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  • What (the heck) is it doing now???

    David? What are you doing David? Sorry David. I can't allow you to pull up David. (Was it David?)

    Super kuddos to the crew (even when perhaps they could have possibly done something better).

    And Evan, let's talk about that "gimme my fucking plane back!" switch.

    Aviation Herald - News, Incidents and Accidents in Aviation


    Incident: Lufthansa A321 near Bilbao on Nov 5th 2014, loss of 4000 feet of altitude

    On Mar 24th 2015 Germany's Büro für Flugunfall Untersuchungen (BFU) reported in their November 2014 bulletin, that the first officer observed an irregularity in the properties of the speed indication just prior to reaching FL310 and disengaged the autopilot, the aircraft in response began a descent that lasted for about one minute before the crew was able to stop the descent at FL270.

    The BFU reported Spain's CIAIAC delegated the investigation to the BFU on Nov 11th 2014.

    The BFU reported that according to flight data and cockpit voice recorder the first officer (35, ATPL, 6,473 hours total, 5,179 hours on type) was pilot flying, the captain (52, ATPL, 16,384 hours total, 12,414 hours on type) pilot monitoring. After the aircraft climbed clear of top of clouds at about FL200 the flight data recorder recorded a fixed value of +4.2 degrees for the left hand AoA sensor, less than a minute later the FDR began to record a fixed value of +4.6 degrees for the right hand AoA sensor. The aircraft subsequently turned to fly direct to LATEK waypoint, during this turn the captain noticed the Alpha Protection Band had unusually and significantly increased. The first officer therefore reduced the climb rate from 800 to 500 feet per minute to enable the aircraft to accelerate. A short time later the first officer disengaged the autopilot and gave a brief nose down input.

    The aircraft however continued to pitch down, inputs to counter the pitch down remained without effect. About 45 seconds after the nose down began the first officer alerted the captain who took control of the aircraft, that at this time had reached a rate of descent of 4000 feet per minute and a pitch of -3.5 degrees. The captain provided a maximum nose up input which caused the aircraft to pitch up again and the rate of descent decreased and the aircraft entered level flight.

    The captain was able to maintain altitude by providing a continuous nose up input deflecting the side stick about 50% of its travel. The autopilot could not be engaged again, and a manual nose up trim was not possible.

    The crew checked for related checklists but did not find any. The crew reset the Flight Augmentation Computers 1 and 2 in sequence with no effect.

    8 minutes after the aircraft began its descent the Aircraft Communications Addressing and Reporting System (ACARS) issued an automated information to dispatch showing the three AoA sensor values amongst other data.

    21 minutes after the aircraft began its descent the crew sent a message to maintenance checking whether a simultaneous reset of all FACs would be possible. Maintenance replied in the positive stating that the aircraft would revert to alternate law as result. Another 7 minutes later the crew reported they needed to constantly pull on the sidestick, trim was inoperative and autopilot could not be engaged and the Alpha Prot Band came up extremely quick. In addition the crew received a message "PH6 AOA3" on the centralized fault display system (CFDS). Upon suggestion by maintenance the crew switched off the air data reference unit (ADR3), however, without effect. ADR3 was reengaged. Another 12 minutes later maintenance wrote a message to the cockpit along the lines "after review of the data we found the values for AoA 1 and AoA2 appear to be frozen and report too high an angle of attack. If the problem persists, disengage ADR1 and ADR2 which will cause the aircraft to revert to Alternate Law however." then followed up "perhaps it is sufficient to just disengage ADR2".

    The crew disengaged ADR2 which immediately prompted the aircraft to revert to Alternate Law and it was no longer necessary to pull the nose up.

    The crew decided to use the remaining hour of flight time to verify the system status and to prepare for landing and landed safely at the destination.

    The BFU reported that the aircraft features three Angle of Attack sensors consisting of a heated movable vane, the movement of the vanes is converted into electrical signals and the actual angle of attack computed by the related air data reference unit.

    If the measured/computer Angle of Attack exceeds the value of Alpha Prot by one degrees, the autopilot is automatically being disengaged. In manual flight if the Alpha Prot Angles is exceeded, the Alpha Protection activates, the position of trim is stored and used as maximum nose up trim, the function of the side stick changes to command a specific pitch angle with the most nose up angle being Alpha Max which can be reached by full nose up deflection of the side stick.

