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  • #16
    Originally posted by Gabriel View Post
    If you are talking about the AA A300 in 2001, it lost the whole fin, not just the rudder.
    And you can brake almost any plane with manual input. Especially something like a 737.
    Yes. That was also one of the conclusions of that "Greg Feith" documentary. I don't think that he wrote the death certificate for these 265 passengers, but I think that I have at least one thing in common with him. Ask yourself where are the limits of (not only) your aircraft. And please don't confuse rudder and aileron.

    Sometimes I sound like a driving instructor. Isn't that bad?! Shame on me. But I own my driver's license since a year when some jetphotos members weren't even born...

    We are real seniors, Gabe!
    The German long haul is alive, 65 years and still kicking.
    The Gold Member in the 747 club, 50 years since the first LH 747.
    And constantly advanced, 744 and 748 /w upper and lower EICAS.
    This is Lohausen International airport speaking, echo delta delta lima.

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    • #17
      Originally posted by LH-B744 View Post
      Wow. A (not yet) online friend who also thinks that a PCU is a servo?
      And here you have another one. It is not?

      --- 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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      • #18
        Originally posted by Nirwanda View Post
        Three incidents I believe were attributed to this phenomena (and a further 4-5 suspected), and in the end it was the PCU from the Pittsburgh accident which investigators finally managed to get to jam through extensive testing.

        It appears to me (a layman, mind you) that the tests they did to the PCU perhaps didn't correctly represent the actual conditions of the accidents. They basically froze the PCU and then injected it with super-hot hydraulic fluid, and while it's great that hardware modifications were made to prevent that event from occuring again, the feeling I get is that perhaps they were barking up the wrong tree.

        I know for a fact that Parker Hannifin (the manufacturer of the PCU) as well as "people within the industry" (quoted from a documentary I saw) claim they didn't get it right. It's understandable that the PH would take this stance, but still...

        Thoughts?
        Thoughts are that the PCU did it.

        You have the whole 3 investigation reports PLUS additional documents in the investigations dockets available in the NTSB web site.

        But if you want to summarize 8 years of investigation in 10 minutes, Wikipedia is your friend.

        United Airlines Flight 585, Colorado Springs, 1991
        Although the flight data recorder (FDR) outer protective case was damaged, the data tape inside was intact and all the data was recoverable. Five parameters were recorded by the FDR: heading, altitude, airspeed, normal acceleration (G loads), and microphone keying. The FDR did not record rudder, aileron or spoiler deflection data, which could have aided the NTSB in reconstructing the plane's final moments. The data available proved insufficient to establish why the plane suddenly went into the fatal dive.[3]:102 The NTSB considered the possibilities of a malfunction of the rudder power control unit servo (which might have caused the rudder to reverse) and the effect of powerful rotor winds from the nearby Rocky Mountains might have had, but there was not enough evidence to prove either hypothesis. Thus, the first NTSB report (issued on December 8, 1992) did not conclude with the usual "probable cause". Instead, it found:
        The National Transportation Safety Board, after an exhaustive investigation effort, could not identify conclusive evidence to explain the loss of United Airlines flight 585.

        USAir Flight 427, Pitsburg, 1994
        Both the CVR and FDR were recovered and used for the investigation. Due to the limited parameters recorded by the FDR, investigators did not have access to the position of the flight-control surfaces (rudder, ailerons, elevator, etc.) for the accident. However, two parameters recorded by the FDR were crucial, one being the aircraft's heading, the other being the pitch control yoke position. During the approach, Flight 427 encountered wake turbulence from Delta 1083; the FAA, however, determined "the wake vortex encounter alone would not have caused the continued heading change that occurred after 1903:00. The abrupt heading change shortly before the dive pointed investigators immediately to the rudder. Due to the absence of rudder pedal positions from the data, investigators had to determine whether the rudder moved hard-over by a malfunction or by pilot command. This in turn led to the CVR being more heavily scrutinized than most other recordings as statements and breathing from the pilots could potentially tell investigators if they were fighting for control over a rudder malfunction or inadvertently stomped on the wrong rudder pedal in excitement from the wake-turbulence. Boeing felt the latter more likely, while USAir and the Pilot's Union felt the former was more likely.
        Investigators later discovered that the recovered accident rudder PCU was much more sensitive to bench-tests than other new PCU's. The exact mechanism of the failure involved the servo valve, which remains dormant and cold for much of the flight at high altitude, seizing after being injected with hot hydraulic fluid that has been in continuous action throughout the plane. This specific condition occurred in fewer than 1% of the lab tests, but explained the rudder malfunction that caused Flight 427 to crash. The jam left no trace of evidence after it occurred and a Boeing engineer later found that a jam under this controlled condition could also lead to the slide moving in the opposite direction than that commanded. In light of this, Boeing felt that the test results were not real-world and not applicable due to the extremes under which the valve was tested. Boeing stated that the rudder reversal was more likely psychological, likening it to examples when a human panics and intends to step on the brake during an automotive accident, but accidentally presses on the gas pedal instead while under duress. The FAA's official position was that there was not enough evidence for probable cause of rudder system failure.
        After the longest accident investigation in NTSB history — lasting more than four and a half years — the NTSB released its final report on March 24, 1999. The NTSB concluded that the accident was due to mechanical failure:
        The National Transportation Safety Board determines that the probable cause of the USAir Flight 427 accident was a loss of control of the airplane resulting from the movement of the rudder surface to its blowdown limit. The rudder surface most likely deflected in a direction opposite to that commanded by the pilots as a result of a jam of the main rudder power control unit servo valve secondary slide to the servo valve housing offset from its neutral position and overtravel of the primary slide.
        The NTSB concluded that similar rudder problems had caused the previously mysterious March 3, 1991 crash of United Airlines Flight 585 and the June 9, 1996 incident involving Eastwind Airlines Flight 517, both Boeing 737s.

