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  • Flight Management

    A curious mental segue from continental masses to air control gave rise to a question. There are some changes in load in airliners during flight. I don't want to bog down things with attempts to list them. I know fuel gets expended, but I have no idea how many other things could change. But it did make me wonder about the flight management computers. I know they get data input from various sources on the plane. Do they process those to decide what changes to make in aviating? Or do they just set goals for things like speed and angle of attack, and adjust control surfaces to achieve those goals? The latter approach seems easier, but I don't want to try to guess which it is.

  • #2
    Both.

    --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
    --- Defend what you say with arguments, not by imposing your credentials ---

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    • #3
      Sucks.

      Take it to ThreadDisaster.com

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      • #4
        Originally posted by Gabriel View Post
        Both.
        I tend to disagree.

        Given that the aircraft encounters all sorts of wind gusts, I don't think the autopilot is "pondering" center of gravity, or fuel burn and how it needs to adjust for the latter.

        I think it's predominately- what control surface input do I need to adjust the current attitude- regardless of how winds or weight is affecting the attitude.
        Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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        • #5
          Originally posted by 3WE View Post
          I tend to disagree.

          Given that the aircraft encounters all sorts of wind gusts, I don't think the autopilot is "pondering" center of gravity, or fuel burn and how it needs to adjust for the latter.
          Autopilot? Where did EC said "autopilot"?
          He mentioned "aviating". And not only that, he mentioned the "flight management computer" too.

          I think it's predominately- what control surface input do I need to adjust the current attitude- regardless of how winds or weight is affecting the attitude.
          And how do you (or the autopilot) knows, "what control surface input do I need"?

          "Let's try this. Oops, no, 7.2Gs, let's lower the elevator a bit, except that we don't have an elevator anymore..."

          Just kidding. Half. You should have seen the thread that I had written before loosing it, AGAIN! (this time I did press "submit" but the internet connection was down).

          Yes, you are right, the autopilot, at least the basic ones, don't know the weight or CG but are calibrated for each airplane so the autopilot knows how much input, more or less, is needed to achieve a give goal.

          In fact, the modern pitch-based digital autopilot in your new Cessna 172 can keep the needles solidly centered in an ILS approach with a small female pilot alone, no baggage and almost dry tanks or with 4 big male adults and as much fuel as will make for maximum gross weight, and there is nothing in a Cessna 172 to tell the autopilot what the current weight or CG is.

          In a big, modern airplane, the flight management computer will take the weight and CG data to tell the pilot a lot of important data, VERY related to "aviating". Things like V1, Vr, V2, Vref, trim setting for take-off, best climb speed, "green dot" speed... Not only that, but the FMC will not only "know" the wight and CG and give information, but also affect the CG by moving fuel from one tank to another to reduce trim drag in cruise or increase the stability during take-off and landing.

          Also, the autothrottle will not let you select speed below a certain threshold, and some autopilots will limit the bank angle below certain speeds. And those limit speed come from "knowing" the airplane''s weight.

          Finally, I really don't know but I suspect that in FBW systems, mainly Airbus type, the weight and perhaps the CG data is part of the flight control logic so when you pull back in the stick 0.2 inches and that commands a 0.2Gs pull-up, that goal is achieved more smoothly, quickly, accurately and with no overshot compared to what would happen with a closed-feedback "trial and error" system (also known as P-I-D because the input is Proportional to the parameter, to the Derivative of the parameter, and to the Integer of the parameter that is to be controlled). The knowledge of the airplane's weight, combined with the knowledge of the airplane's flight dynamics, will give an initial value of control input. Of course the flight control computer will not be happy with just applying the predicted input: it will monitor the response of the plane and adjust the input as/if needed to achieve the parameter's goal, based now yes on a close-feedback control loop.

          --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
          --- Defend what you say with arguments, not by imposing your credentials ---

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          • #6
            I wasn't specifically referring to autopilot. If the plane was on autopilot, it would have to be running some sort of routine to keep course and altitude. But if the software stands between the pilot and the controls, then it would have to interpret whatever the pilot does to achieve the same result as autopilot. That is, not just literally do whatever the pilot tells it to do or why have software at all?

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            • #7
              Originally posted by Gabriel View Post
              ...loosing it, AGAIN! (this time I did press "submit" but the internet connection was down)...
              And to think, we want this computer stuff living between the yoke and the control surfaces...
              Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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              • #8
                Originally posted by 3WE View Post
                And to think, we want this computer stuff living between the yoke and the control surfaces...
                In the 1970's, I was working in a bank in the check processing department. We had a mainframe that regularly froze and had to be cycled. When ever the 747 came out and I heard "computers will now control the plane" (I didn't read into it in any depth), I thought: Oh, that's really reassuring. Well, it turned out to be much, much safer than I thought, but I didn't have much faith in computing technology at the time. When a pilot typed in the wrong code for an airport in South America and flew into a mountain range, that seemed like a GIGO situation.