    The BFU reported that all three AoA sensors were examined by the manufacturer, no damage, malfunction or anomaly was identified with either of the sensors.

    Airbus analysed the data and stated: "all three sensors worked normally until about 8 minutes into the flight, when the aircraft climbed through FL195. At that point, at an ambient temperature of -35 degrees C, AoA sensors 1 and 2 froze up at a position of approximately 4.5 degrees nose up and remained in this position until the aircraft descended towards the destination airport. At the time, when the autopilot disengaged the aircraft was flying at 0.675 mach, the Alpha Prot angle was 4.2 degrees, the Alpha Max 5.8 degrees. Within 15 seconds the first officer made increasing nose up input until reaching 75% of the maximum travel of the side stick, the attitude however changed from 4.5 degrees to -3.5 degrees against this input. The system disregarded/turned off the AoA 3 sensor because it disagreed more than the permitted value with the other 2 sensors.

    When later ADR2 was disengaged, the system immediately reverted to Alternate Law because ADR3 had already been disengaged by the system and now two ADRs were offline.

    The BFU reported that they are working to establish how reliable AoA sensors are but annotated: "The algorithms and boundary conditions differ from each other and are not entirely known to the BFU. The investigation is aiming to establish the probability of a repeat of this occurrence."

    --- 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 ---

  • #2
    The interest in this incident for me is in the 'loss of airspeed' situation.

    After AF447, there was quite a push along the lines of 'always obey the stall warning'. We've now had a couple of incidents where the AoA has been giving erroneous information. Its interesting that we're having so many probe failures! I'd be interested to know if the rate is increasing, or it is simply that the aircraft (/pilots) are less able to cope with them.

    Comment


    • #3
      Originally posted by MCM View Post
      The interest in this incident for me is in the 'loss of airspeed' situation.

      After AF447, there was quite a push along the lines of 'always obey the stall warning'. We've now had a couple of incidents where the AoA has been giving erroneous information. Its interesting that we're having so many probe failures! I'd be interested to know if the rate is increasing, or it is simply that the aircraft (/pilots) are less able to cope with them.
      Good point but...

      If the AoA get's "frozen", that would hardly happen at stall warning AoA.
      Rather to reacting to a false stall warning the greatest risk is, I think, not recognizing other cues of the stall and not reacting to it when the stall warning FAILS. (like in that test flight that I don't remember the details).

      Regarding AF, if the near zero roll stability, control and authority + 10000 fpm rates descent + the 10 deg nose up + the plane not rising the nose anymore when you pull full aft + the fact that "I've been pulling up all the time" don't combine to form in your head an idea that the stall warning that has been sounding uninterruptedly for one minute and half might be not false, I don't know what will.

      --- 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


      • #4
        Originally posted by MCM View Post
        The interest in this incident for me is in the 'loss of airspeed' situation.

        After AF447, there was quite a push along the lines of 'always obey the stall warning'. We've now had a couple of incidents where the AoA has been giving erroneous information. Its interesting that we're having so many probe failures! I'd be interested to know if the rate is increasing, or it is simply that the aircraft (/pilots) are less able to cope with them.
        I've seen speculation that changing global weather patterns might be responsible for greater humidity at altitude resulting in more probes unable to cope with certain conditions, but I think it is nothing more than speculation at the moment. It definitely seems like these instances are happening pretty often the last few years.

        Comment


        • #5
          It depends on the failure associated with the AoA vane, and the manner in which it has frozen. There was information recently associated with an A330 where they received false stall warning information due to probe failure.

          Couple this with false Alpha Prot events where the aircraft pushes nose down... and we know about the airspeed issues... and it is starting to become a significant issue.

          It was one thing to say 'the airspeed might fail, but obey the stall warning' (easy to do) to now 'the airspeed might be overreading and you might be in a stall - or the airspeed might be correct and you are getting false stall warning indications'.

          It is massively complicating the unreliable indications scenario, and doing it on an aircraft type that makes life even more complicated to handle these issues due to its inherent design. But that doesn't answer the question - why are all these probes failing!

          Comment


          • #6
            Originally posted by MCM View Post
            It depends on the failure associated with the AoA vane, and the manner in which it has frozen. There was information recently associated with an A330 where they received false stall warning information due to probe failure.