        Eastwind Airlines Flight 517, Richmond, 1996 (i.e. while USAir investigation was ongoing, so the NTSB re-opened the United investigation and made a combo of the 3 accidents / incidents in one investigation)
        Flight 517 departed Trenton without incident and encountered no turbulence or unusual weather en route to Richmond. While on approach to Richmond International Airport, at an altitude of about 5,000 feet (1,500 m) MSL, the captain felt a brief "kick" or "bump" on the right rudder pedal. Around the same time, a flight attendant at the rear of the plane heard a thumping noise underneath her. As the plane continued to descend through 4,000 feet (1,200 m), the captain suddenly experienced a loss of rudder control and the plane rolled sharply to the right.
        Attempting to regain control, the captain tried to apply full left rudder, but the rudder controls were stiff and did not respond to his commands. The captain applied left alieron and increased power to the right engine to try to stop the roll. The airplane temporarily stabilized, and then rolled to the right again. The crew performed their emergency checklist and attempted to regain control of the aircraft, and after several seconds they abruptly regained control. The airplane operated normally for the duration of the flight.
        No damage occurred to the aircraft as a result of the incident. One flight attendant suffered minor injuries. No other passengers or crew aboard Flight 517 were injured.
        The NTSB investigated the incident, with a particular focus on determining if the events of Flight 517 were related to prior Boeing 737 crashes.
        During its investigation, the NTSB determined that prior to Flight 517, flight crews had reported a series of rudder-related events on the incident aircraft, including uncommanded "bumps" on the rudder pedals and uncommanded movement of the rudder.
        Investigators removed rudder components from the incident aircraft, which combined with interviews with the pilots of Flight 517, helped investigators establish the cause of the prior crashes United Flight 585 and USAir Flight 427. The NTSB determined that all three incidents could only be explained by pilot error or a malfunction of the rudder system, and based partly on post-accident interviews with the Flight 517 pilots, determined it was likely that rudder malfunctions had caused all three incidents.
        The NTSB also determined that, unlike the United or USAir accidents, the rudder problem on Flight 517 occurred earlier in the landing process and at a higher speed, which increased airflow over the aircraft's other control surfaces and allowed the pilots to overcome the rudder-induced roll.

        --- 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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        • #19
          Originally posted by Evan View Post
          No, I mean the design using a single rudder actuator with a single servo. That obviously lacked redundancy.
          So you're saying that if they'd installed two of those reversal-prone servos, the problem would not have happened?

          Obviously I'm being a bit sarcastic, but if the PCU failures are dependent on operating conditions rather than being random, installing two (or three or forty-seven) of them would not alleviate the problem.
          Be alert! America needs more lerts.

          Eric Law

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          • #20
            Originally posted by elaw View Post
            So you're saying that if they'd installed two of those reversal-prone servos, the problem would not have happened?

            Obviously I'm being a bit sarcastic, but if the PCU failures are dependent on operating conditions rather than being random, installing two (or three or forty-seven) of them would not alleviate the problem.
            Yes it would. The fail rate was very small even in similar conditions. It is very likely that if you had 2 PCUs one of them would not have been affected. As a comparison, British Airways at Heathrow lost (sort of) both engines due to to ice crystals in the fuel, but there were several incidents where only one engine was affected.

            That said, I really don't know what wold happen if you have one failing PCU trying to turn left and another good PCU trying to turn right.

            --- 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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            • #21
              Ok, let me clarify that post because you got it wrong

              Originally posted by LH-B744 View Post
              Wow. A (not yet) online friend who also thinks that a PCU is a servo?
              And here you have another one. It is not?

              Meaning:
              And here you have me: another online friend who thinks that the PCU is a servo. Isn't the PCU a servo.