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                • #9
                  Originally posted by EconomyClass View Post
                  In the 1970's, I was working in a bank in the check processing department. We had a mainframe that regularly froze and had to be cycled. When ever the 747 came out and I heard "computers will now control the plane" (I didn't read into it in any depth), I thought: Oh, that's really reassuring. Well, it turned out to be much, much safer than I thought, but I didn't have much faith in computing technology at the time. When a pilot typed in the wrong code for an airport in South America and flew into a mountain range, that seemed like a GIGO situation.
                  If this is a topic about automation capabilities and/or dependence thereon, I can see this being a VERY long thread.

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                  • #10
                    Originally posted by EconomyClass View Post
                    Do they process those to decide what changes to make in aviating? Or do they just set goals for things like speed and angle of attack, and adjust control surfaces to achieve those goals? The latter approach seems easier...
                    Gabriel: I don't disagree with your comments- and part of it is my good ole "absolute statement rule".

                    When I see these words from Economizer, I have to say that "they" basically do the "latter, easier approach"...what do I need to do to address what I see before me right now.

                    Of course, I'm not 100% sure who "they" are (Otto specifically, or a grander piece of FMS software).

                    No doubt there are computers and routines looking deeper and providing output- but those things probably aren't analyzing weight and balance and then going directly to the elevators with control inputs.

                    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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                    • #11
                      I just think a computer routine that is goal-oriented is less vulnerable to complexity-related errors. Though the data for all the important stuff ought to be there for the human to read in case the plane struggles in meeting the goals. I wonder how this relates to factory automation. An automated factory has many machines working in synch, but being separate pieces of equipment, each one could plausibly vary from its individual directives.

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                      • #12
                        Maybe I'm misunderstanding the OP

                        ..But the FMS is like the olden day navigator and the auto pilot is like the olden day pilot in command. The auto pilot will steer/climb/descend wherever and whenever the FMS tells to. They can be linked ( or unlinked) with the flip of a switch and called an auto flight control system. The FMS is loaded with the latest airport info and navigation map info by maintenance crews every couple months. FMS' know where they there are by the inertial reference units, or maybe these days by GPS. Been awhile since I was an Avionics guy. The autopilot takes data from accelerometers/cadc/IRU's etc and adjust the flight surfaces as needed-except for flaps and spoilers. That's the pilots job.

                        As far as load changes, passengers will for the most part, stay put as will cargo and luggage. Fuel moves from center tank and each wing uniformly unless the crew doesnt want it to or there is a malfunction. No automation with fuel pumps or tank selection. That's why we need the crew. The AFCS lightens the crew's load but it doesnt do everything.

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                        • #13
                          Originally posted by T.O.G.A. View Post
                          Fuel moves from center tank and each wing uniformly unless the crew doesnt want it to or there is a malfunction. No automation with fuel pumps or tank selection.
                          T.O.G.A.,
                          How old are you? Or how long not involved with airplane systems, even for curiosity purposes?

                          The fuel management is automatic in most modern planes. Some of them not only switch tanks "by themselves", but actively transfer fuel from one tank to another to adjust the CG, without pilot intervention.

                          Of courser the crew has overriding authority, and can stop this automation and do it manually.

                          --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                          --- Defend what you say with arguments, not by imposing your credentials ---

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                          • #14
                            Well, if you must know

                            I am fifty, and my last day as a line mechanic/avionics technician was September 12, 2001.

                            Still interested in aviation to this day-from a curiosity standpoint.. I learned rapidly that there is waaaaaaaay more money in manufacturing automation. To EC, factory automation is nowhere near as synchronized as a modern aircraft's, yet, in many instances way more advanced.

                            It is good to know this fuel transfer capability exists today but the planes I worked on did not have that capability. Could be one reason why the media says pilots dont know how to fly anymore.

                            I love learning new things about flight, and even relearning things That were rote in 1982, but fuzzy today.

                            How old are you? oooooh, nevermind. it's the tone of your posts that make you sound much younger than you probably are.

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                            • #15
                              Originally posted by T.O.G.A. View Post
                              How old are you? oooooh, nevermind. it's the tone of your posts that make you sound much younger than you probably are.
                              I'm 42, and never worked in aviation. I'm a low time PPL currently not active and an aeronautical engineer that never worked as such but where I learned the things about physics, structures, aerodynamics, performance and flight mechanics that are my strong points in aviation. The rest is part self learning for curiosity and part not knowing really what I'm talking about.

                              And sorry for the tone. It was not the intention.

                              --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
                              --- Defend what you say with arguments, not by imposing your credentials ---

                              Comment

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