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  • Pitot Tube Failure

    Hello everyone,,
    Involved in a discussion of the computerization of recently produced automobiles, trying to make an analogy to fly by wire. Attempting to reference pitot tube failure. Got hung up on how fly by wire systems compensate for loss of pitot tube sensing. I know that not all systems are dependent on working pitot tubes. But all I remember of the long detailed discussions of crashes involving pitot tubes is something about power and attitude. So maybe someone can remind me of where the pilots turn when they no longer have air speed data. It is my impression that air speed is very critical to flight management, but pilots can still aviate after the air speed is unreliable.

  • #2
    Originally posted by Jhonmicky View Post
    Got hung up on how fly by wire systems compensate for loss of pitot tube sensing.
    They don't, unless you consider losing protections, degrading the flight law and the autopilot stopping working a "compensation".

    But all I remember of the long detailed discussions of crashes involving pitot tubes is something about power and attitude. So maybe someone can remind me of where the pilots turn when they no longer have air speed data.
    Exactly. Pilots (not fight automation or fly-by-wire systems) turn to P+P=P, which means "Power + Pitch = Performance", which in turn means what combination of power (thrust) setting and pitch attitude will give you the desired airspeed and vertical speed.

    A good question could be why, to this day and to my knowledge, P+P=P has not been incorporated in the flight automation logic. It doesn't seem to be so complicated.

    --- 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


    • #3
      Originally posted by Jhonmicky View Post
      Hello everyone,,
      Involved in a discussion of the computerization of recently produced automobiles, trying to make an analogy to fly by wire. Attempting to reference pitot tube failure. Got hung up on how fly by wire systems compensate for loss of pitot tube sensing. I know that not all systems are dependent on working pitot tubes. But all I remember of the long detailed discussions of crashes involving pitot tubes is something about power and attitude. So maybe someone can remind me of where the pilots turn when they no longer have air speed data. It is my impression that air speed is very critical to flight management, but pilots can still aviate after the air speed is unreliable.
      Redundancy and reversion. FBW needs agreement from two pitot tube sensors to function in Normal Law (with flight envelope protections). Therefore there are at least three pitots. If one pitot is lost, there are still at least two in agreement, so the condition is considered fail-operational, the FBW remains in Normal Law. However, if multiple pitots are lost, and there is no agreement between at least two of them, FBW degrades to a reversion control law where certain protections are no longer in force. FBW continues to function however.

      Some aircraft have a red-green speed range scale that replaces the normal speed-tape on the primary flight display in the event of a loss of pitot airspeed data. The scale is driven by calculations using angle-of-attack data instead of pitot data, and allows the pilots to fly within a safe (green) speed range until the pitot data restores (multiple pitot failure is almost always the result of ice ingestion that overwhelms the pitot heater's ability to clear it, but typically only lasts a short time before the heater can restore function.) Otherwise, pilots must use the P + P = P formula Gabriel mentioned. Values for pitch and power are provided in the Quick Reference Guide that pilots keep close at hand for emergencies.

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      • #4
        Originally posted by Gabriel View Post
        A good question could be why, to this day and to my knowledge, P+P=P has not been incorporated in the flight automation logic. It doesn't seem to be so complicated.
        Maybe because that's not a super accurate way to measure speed: it takes a while for a plane to actually settle on an airspeed. (Ever follow any of the Garrison stuff about getting the airplane "on the step" like you do a speed boat?...it's kind of a fallacy, but then again, there's some fundamental truths, too).

        Don't get me wrong, it's much preferred to a relentless pull up.

        And, we have a new insight into Evan...so if the system goes crazy and you don't have airspeed, you cannot choose a known and familiar power and pitch from memory, but you CAN look it up in an acronym book. I guess that eliminates you from accidentally using a combination from your 150 days. Fascinating.
        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
          And, we have a new insight into Evan...so if the system goes crazy and you don't have airspeed, you cannot choose a known and familiar power and pitch from memory, but you CAN look it up in an acronym book. I guess that eliminates you from accidentally using a combination from your 150 days. Fascinating.
          First the known (memorized) and familiar settings to stabilize, then the more weight/altitude specific ones from the QRH. It's not a new insight 3WE, and it's not that fascinating.

