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Discrepancy between ET and ACARS flight time?

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    Discrepancy between ET and ACARS flight time?

    Hi folks,

    has somebody else noticed on long flight, that with the digital clock type there is a discrepancy between the ET and ACARS' flight time?
    On my last flight UA840 YSSY-KSFO I noted ET 11:52 and a flight time of 11:54 about 12 hours into the flight.

    Is this true 77W aircraft behaviour?
    ACARS' flight time was closer to the 'true' flight time.

    Thanks.
    Andre(as) Berg --- i7 8700K 12MB 3.7G. MSI Z370 Tomahawk. DDR4 16G. SSD 500G M.2. MSI RTX2080 DUKE 8G

    #2
    Provided the 777 works the same as the 737MAX: The ET starts when the weight on wheels sensor reports the aircraft being in flight and ends 30 seconds after touchdown. Maybe that's where the difference comes from.
    Takeoffrun usually takes around 30 seconds, landing run around the same time. Add both together it if you round it up you have two minutes difference.

    Comment


      #3
      Originally posted by Emi View Post
      Provided the 777 works the same as the 737MAX: The ET starts when the weight on wheels sensor reports the aircraft being in flight and ends 30 seconds after touchdown. Maybe that's where the difference comes from.
      Takeoffrun usually takes around 30 seconds, landing run around the same time. Add both together it if you round it up you have two minutes difference.
      Thanks, Emi,

      however, that was 12 hours into the flight, so only half of your calc.
      The question is, does ACARS start the timer differently?
      Andre(as) Berg --- i7 8700K 12MB 3.7G. MSI Z370 Tomahawk. DDR4 16G. SSD 500G M.2. MSI RTX2080 DUKE 8G

      Comment


        #4
        I thought you were going to say there was a significant difference (like tens mins or something)

        If there is a rounding then you could have 11:52:29 and 11:53:31 (ie 62 sec) deriving the 2 minute indicated error

        I will have a look at the FCTM and Refresher manuals and try to figure out how the systems derive the numbers (I don[t think they go to that depth but will check)

        Not sure if there are any current 777 drivers on here, maybe Brian or Vlad will know how the system works on the -8F and be able to cast some light on it
        Adrian Howe

        Comment


          #5
          Originally posted by 19ab67 View Post
          Hi folks,

          has somebody else noticed on long flight, that with the digital clock type there is a discrepancy between the ET and ACARS' flight time?
          On my last flight UA840 YSSY-KSFO I noted ET 11:52 and a flight time of 11:54 about 12 hours into the flight.

          Is this true 77W aircraft behaviour?
          ACARS' flight time was closer to the 'true' flight time.

          Thanks.
          I asked this question of a beer drinking buddy of mine who has about 20,000 hours on the B777 and he laughed at me. 🤣
          Bode Bridges
          I Earned my Spurs in Vietnam

          Comment


            #6
            Originally posted by Bluestar View Post

            I asked this question of a beer drinking buddy of mine who has about 20,000 hours on the B777 and he laughed at me. 🤣
            Because two minutes discrepancy on a 12 hour flight is nothing?
            Because there is no discrepancy on the true bird?
            Because ET and ACARS Flight Time are / should be the same?
            Andre(as) Berg --- i7 8700K 12MB 3.7G. MSI Z370 Tomahawk. DDR4 16G. SSD 500G M.2. MSI RTX2080 DUKE 8G

            Comment


            • DDowns
              DDowns commented
              Editing a comment
              Probably the first..... I think Emi answered you.

            #7
            Originally posted by 19ab67 View Post

            Because two minutes discrepancy on a 12 hour flight is nothing?
            Because there is no discrepancy on the true bird?
            Because ET and ACARS Flight Time are / should be the same?
            Because aircraft aren't perfect machines. There are thousands of discrepancies you see every day and honestly, who cares about such two minutes?
            Here's another one I'm sure many flight simmers would love:
            Often the FMC commands a speed of let's say 257kt while the bug on the PFD shows 258. So what, doesn't matter to us, but I'm very sure if PMDG modelled this there'd be a ton of bugreports on the forums and ticket system.

            In fact, there's even an IATA Delay Code for "incorrect time recorded by ACARS system".
            Now how many simmers will even know what a delay code is?

            Comment


              #8
              Emi I agree, but didn't want to go down the who cares route...but you're right from an ops and engineering stand point it's not significant, there's sufficient safety factor in the policies to cover it

              On my EA300 we have four group members. The Hobbs meter kicks in at 60 knots and we always round up if it's going to click over to the next decimal....so you could end up with 6 mins in 60 of discrepancy. Reality is it evens out over time. I'd take 2 mins in 720 anytime

              I've only just thought though - when I am going at less than 60 knots whether it stops?

              ...or if I go backwards, would I be as foolish as Ferris Bueller in thinking that to drive the Ferrari 250GTO in reverse will unwind the odometer?

              Now I digress

              Adrian Howe

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