              --- 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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              • #22
                Originally posted by Gabriel View Post
                That said, I really don't know what wold happen if you have one failing PCU trying to turn left and another good PCU trying to turn right.
                So maybe you need 3 of them, but then you need 3 pressure release systems so nothing breaks, but then those could leak or fail and 3 sensor systems which could fail and give triple the false warnings so guys pull circuit breakers... and someone tried to tell me that elevators have reversed and then you have dual ELACAS failures...

                NO WIN SCENARIO, BAN ALL AIRPLANES!
                Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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                • #23
                  Originally posted by Gabriel View Post
                  Isn't the PCU a servo.
                  I checked Wikipedia. The distinctions were not particularly clear. Perhaps some secret club insider jargon?
                  Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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                  • #24
                    Originally posted by elaw View Post
                    So you're saying that if they'd installed two of those reversal-prone servos, the problem would not have happened?

                    Obviously I'm being a bit sarcastic, but if the PCU failures are dependent on operating conditions rather than being random, installing two (or three or forty-seven) of them would not alleviate the problem.
                    No, this isn't a common environmental vulnerability issue (like icing on pitots).

                    The problem is this: a) the 737 was designed in the mechanical age when certain innovations had not yet developed and thus has a very complicated rudder control system, and b) It it the only multi-engine transport jet with a rudder controlled by a single actuator.

                    So you have two major problems there. The complexity creates many scenarios for failure and the lack of redundancy prevents these from being fail-operational.

                    The real solution, the only truly safe one is to redesign the entire system into a modern incarnation (or better yet, design a new, modern-era airframe such as the Y1).

                    The solution Boeing took, following an AD released in 2002, addresses the issue with the actuator slides that is suspected to have caused these accidents and provides added redundancy for the single actuator design. It is still a single actuator design however, driving a single panel rudder.

                    The original redundancy Boeing built into the single PCU was a dual-concentric servo powered by two hydraulic systems. There is also a back-up PCU used provisionally if both of those hydraulic systems fail. It did not, however, provide redundancy for a jammed PCU.

                    The revised system adds mechanical redundancy and (AFAIK) replaced the dual-concentric servo design of the PCU with a second servo. Apparently this, plus failure indications in the cockpit and pilot training to avoid certain control errors eliminates the suspected causes of these accidents. It does not eliminate other failure scenarions however, for which there is still no redundacy.

                    If you want to understand this complex issue better, my advice (as always) is to read the reports. This one is quite detailed:



                    The 2002 AD required the following:

                    Install a new rudder control system that includes new components such as an aft torque tube, hydraulic actuators, and associated control rods, and additional wiring throughout the airplane to support failure annunciation of the rudder control system in the flight deck. The system also must incorporate two separate inputs, each with an override mechanism, to two separate servo valves on the main rudder power control unit (PCU); and an input to the standby PCU that also will include an override mechanism.
                    The reports summarizes the failure condition as follows:

                    Testing showed that, when the secondary slide was jammed to the servo valve housing and a sufficiently high-rate force was applied on the input crank, compliance within the rudder system could allow the primary slide to overtravel and result in a reverse rudder command. Therefore, the Safety Board concludes that it is possible that, in the main rudder PCUs from the USAir flight 427, United flight 585, and Eastwind flight 517 airplanes (as a result of some combination of tight clearances within the servo valve, thermal effects, particulate matter in the hydraulic fluid, or other unknown factors), the servo valve secondary slide could jam to the servo valve housing at a position offset from its neutral position without leaving any obvious physical evidence and that, combined with a rudder pedal input, could have caused the rudder to move opposite to the direction commanded by a rudder pedal input.
                    However, it also revealed:

                    In addition to this reversal potential, the Safety Board’s investigation revealed two other potential failure mechanisms within the 737 rudder control system that could result in a deflection to the rudder’s blowdown limit. One of these potential failure mechanisms is a physical jam in the rudder system input linkage (between the PCU’s input crank and body stop), preventing the main rudder PCU control valve from closing; the other is a jam of the primary to the secondary slide of the main rudder PCU servo valve combined with a jam of the secondary slide to the servo valve housing at positions other than neutral (known as a dual jam). These failure mechanisms probably did not play a role in the USAir flight 427, United flight 585, and Eastwind 517 upsets. 354
                    And concluded:

                    Nonetheless, the failure mechanisms are cause for concern because they further illustrate the vulnerability of the 737 rudder system to jams that could produce rudder deflections and result in catastrophic consequences.
                    The report also stated:

                    The 737 has a history of rudder system-related anomalies, including numerous instances of jamming. Examples of jamming events include the following:
                    It goes on to list 11 other jamming scenarios.