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          • #6
            Originally posted by 3WE View Post
            Maybe because that's not a super accurate way to measure speed: it takes a while for a plane to actually settle on an airspeed.
            I won't argue that. But will you argue that a flight control computer + autopilot can do it better, more accurately, and more reliably than a human pilot startled by he sudden disconnection of the autopilot out of nowhere?

            --- 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


            • #7
              Originally posted by Evan View Post
              Some aircraft have a red-green speed range scale that replaces the normal speed-tape on the primary flight display in the event of a loss of pitot airspeed data. The scale is driven by calculations using angle-of-attack data instead of pitot data, and allows the pilots to fly within a safe (green) speed range until the pitot data restores.
              This system was optional in the planes types that even offered it as an option.
              And then I heard critiques because AoA is very non-sensitive to airspeed at high speeds, meaning that you need big variations in airspeed to make a meaningful change in AoA (always keeping 1G) and that, with that considered, it was even less accurate than P+P=P (although I don't understand why, since pitch -one of the P- is as sensitive as AoA, in delta-pitch = delta-AoA if flight path remains constant).
              But anyway, I've seen this system being dis-recommended by some experts.

              --- 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


              • #8
                Originally posted by Gabriel View Post
                ***will you argue that a flight control computer + autopilot can do it better, more accurately, and more reliably than a human pilot startled by he sudden disconnection of the autopilot out of nowhere?***
                That wasn't what you asked earlier.

                The answer to your question is very simple: "No- computers are generally a lot faster and more accurate and therefore, generally do a better job"

                The answer to the real question is not so simple: "Something has gone awry and we don't know what the airspeed is- do I invoke a second system (which may or may not be functioning correctly) and which will have +/- 10 knots accuracy as the plane settles, OR do I turn the system over to the human who often has a little bit better perception of what's actually going on.

                I would further argue that the loss of an airspeed indication usually comes from a rather minor problem, so maybe it's generally better to dump it in the pilots lap so he can look up pitch and power values in Evan's acronym book since he can't revert to familiar settings.

                Are there some exceptions...certain persons in stormy weather in the dark of night, overload, panic...yes?.

                I guess we need more lines of code, but be sure to not let a DCAS type flaw sneak in.
                Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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                • #9
                  Originally posted by Gabriel View Post
                  Blah blah blah- I heard critiques blah blah- this system being dis-recommended by some experts.
                  Because???

                  Blah blah blah Translation: [It's not all that accurate.]

                  Also- watch a plane land in brisk, but-not-extreme wind...lot's of pretty remarkable pitch changes...Guess what the airspeed and glideslope and lateral alignment looks like: Amazingly dang almost dead on AND a hell of a lot better than you or I could ever do.
                  Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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                  • #10
                    Originally posted by 3WE View Post
                    The answer to the real question is not so simple: "Something has god awry and we don't know what the airspeed is- do I invoke a second system (which may or may not be functioning correctly) and which will have +/- 10 knots accuracy as the plane settles, OR do I turn the system over to the human who often has a little bit better perception of what's actually going on.
                    You do nothing. The autoflight reacts to the loss of airspeed data by entering a flashing amber UAS mode, essentially a speed hold on the autothrottle and a pitch hold on the vertical guidance, using values exactly right for the current weight and altitude to maintain the previous airspeed at the previous level altitude (more or less, but within RVSM tolerance). The system also monitors AoA to ensure that the plane remains safely in the envelope while FADEC or EEC assures that power setting is resulting in expected thrust. The system knows attitude, AoA, static air data, weight, configuration and thrust, thus it can fly a safe airspeed derived from this data. The pilots are alerted to the mode with an aural alert and may take over manual control or opt to work the problem while autoflight does the flying. Guess which they would choose...

                    No upset = no pilot error.

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                    • #11
                      Originally posted by Evan View Post
                      You do nothing.
                      Evan, stop the Pavlovian regurgitation of acronym manuals and read the words.

                      Gabieee asked a very specific question as to why P+P=P isn't used as a computerized speed calculation method. He also discussed autopilot disconnects.

                      Based on ~75 personal experiences of watching P+P take a few minutes to settle on an airspeed, I offered the discussion that MABYE because it wasn't super accurate over the short term. What is your experience on using P+P=P to arrive at a target airspeed?