            Couple this with false Alpha Prot events where the aircraft pushes nose down... and we know about the airspeed issues... and it is starting to become a significant issue.

            It was one thing to say 'the airspeed might fail, but obey the stall warning' (easy to do) to now 'the airspeed might be overreading and you might be in a stall - or the airspeed might be correct and you are getting false stall warning indications'.

            It is massively complicating the unreliable indications scenario, and doing it on an aircraft type that makes life even more complicated to handle these issues due to its inherent design.
            Yes, agree.

            But that doesn't answer the question - why are all these probes failing!
            That's easy to answer: not a clue.

            --- 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 Gabriel View Post

              And Evan, let's talk about that "gimme my fucking plane back!" switch.
              P L E A S E ! This crew did the right thing (eventually) by shutting down two of the ADR's. As I've been saying for years, THERE'S YOUR GIMME THE F_ING PLANE BACK SWITCH. Why do Airbus crews still not understand that, so many years after the Qantas events and the subsequent AD outlining the correct procedure, that's the real question here...

              As I understand it, since this event, LH crews are becoming intimately acquainted with the procedure. I hope this is true across the industry.

              Originally posted by MCM
              But that doesn't answer the question - why are all these probes failing!
              MCM, the investigation following the XL AIrways crash involving the same issue found that improper maintenance was the cause - specifically the failure to keep the AoA probes masked during high-pressure washing after a repaint (In the XL investigation it was also pointed out that procedure forbid the use of high-pressure washing to remove paint dust. This was another case of the fatal 'why the heck not' mentality, sort of like using a fork-lift to mount an engine or getting your deicing nozzle at Home Depot). Although they are protected against water incursion from the worst expected flight-related environmental conditions, you should ABSOLUTELY NEVER direct high-pressure water at the AOA sensors. This caused some water to permeate the seal and remain there for several days until the accident flight, causing them to freeze during climb. I suspect the incident here is of a similar cause.

              The XL report recommends a more robust seal design for the sensors and I absolutely concur with that idea, (although it shouldn't be necessary if ground ops is competent). Once again, the problem isn't with the AIrbus systems, it is with the environmentally exposed sensor that the systems depend on. These sensor designs must be constantly improved but more importantly here, ground crews must learn to respect them as very fragile, very critical, very precious objects. You wouldn't aim a pressure washer at a Ming vase, and that's how they should think of them.

              But the real issue is that pilots need to know their aircraft and its specific abnormal procedures. Frozen AoA sensors is a fault-passive situation.

              Comment


              • #8
                Once again, the problem isn't with the AIrbus systems, it is with the environmentally exposed sensor that the systems depend on.
                Pitot-static and AoA probes/vanes have been in use for decades, quite successfully. But now we're having incidents and accidents resulting from their failure.

                In the past the probes probably didn't need to be as reliable as they now need to be. We're designing systems for which probe information is critical for flight, rather than 'just' very important.

                If the probes are not reliable enough, is it then an Airbus design flaw to obtain critical information from sources that aren't up to the required reliability? Is that the probe's fault, or poor system design?

                There's no doubt that Airbus aircraft are very sensitive to fault in general. They have significant redundancy, and self-notify failures very quickly - but don't seem to be very good at giving reliable failure indication for their probe-based information. Why?

                In the beginning I was of the feeling that it was one or two unlucky crews to cop these failures, and they handled them badly. But as time goes on, it becomes more and more evident there's a serious problem regarding air data reliability. It needs to be fixed, and now.

                P.S - It doesn't matter how many times you train for probe failures, they're still one of the most confusing situations a pilot can face.

                Comment


                • #9
                  Originally posted by MCM View Post

                  There's no doubt that Airbus aircraft are very sensitive to fault in general. They have significant redundancy, and self-notify failures very quickly - but don't seem to be very good at giving reliable failure indication for their probe-based information. Why?
                  The 'why' is easy: Probes of similar design exposed to the same environmental conditions can be expected to fail in a similar manner. The Airbus logic is designed to identify failures by a 'voting' process whereby either the majority wins or, if all three vote differently, everyone loses. In these instances the AoA vanes will freeze in agreement, so the logic votes them valid and the fault goes undetected. I don't know of a more reliable way for logic to determine failure. I suppose logic could be improved to also compare IR data with other air data to verify that the AoA vane data is valid. Maybe... but then we are adding complexity...