                    In other words, the thing is still inherently too complex, outdated and vulnerable to failure compared to newer aircraft designs. The news that Boeing was killing off the Y1 and merely adding modernizations to the B737 was a real disappointment, especially since the industry subsequently recovered with a more short-haul and single-aisle medium haul oriented business model.

                    Comment


                    • #25
                      Originally posted by LH-B744
                      Another (not yet) friend? Oh, don't you lie, Gabe.

                      No, HalcyonDays was a (not yet) friend. Is he so much older than the two of us? So much older so that the word "online friendship" since 1920 was never important in his whole life?
                      Back on topic.
                      Let me be honest, wow, now I know when you write all your 59 zillion forum entries. When I (normally) sleep. LH # 510 still is not a "normal" time for me to be awake, not after all this time (almost ten years, here), and not with Randazzos LH-B744 simulator, which is flown by me with real LH flight numbers... A good a/c definitely makes things easier.

                      But after all, I've never wondered of how many parts such a 747 would consist, or, how many different names there are on the planet for all the 2 zillion parts.

                      What would you say, a pilot should be able to distinguish between aileron and rudder.
                      And I think, until today I was quite successful to handle this difference, it's only geometry, isn't it. Elevator is the x-axis, aileron is the y-axis and rudder is only z, the last axis which I'd use. I'd rather try to use the throttle quadrant for steering - and a 747 there again has four possibilities..- instead of the rudder!

                      A chief engineer like Sutter, or a quite famous LH chief engineer who recently left his airline after almost 48 years, should think about how to design a tail fin and rudder servos.
                      A pilot should think how to use it, in the best case without destruction of the whole aircraft...
                      And one thing remains uncommon since Spohr is the LH CEO, don't know if former CEOs also had this broad range of abilities, sometimes it is not easy to name what Spohr is doing. Sometimes he is a jet pilot. Sometimes he is the man who says 'We buy Air Berlin.' .
                      And what if also the quite famous LH chief engineer (in the airline 1970-2017) is sometimes a pilot.

                      Most of the time, there's more than 1 thing in a man (or a woman), isn't it. A crossover between NTSB investigator and pilot, or flight instructor and engineer, or...

                      Welcome in the new year, Gabe.

                      As ATL would say "WHAT"?

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                      • #26
                        @evan interesting

                        The rudder system on the 737 does indeed seem fallable. I thought the design philosophy in aviation was to have redundancy, and while I'm sure there are built-in redundancies _within_ the rudder system, the fact remains that it's a ONE rudder when it doesn't have to be.

                        I think other aircraft has two (or more?) rudders. There was some incident (can't remember which one) where one rudder jammed (think it was the top one) at some angle, but the other rudder still functioned properly and the aircraft remained intact and controllable.

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                        • #27
                          AA-587 Busted Rudder from control pedal inputs
                          Originally posted by Nirwanda View Post
                          That accident bothers me. If you can break the rudder by very-low-force and very-low-magnitude manual inputs, to me that's a design flaw. I don't care who's piloting the airplane.
                          Fixed and Concur with your premise.
                          Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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                          • #28
                            Originally posted by 3WE View Post
                            Fixed and Concur with your premise.
                            The A300 (and every other certified airliner) can withstand a full, rapid rudder deflection to the mechanical stops (push the pedal as hard as you want) in a single direction.

                            No certified airliner is designed to withstand full, rapid rudder reversals however. But of course, any pilot doing that has a very flawed understanding of rudder.

                            You can't break the rudder by very-low-force and very-low-magnitude manual inputs unless those inputs are very-rapidly-reversed and very, very wrong.

                            (bold font becuase 3WE still hasn't gotten this after the 11,000,000th time in regular font)

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                            • #29
                              Originally posted by Evan View Post
                              ...bold font becuase 3WE still hasn't gotten this after the 11,000,000th time in regular font..
                              as you still have no clue that some quick, low displacement (and yes, even reversed) control inputs are not uncalled for (see YouTube) and that there's not a huge black and white line between dealing with a wake encounter and making mindlessly slamming the rudder back and forth to the full extent of travel...

                              ...the very short travel distance and very small input forces really gray that distinction...

                              TWA B727-200 LANDING R/W 30L STL, VISUAL, (Added Dec. 2) When this video was made, the intro music of "St. Louis Blues" seemed appropriate. The source of t...


                              Yeah, you can't see the dude's feet in this picture, but I am willing to bet there are rudder inputs during this landing, and I find the aileron reversals to be most interesting...it's just a typical summer afternoon with a few typical thermals...
                              Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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                              • #30
                                Originally posted by Evan View Post
                                The A300 (and every other certified airliner) can withstand a full, rapid rudder deflection to the mechanical stops (push the pedal as hard as you want) in a single direction at a speed lower than the maneuver speed Va.
                                Just for completeness.

                                --- 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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