                      I love the concept of P+P=P being a powerful way to keep between stall and overspeed...I concede, it's not perfect for nailing a target speed. If there's a problem on the plane, follow procedures, but it's OK if you happen to know some fundamental stuff that's behind the procedures.

                      Gabieee agreed.

                      This is fundamental aeronautical stuff...I know you love to have it buried in cryptic acronyms in a computer, but the USA and forum polices allow discussion of that which you disdain.
                      Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

                      Comment


                      • #12
                        Originally posted by 3WE View Post
                        Evan, stop the Pavlovian regurgitation of acronym manuals and read the words.

                        Gabieee asked a very specific question as to why P+P=P isn't used as a computerized speed calculation method. He also discussed autopilot disconnects.

                        Based on ~75 personal experiences of watching P+P take a few minutes to settle on an airspeed, I offered the discussion that MABYE because it wasn't super accurate over the short term. What is your experience on using P+P=P to arrive at a target airspeed?

                        I love the concept of P+P=P being a powerful way to keep between stall and overspeed...I concede, it's not perfect for nailing a target speed. If there's a problem on the plane, follow procedures, but it's OK if you happen to know some fundamental stuff that's behind the procedures.

                        Gabieee agreed.

                        This is fundamental aeronautical stuff...I know you love to have it buried in cryptic acronyms in a computer, but the USA and forum polices allow discussion of that which you disdain.
                        I think you are playing devil's advocate. An automated flight version of P + P is going to arrive at the target airspeed faster than a human pilot and it's going to target that airspeed better and it's going to remain there more reliably. Now consider the scenario: You are 98% N1 and 0° pitch when the airspeeds are lost. The system instantly reorients the aircraft to 99% N1 and 2.5° pitch. How long is it going to take for the speed change to settle?

                        I have long agreed with Gabriel on this. I don't see a good reason not to have an autopilot reversion mode for UAS.

                        Comment


                        • #13
                          Originally posted by Evan View Post
                          You do nothing. The autoflight reacts to the loss of airspeed data by entering a flashing amber UAS mode, essentially a speed hold on the autothrottle and a pitch hold on the vertical guidance, using values exactly right for the current weight and altitude to maintain the previous airspeed at the previous level altitude (more or less, but within RVSM tolerance). The system also monitors AoA to ensure that the plane remains safely in the envelope while FADEC or EEC assures that power setting is resulting in expected thrust. The system knows attitude, AoA, static air data, weight, configuration and thrust, thus it can fly a safe airspeed derived from this data. The pilots are alerted to the mode with an aural alert and may take over manual control or opt to work the problem while autoflight does the flying. Guess which they would choose...

                          No upset = no pilot error.
                          3we will not like it, but I 100% agree with the above. Even more, to ensure you stay within RVSM, the pitch mode can be actually an altitude control mode and the thrust can be adjusted so as the plane flies the selected altitude with the target pitch. It would be very difficult (I would venture impossible) to control altitude within RVSM compliant margins if you try to control altitude with an open loop or even with a closed loop that relies on the throttles, because engine and speed react more slowly than pitch and vertical speed. With this alternative that I am proposing, you really don't care too much if the pitch wanders 0.3 degrees above and below the target while the throttle adjusts up and down trying to find the right speed that will keep the altitude nailed, while the elevator takes care of assuring that the altitude remains nailed.

                          Even more, this would be easily scalable to use the autopilot and autothrottle in a wide range of speeds and vertical speeds, which would allow not just to keep the current speed and altitude but also to change it in case the issue doesn't clear soon or in case you need to climb or descent to clear or get out of whether. Just let the system pick the right pitch goal for the selected speed and add or subtract from that speed the flight path angle that would result from te selected vertical speed combined with the speed. And even more, the manual flight can also take advantage of this by G-on stick-or pitch-on-stick, but always with autothrottles in selected speed, and the pitch can be limited with a good margin up and down from the points where the throttles would not be able to maintain the selected speed at idle (lower pitch limit) or at climb thrust (upper pitch limit).

