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Cannot change FBW Trim Ref Speed

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    Cannot change FBW Trim Ref Speed

    Hey guys,

    I have a problem with my 777. I haven't flown it in ages, so I'm not sure if I'm doing something wrong, or if it's a bug (which I don't think).

    When I get airborne, my FBW Trim Ref Speed is set to 181kn, regardless of my weight. Always. I did 5 flights now, all with different fuel and cargo amounts (777F) and on every flight it was behaving like that. As far as I know, the ref speed should normally be changeable by using the trim switches. At least that's was the introduction manual says about this topic. However, when I'm in the air shortly after takeoff (and above 100ft, so FBW kicks in), I try to use the trim switch to raise the ref speed, because I don't want to have it at 181kn. Nothing happens. Regardless in which direction I try to "trim", the ref speed will stay at 181kn. I already checked my control setups, if my assigned buttons work, and yes, they do. They are correctly assigned and work well on the ground for the stab trim (when FBW is not engaged) and for any other PMDG aircraft. But in the T7, once FBW is engaged, they just stop working.

    I can see on the stab trim indicator to the left of the throttles that I'm moving the stab trim (when I'm using the trim switches), so something happens, but it's not the right thing that is happening. Obviously.

    If someone could point me in the right direction of a fix (or tell me what I might be doing wrong), so I can have fun flying this thing again. Right now, handflying is borderline impossible due to this issue.
    Regards,
    Jan Mebus

    #2
    Try changing the panel state... maybe the one you are using has a problem.
    Dan Downs KCRP

    Comment


      #3
      Will try that tomorrow, thanks for the suggestion!
      Regards,
      Jan Mebus

      Comment


        #4
        I’m confused as to why you think the fly-by-wire is inactive below 100ft. What happens when the jet rotates? Cables are moving the elevator? (Trick question)
        Regards,

        Aaron Zylman

        Comment


          #5
          Originally posted by DDowns View Post
          Try changing the panel state... maybe the one you are using has a problem.
          Thanks, that helped. I always used a custom Cold&dark panel state. Now I tried it with the default cold&and dark, and now it works as expected!

          Originally posted by azylman1 View Post
          I’m confused as to why you think the fly-by-wire is inactive below 100ft. What happens when the jet rotates? Cables are moving the elevator? (Trick question)
          I should've phrased that better. Below 100ft AGL, the control surfaces will just simply do what the pilot commands via the control column. Above 100ft AGL, the computer logic kicks in and starts calculating the control surfaces movement based on the inputs of the pilot, the state of the aircraft and the FBW trim ref speed.

          For the same reason I haven't had the trim problem below 100ft AGL, just because the C*U logic isn't operating at that moment, so there was no trim ref speed that the FBW tried to maintain.
          Regards,
          Jan Mebus

          Comment


            #6
            Originally posted by Learjet View Post
            Thanks, that helped. I always used a custom Cold&dark panel state. Now I tried it with the default cold&and dark, and now it works as expected!


            I should've phrased that better. Below 100ft AGL, the control surfaces will just simply do what the pilot commands via the control column. Above 100ft AGL, the computer logic kicks in and starts calculating the control surfaces movement based on the inputs of the pilot, the state of the aircraft and the FBW trim ref speed.

            For the same reason I haven't had the trim problem below 100ft AGL, just because the C*U logic isn't operating at that moment, so there was no trim ref speed that the FBW tried to maintain.
            Old panel states should be used with care when a version change took place, either due to aircraft or sim updates. If something regarding the airplane usage that has worked before ceases working after an update the panel state is the first thing to check. Glad you got it solved.
            i7-6700k, GTX 1080TI, 32GB DDR4 RAM @1600MHz, 4k

            Comment


              #7
              Yeah, I know, not really smart. I checked everything, control assignments, calibration, etc. But I just didn't think about that at the moment, even though I know that corrupted panel states can cause issues.
              Regards,
              Jan Mebus

              Comment


              • DDowns
                DDowns commented
                Editing a comment
                Generally if the problem doesn't make sense then always suspect the panel state.

              #8
              Originally posted by Learjet View Post
              I should've phrased that better. Below 100ft AGL, the control surfaces will just simply do what the pilot commands via the control column. Above 100ft AGL, the computer logic kicks in and starts calculating the control surfaces movement based on the inputs of the pilot, the state of the aircraft and the FBW trim ref speed.

              For the same reason I haven't had the trim problem below 100ft AGL, just because the C*U logic isn't operating at that moment, so there was no trim ref speed that the FBW tried to maintain.
              Where is that in the FCOM? Never heard of that before.
              Regards,

              Aaron Zylman

              Comment


                #9
                FCOM 9.20.10 Starts a discussion on pitch control modes and includes how normal mode on the ground differs from airborne. The details of when it transitions from on ground to airborne are not included by Boeing as too much information for the pilot. Jan reveals some of that detail and it makes sense that the control laws should not change during rotation or initial lift off and not until 100 AGL. There was a lot of information provided by RSR to the test team regarding C*U but no where in the FCOM or FCTM will you find the term C*U. Boeing doesn't seem to want to reveal to many details for the poor overburdened pilots who may not understand all that technical stuff ..... tongue in cheek of course.
                Last edited by DDowns; 03Dec2019, 20:23.
                Dan Downs KCRP

                Comment


                  #10
                  Interesting.
                  Regards,

                  Aaron Zylman

                  Comment


                    #11
                    Ok so we are discussing here the pitch trim system.

