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  • Pitot Tube Failure

    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 EconomyClass View Post
    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.
    Well as of late, SOP appears to be "crash the airplane".

    But seriously... there's a saying in piloting that goes "pitch plus power equals performance". Basically if you know what you're doing and about how much power the engines are putting out, you can adjust the pitch attitude of the airplane and end up with a) airspeed approximately what you want it to be and b) the aircraft not stalling.

    The above is a bit of an oversimplification... you do need some other info, need to know the airplane's operational parameters pretty well, and have good piloting skills. But if the circumstances are right it's definitely doable.
    Be alert! America needs more lerts.

    Eric Law

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    • #3
      Originally posted by elaw View Post
      Well as of late, SOP appears to be "crash the airplane".

      But seriously... there's a saying in piloting that goes "pitch plus power equals performance". Basically if you know what you're doing and about how much power the engines are putting out, you can adjust the pitch attitude of the airplane and end up with a) airspeed approximately what you want it to be and b) the aircraft not stalling.

      The above is a bit of an oversimplification... you do need some other info, need to know the airplane's operational parameters pretty well, and have good piloting skills. But if the circumstances are right it's definitely doable.
      What elaw says is correct.

      An airplane needs airspeed, not airspeed INDICATION, to fly.

      If you were happily flying straight and level at cruise speed with a given pitch and a given power, then the plane will keep happily flying in the same condition if you lose valid airspeed indication and do just nothing, or better, just do as to keep the same pitch and power.

      To be even more conservative (because airspeed can become unreliable when the plane is not flying straight and level (or in any steady state in general), the airplane manuals have very simple first reaction memory items that go something like "set climb thrust, keep pitch 10° up if below 15000 ft or 5° up if above 15000 ft". That will not keep the plane steadily flying in the same state, but will ensure a climb at a speed will between stall and overspeed. Once stabilized in that condition, the procedure calls to consult a "P + P = P" table (the "pitch plus power equals performance" that elaw mentioned) where you get, in function of the altitude and weight, the pitch and power required to fly straight and level. Once established in that condition, you make small power adjustments (while keeping the pitch) to fine-tune the vertical speed at zero.

      All that said, modern FBW planes and automation will detect the error and call it quits leaving the pilot in manual flight with no previous warning. This, in my opinion, is an unnecessary risk. In older planes, the autopilot could be limited not to exceed a given angle of attack even if that means not complying with the required performance- The P+P=P tables could be loaded in the system and enforced by the autopilot (once the system detects the problem) and at least give time to the pilot to asses the situation and react.

      Another related subject discussed in these pages was the necessity (or not) of a really redundant airspeed system. Airplanes have three pitot tubes and airspeed indicators for redundancy. However, there is no full triple redundancy really. To begin with, if only two airspeed indications are erroneous then you cannot tell which is the correct one, not at least until you do some crosschecking to complete the P+P=P equation (even if mentally: "we have normal cruise pitch and power, the altitude is stable, so the one airspeed that is consistent with a typical cruise speed is the correct one"). Second, you cannot call "fully redundant" to three identical systems that all be failed at the same time by a single event: icing. In the Air France thread, there was a valid discussion regarding alternate means to measure airspeed that don't involve a pitot tube and that are immune to icing, to have one really fully redundant alternate way to get an airspeed indication.

      --- 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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      • #4
        Pitot tubes go back to the beginnings of aviation and the need to measure airspeed. At some point, AUTOPILOT came into being, and then electronic autopilot. Electronic autopilot depends on data input, principally inertial reference and air data. Part of the air data is supplied by the pitot tubes and the autopilot systems cannot function without it. That data is collected by a subsystem called an ADIRU (Air Data Inertial Reference Unit). If the ADIRU detects a deviation between pitot probe values, it declares a fault and goes offline. With the ADIRU's offline, the AUTOPILOT cannot function and also declares fault and goes offline as well. This is not a problem per se because the autopilot is merely a tool to be used when things are going as they should. When they aren't, the pilots are there to fly the plane. All of this predates FBW. The non-FBW A300 uses an ADIRU. The non-FBW 767 uses an ADIRU. When a 767 lost pitot tube function over the Great Lakes, it had to declare MAYDAY and could not hold altitude.