                          And even when the direct speed measurement is lost, the system will still have 3 sets of static(altitude and vertical speed), AoA , attitude, magnetic heading, turn rate, and 3D linear and angular acceleration to make all the calculations and crosschecks that you want, combined with the weight, CG and config information together with the FADEC data, to avoid that the system is responding to inaccurate data to cover for the scenario presented by 3we. You still can loose quite a bit of other things before you would not have enough data to keep this system working, at which point there would be no chance other than handling the controls to the pilot and God helps us.

                          It really sounds so simple that I don't understand why it is not done, other than the manufacturer preferring a bigger risk of a deadly crash where the airline is liable rather than a very much smaller chance of a deadly crash where the manufacturer is liable.

                          --- 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


                          • #14
                            Originally posted by 3WE View Post
                            Gabieee asked a very specific question as to why P+P=P isn't used as a computerized speed calculation method. He also discussed autopilot disconnects.

                            Based on ~75 personal experiences of watching P+P take a few minutes to settle on an airspeed, I offered the discussion that MABYE because it wasn't super accurate over the short term. What is your experience on using P+P=P to arrive at a target airspeed?
                            So you prefer the human pilot to use P+P=P? Will the plane settle any faster?

                            Plus...
                            1- I don't necessarily agree that it takes so much to settle, especially taking into account that initially you will want mainly to maintain the performance tat you already had at the moment of loosing the speed indication, so you are already settled.

                            2- When you want to change from P1+P1=P1 to P2+P2=P2 the transition happens witha perfromance (speed and vertical speed) that remains within P1 and P1. For example, if you want to transition from straight and level at 280 knots to -1000 FPM at 250 knots, you set the pitch and power for -1000 FPM at 250 knots and, whatever it takes, the plane will smoothly transition from level to -1000 FPM and from 280 knots to 250 knots. So what's the risk?

                            3- Furthermore, the computer, much better than a human pilot, can make it more aggressive. In the previous example, the computer can idle the throttles and let the plane slow down to 250 knots while the autopilot smoothly increases the pitch to remain level and when approaching 250 knots the pitch can be let go down to keep the desired vertical speed while the throttle is increased to keep 250 knots. It doesn't get any better (or quicker) than that unless you start the descent by climbing to slow down faster.

                            I love the concept of P+P=P being a powerful way to keep between stall and overspeed...I concede, it's not perfect for nailing a target speed. If there's a problem on the plane, follow procedures, but it's OK if you happen to know some fundamental stuff that's behind the procedures.

                            Gabieee agreed.

                            This is fundamental aeronautical stuff...I know you love to have it buried in cryptic acronyms in a computer, but the USA and forum polices allow discussion of that which you disdain.
                            Yes, I agree conceptually. But the AF investigation revisited about a dozen of UAS events where, in 100% of them, the crews did not crash but did not either follow the prescribed P+P=P which I would almost say it would be still ok if you keep healthy traditional pitches and powers for the condition, except that all the interviewed crews declared that they had difficulty controlling the plane.

                            --- 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


                            • #15
                              What are you talking about???

                              You asked this:


                              Originally posted by Gabriel View Post
                              A good question could be why, to this day and to my knowledge, P+P=P has not been incorporated in the flight automation logic. It doesn't seem to be so complicated.
                              I answered 'because maybe it takes a few minutes for the plane's speed to stabilize'.

                              To which you asked aren't computers better than humans?

                              That's a totally different question-but yes. (refer back a few posts where I say yes.)

                              Then you say, so you prefer humans? Where the hell did that come from?

                              Is this discussion or are you on a misplaced mission to tell the world that flight management systems MUST be redone everywhere to include P+P=P airspeed indications?

                              If so, I'm the wrong person, and this forum is the wrong place. I'd prefer all that crap be left alone, INCLUDING the removal of MCAS... In the meantime, I'll leave it to the existing aeroengineers as to how pitot problems are handled by computers OR people.

                              Please note- if you change pitch and power, it doesn't matter if a human or computer does it- the plane moves towards the new speed at the same rate and settles on it asymptotically (unless you tweak it) computer OR human. This may not be a very precise way to control airspeed- so maybe that's why your colleagues haven't invested in writing computer code?

                              'Tweak' is key word. If the pitot is working things can be adjusted (over controlled if you will) to promptly establish a new speed. If it's not, you 'wait' through the asymptotic approach.
                              Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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