                    On the ground, in with the PFCs in normal mode, moving the trim switches on the yoke moves both the stabilizer and the trim reference speed. In the air, it only changes the trim reference speed.

                    Alternate pitch trim works just a little bit differently but just so there’s no further confusion, the Primary Flight Computers in Normal mode are still controlling the primary flight controls whether on the ground or in the air.

                    The elevator, whether controlled on the ground or in the air is still given commands from the yoke to the ACES, then to the PFCs, back to the ACES, then to the control surfaces. Assuming again the PFCs are in Normal mode. Regarding the elevator, the PFCs are always looking at the trim reference speed in Normal mode whether you’re on the ground or in the Air.
                    Last edited by azylman1; 07Dec2019, 14:34.
                    Regards,

                    Aaron Zylman

                    Comment


                      #12
                      You don't seem to understand my point. Sure the PFC controls the control surfaces while on the ground (and below 100ft AGL), but it won't compute it's inputs in regard to the situation of the aircraft. If I pull on the yoke, it will just "replicate" this movement with the elevators, whilst it won't do that once you're high enough for the C*U logic to engage. Then it will just move the elevators (and other control surfaces) in regard to the state (speed, AOA, bank, etc.) of the aircraft.

                      At least that's how I understand the system, and that's how it would make the most sense.
                      Regards,
                      Jan Mebus

                      Comment


                      • azylman1
                        azylman1 commented
                        Editing a comment
                        Ok, after further digging and reading I found an article, mostly about airbus law but does reference this 777 C*U stuff. You are correct in that C*U doesn’t engage until airborne but that’s so much deeper than Boeing intended for the pilots to understand. In a nutshell this C*U is basically providing information to the PFCs for calculating the correct amount of control input which in turn is offering feedback for the pilots because there’s no physical connection to the flight controls through the yoke.

                        And it seems to be completely unrelated to the pitch trim reference speed issue you had before.

                        Moving the pitch trim switches on the ground still adjusts the pitch trim reference speed and physically changes the angle (attitude) of the horizontal stabilizer. In the air, they do not adjust the physical attitude of the horizontal stabilizer; only the trim reference speed.

                      #13
                      Originally posted by Learjet View Post
                      You don't seem to understand my point. Sure the PFC controls the control surfaces while on the ground (and below 100ft AGL), but it won't compute it's inputs in regard to the situation of the aircraft. If I pull on the yoke, it will just "replicate" this movement with the elevators, whilst it won't do that once you're high enough for the C*U logic to engage. Then it will just move the elevators (and other control surfaces) in regard to the state (speed, AOA, bank, etc.) of the aircraft.

                      At least that's how I understand the system, and that's how it would make the most sense.
                      Which trim switches are you using? You need to use the yoke switches, not the standby trim switches on the pedestal.

                      Comment


                        #14
                        Kevin, I have absolutely no clue what you're talking about. If you're referring to my problem, I solved that a few days ago (as stated above).
                        Regards,
                        Jan Mebus

                        Comment


                        • Kevin Hall
                          Kevin Hall commented
                          Editing a comment
                          Apologies, Jan. For some reason I had it in my head that the alternate trim switches moved the stabiliser directly but didn't change trim speed. Although that makes idea no sense at all (the trim speed has to change as well) I never questioned it as I don't display the trim ref speed and rarely touch the alternate switches. Looking at the FCOM just now my mistake is clear.
                          Last edited by Kevin Hall; 09Dec2019, 22:18.

                        #15
                        He means alternate pitch trim switches on the center pedestal next to the speed brake handle. The primary pitch trim switches are on the yokes.

                        If your problem is solved then congratulations, because that can be frustrating when it doesn’t work right.

                        Regarding the flight controls and pitch trim switches, What I stated above comes directly from my real world 777 FCOM. I didn’t intend to spark a debate, just shed light on how the system works and what it’s doing.
                        Regards,

                        Aaron Zylman

                        Comment


                          #16
                          I'm very aware of the fact that he meant the trim switches on the center pedestal. I still have no idea why he was referring to them though, because I made clear that that wasn't the problem, way earlier in this thread.

                          Regarding your comment on my post Aaron. As Dan already said, the FCOM doesn't go into the deeper parts of the C*U logic. And that was the reason why neither Dan or I could provide you with a quote from the FCOM. As you correctly said, this is beyond all knowledge that a pilot need to know, but Dan and I stated that way before. And yes, it is also true that the stabilizer is only directly movable by the pilots while on ground. As you said, in the air it is fully controlled by the PFC and the FBW trim ref speed. Never said anything else.

                          That this is of course unrelated to my problem is obvious as well, this was just an interesting discussion regarding the airliners systems.
                          Regards,
                          Jan Mebus

                          Comment

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