        FBW, on the other hand, has nothing directly to do with pitot tubes, It is merely a modern means of moving control surfaces, using electrical signals over wires instead of manual force over cables. It does not make important flight control decisions (within the safe flight envelope), the AUTOPILOT does that. FBW is lighter, more accurate and requires less maintenance. It also allows for a feedback loop to fine-tune flight control. If the aircraft is a non-FBW 767, and it loses pitot tube functions, it also loses the ADIRU and the AUTOPILOT and then the pilot is in control in the same way a the pilot of a FBW aircraft is. If he pulls up relentlessly and ignores stall warnings, he will crash it.

        The procedure is as Gabriel describes, using first memory items and then QRH charts to get a pitch/power combination. Doing nothing is NOT always enough because the event can happen when the aircraft is transitioning to a different airspeed and the power setting will then be wrong to maintain steady flight. This happened with AF447 but it did not cause the crash. The PNF noticed this and corrected it by adding power. If the PF had not radically altered pitch the crash would have been averted.

        Again, nothing specific to FBW.

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        • #5
          Originally posted by Evan View Post
          The procedure is as Gabriel describes, using first memory items and then QRH charts to get a pitch/power combination. Doing nothing is NOT always enough because the event can happen when the aircraft is transitioning to a different airspeed and the power setting will then be wrong to maintain steady flight.


          Wrong that you need charts...

          ...and wrong that the plane does not return to steady flight (especially the all caps in "NOT always enough").

          Most pilots have a whole number of configurations memorized- and the ability to interpolate. It's a very common instrument flight technique that gives pilots the ability to establish just the right descent rate at just the right speed. (Did Eric and Gabe hit on this, perhaps?)

          And read a book...if the power/speed is a little bit out of whack- the typical airplane 'automatically' adjusts to fix it in a very robust manner.

          Too much power/speed....the plane simply climbs or speeds up. Too little, it descends....point the nose too high, the plane is going to slow and the nose will drop all by itself...and vice versa. The vast majority of the time- the plane will take very good care of itself.

          The attitude indicator and the power gauge is a damn powerful speed control...

          (Did Eric and Gabe hit on this, perhaps?)

          If you know five power/attitude combinations- you've got plenty of speed control to keep a plane flying! And I dare say that Boeing Bobby, Snyder, ATL, MCM, VNav probably have 20 power attitude trim combos....(including some weight variations).

          The plane doesn't need charts nor do the pilots!

          Originally posted by Evan
          This happened with AF447 but it did not cause the crash. The PNF noticed this and corrected it by adding power. If the PF had not radically altered pitch the crash would have been averted.

          Again, nothing specific to FBW.


          Airbus FBW logic was likely a factor.

          It basically eliminates all that stuff I said above...that if you have a bit too much power/speed for the trim, the plane climbs and burns it off...and vice versa if you are underpowered or slow...

          ...so the bus pilots may be 'unable' or at least grossly unpracticed in thinking pitch/power/performance. So, it may have been a much more strange affair than for a 787 or Piper cub pilot.

          ...and instead of dealing with a bad airspeed indication, they were dealing with an unhappy FBW system- giving them big time warnings- and they might not have known that it was basically just iced-over pitot tubes...

          I think there's consensus that they didn't really know what was wrong at that time. Maybe that's not the FBW fault- BUT it was quite a factor...

          Instead of flying a plane known to follow broadly applicable aerodynamic principles they were flying a computer known to defy many aerodynamic principles...the theory hat the guy might have felt pulling up was a good option because that would prevent an overspeed- while the computer prevented a stall...we'll never know, but it's a very reasonable theory.
          Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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          • #6
            As I say, these detailed aeronautical explanations are easy to get lost in. But something that stuck sometime back was if you set the power at X and the angle of attack at Y, you can prevent a stall. Maybe at some point the tubes defrost and you get airspeed back. And if not, then maybe you can aviate to your next landing and a mechanic can look at them. Everything else was lost on me. Maybe it was the crash in Buffalo with the inexperienced pilots? At this point, having seen a couple of different crashes explained, it gets hard to know when someone said that.

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            • #7
              EconomyClass, you got the picture right, except that in all (general aviation, private) aircraft I have flown, you can monitor and adjust Power "X", but there is no way to know Angle of Attack "Y". What the artificial horizon indicates you is the Pitch angle "Z".

              However, through relevant training (and regular practice), it's basic airmanship to control, recover from unusual attitudes, and land those (single engine piston) aircraft with no airspeed indication.

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