #11
                      Originally posted by GerdD View Post

                      Thanks for the detailed explanation. Anyway it´s confusing if both planes are calculating different.
                      Don't assign too much value to that information by the FMS. It is useful to verify that the GS is 3 deg and not 6; otherwise, just round off to the nearest tenth.

                      Something I thought of later after my first post that might help your understanding is knowing where information comes from. The LOC/GS deviation comes from the scenery, as do all simulated ground based radio signals. The FMS information comes from the navdata and the FMS has no knowledge of what is in the scenery. It is not unusual for the scenery, which can be a decade old, to have differences with the navdata, which can be updated every 28 days. Just as in real world instrument flight, it is a good idea to verify the FMS data (freq/crs/GS deg) with the scenery data (check the simulator's map and drill down to the airport and list the ILS) if you have any suspicions such as not receiving the LOC when you expect to.
                      Dan Downs KCRP

                      Comment


                        #12
                        Originally posted by GerdD View Post

                        Thanks for the detailed explanation. Anyway it´s confusing if both planes are calculating different.
                        It's really not though. As mentioned earlier: ignore it.

                        This is like getting confused your can of Coke on Lufthansa Regional got served with a yellow napkin, and the one on Lufthansa came with a blue one. You're still on Lufthansa, and you still got your can of Coke. The color of the napkin is of no real value, much like this calculated angle that is of no consequence, since you follow the magenta diamonds, and nothing else.
                        Kyle Rodgers
                        PMDG Developer Emeritus

                        Comment


                          #13
                          Originally posted by GerdD View Post
                          Is GS angle not a fixed defined value?
                          Well, sort of

                          For an ILS glidepath it is obviously a fixed path in space defined by the electronic glideslope beam.

                          However, with an uncompensated baro-VNAV system like that installed on the B744/B777, a VNAV glidepath is... not fixed in space.

                          The navdata in theory should always show the charted glidepath angle, but the actual angle flown by the aircraft will depend upon temperature (the colder, the flatter as the aircraft will be lower than indicated over the FAF).

                          In this case, I suspect Dan has hit the nail on the head: I am relatively sure that in the real FMCs the glidepath angle is part of the navdata, whereas I suspect the PMDG navdata format doesn't contain a field for glidepath angle and therefore this value is being calculated dynamically from the TCH and the FAF crossing height. If this calculation is performed differently (or there is a difference in the way figures are rounded before/during/after the calculation) on different aircraft then it is understandable that there might be small discrepancies.
                          Simon Kelsey

                          Comment


                            #14
                            777 real world:

                            Calculated angle on the CDU by the FMC is really only important on the non-precision approaches. If the calculated angle isn’t the same as the published chart, don’t use it. Select a different approach. ILS will vary with temp and pressure deviation from standard.
                            Regards,

                            Aaron Zylman

                            Comment

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