                  If the probes are not reliable enough, is it then an Airbus design flaw to obtain critical information from sources that aren't up to the required reliability? Is that the probe's fault, or poor system design?
                  Well, in this case I think the probes are reliable enough as long as they are not mistreated. If we can make them more abuse-proof, than let's certainly do that. But my point, as always, is that an Airbus pilot IS the redundancy for system failure and at the first sign of control response misbehavior should shut down the ADR's and get it on the ground in alternate law. My point is that every Airbus pilot should know how to do this.

                  And I'm not sure how more robust the B777 of B787 are in respect to dependence on air data probes...

                  Comment


                  • #10
                    Originally posted by Evan View Post
                    And I'm not sure how more robust the B777 of B787 are in respect to dependence on air data probes...
                    The difference is that in the 777 the pilot can always readily override the computer.

                    Pull up!
                    No sorry, I will not comply with that.

                    Trim up!
                    No sorry, I will not comply with that,

                    And this overriding computer decisions can be made based on incorrect data.
                    And "click click, clack clack" ALONE will not work.

                    --- 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


                    • #11
                      Originally posted by Gabriel View Post
                      The difference is that in the 777 the pilot can always readily override the computer.

                      Pull up!
                      No sorry, I will not comply with that.
                      True. And which is far far far more often the cause of crashes today?

                      a) Frozen AoA vanes

                      b) Pilots pulling up into a stall

                      Comment


                      • #12
                        Originally posted by BEA Final Report, XL AIrways 888T
                        During the investigation, exposure to running water, based on the criteria of the impermeability corresponding to category R of the RTCA DO 160 C standard, on an angle of attack probe installed with an attachment plate identical to that used on the aeroplane revealed no traces of water inside the body of the angle of attack sensor. To cause water penetration inside the angle of attack sensors on D-AXLA, the conditions of exposure to water of these angle of attack sensors during rinsing of the aeroplane with fresh water three days before the accident flight with no application of the rinsing task in the cleaning procedure, and in particular without any sans protection on the angle of attack sensors, were thus more severe than those of the impermeability tests or those encountered during an aeroplane’s life.
                        WIthout water penetration, the AoA vanes cannot freeze, therefore there is no threat if these sensors are not abused through non-standard maintenance.

                        The attached image is from the first page of the Airbus Aircraft Maintenance Manual section titled 'External Cleaning - Servicing'.

                        Pretty much right there in black and white isn't it? And yes, this pre-dates both incidents.

                        Now should we make a list of all the Boeing threats that can result in loss of control from bonehead non-standard maintenance?

                        Comment


                        • #13
                          Originally posted by Evan View Post
                          You wouldn't aim a pressure washer at a Ming vase
                          I've met people who would!

                          Originally posted by Evan View Post
                          The attached image is from the first page of the Airbus Aircraft Maintenance Manual section titled 'External Cleaning - Servicing'.

                          Pretty much right there in black and white isn't it?
                          It is, but its wording also plays into the hands of people who are inclined to deliberately "misunderstand" things, especially when motivating factors such as profit or laziness are present. Such as the manager who realizes a plane can be cleaned in half the time with a pressure washer than with traditional methods. Or the worker who'd much rather pull a trigger than grab a bucket and sponge and do actual work.

                          Such a person could look at that and say "Oh, it's okay to pressure wash as long as you don't hit electrical equipment, gear box seals, etc.".
                          Be alert! America needs more lerts.

                          Eric Law

                          Comment


                          • #14
                            Originally posted by elaw View Post
                            Such a person could look at that and say "Oh, it's okay to pressure wash as long as you don't hit electrical equipment, gear box seals, etc.".
                            Or "Oh, it's okay to skip the procedure and just hold known pitch and power settings". I know, what can you ever do about that?

                            I just feel bad for the engineers who do everything they can to make aviation safer and then it comes down to some knucklehead who can't be bothered.

                            Comment


                            • #15
                              Originally posted by Evan View Post
                              Or "Oh, it's okay to skip the procedure and just hold known pitch and power settings".
                              Who says that's not the procedure?

                              I just say that although I don't know THE procedure, somehow I know that relentless pull ups are NOT the procedure.
                              Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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