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Thread: 737 rudder hardover

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    Default 737 rudder hardover

    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?

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    Quote Originally Posted by Nirwanda View Post
    Thoughts?
    Yes.

    We may not know exactly what happened.

    The thing was super carefully designed to work correctly almost 100% of the time...

    They proved that reversals COULD occur...they did not necessarily replicate exactly what happened in the crashes...

    Works for me.
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    Quote Originally Posted by 3WE View Post
    Yes.

    We may not know exactly what happened.

    The thing was super carefully designed to work correctly almost 100% of the time...

    They proved that reversals COULD occur...they did not necessarily replicate exactly what happened in the crashes...

    Works for me.
    It is pretty much established. Remember there was a case where the plane DID NOT crash. In that case they had the intact system and the intact pilots to get all the information needed.

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    [QUOTE=Gabriel;658582]It is pretty much established. Remember there was a case where the plane DID NOT crash. In that case they had the intact system and the intact pilots to get all the information needed.[/QUOTE

    The problem stopped happening as well...

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    Quote Originally Posted by Gabriel View Post
    It is pretty much established. Remember there was a case where the plane DID NOT crash. In that case they had the intact system and the intact pilots to get all the information needed.
    Indeed, the overall theory seems to be strongly supported by the alignment of many factors and a couple other incidents beyond the two crashes.

    However, I perceived that the original post was focused on the hydraulic actuator and specifically that the super extra cold actuator with super extra hot oil did not represent normal conditions, and thus questioned if such an extreme test was valid...

    I think the answer is that the hot-cold hydraulic test does NOT say, "The actuator might reverse in normal conditions". Instead it says, "The actuator CAN reverse, and maybe...in some small set of extremely unusual conditions including, but maybe not limited to super cold-super hot, it might reverse...maybe...

    ...and even with all that wishy washyness, it is consistent with the overall theory, even though by itself, is not exact proof of what happened.
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    Quote Originally Posted by 3WE View Post
    Indeed, the overall theory seems to be strongly supported by the alignment of many factors and a couple other incidents beyond the two crashes.

    However, I perceived that the original post was focused on the hydraulic actuator and specifically that the super extra cold actuator with super extra hot oil did not represent normal conditions, and thus questioned if such an extreme test was valid...

    I think the answer is that the hot-cold hydraulic test does NOT say, "The actuator might reverse in normal conditions". Instead it says, "The actuator CAN reverse, and maybe...in some small set of extremely unusual conditions including, but maybe not limited to super cold-super hot, it might reverse...maybe...

    ...and even with all that wishy washyness, it is consistent with the overall theory, even though by itself, is not exact proof of what happened.
    As you said. the test only proves that a rudder reversal hard-over CAN happen, not that it happened.
    At the same time, the accidents and incidents investigated proof that a rudder reversal hard-over was the only thing that could explain, for example, the plane yawing to the left with the pilot applying full pressure on the right rudder pedal (to the point of stepping so hard on it that he overcame his won weight raising himself from the seat).
    Now connect the dots...

    It is pretty much established that the PCU caused a rudder reversal hard-over. Even if what caused that PCU behavior is not 100% clear.

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    Quote Originally Posted by Gabriel View Post
    It is pretty much established that the PCU caused a rudder reversal hard-over- AND THAT THERE WAS A FAILURE MODE WHERE THE THING WOULD OPERATE 'IN REVERSE'.
    Fixed.

    Apologies for being pedantic with the emphasis, but I think that's sort of what the initial post was hinting at- someone felt that the hot + cold test wasn't 100% valid...The fact that the thing could operate in reverse was a critical discovery.
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    Quote Originally Posted by Nirwanda View Post
    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.
    Well, I think adding redundancy where it was needed in first place was barking up the right tree, even if the accidents weren't caused by this weakness. They proved that a vulnerability existed and closed that vulnerability. I still would like to know why Boeing didn't make it redundant to begin with, and why they spent almost ten years in denial aftert he first incident while still not making it redundant.

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    Quote Originally Posted by Evan View Post
    Well, I think adding redundancy where it was needed in first place was barking up the right tree, even if the accidents weren't caused by this weakness. They proved that a vulnerability existed and closed that vulnerability. I still would like to know why Boeing didn't make it redundant to begin with, and why they spent almost ten years in denial aftert he first incident while still not making it redundant.
    Because the vulnerability was discovered only after a plane survived the incident. The previous cases had been closed without identifying what went wrong, there were suspicions that a wrong pilot reaction to an upset generated by wake turbulence could be the cause.

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    Quote Originally Posted by Gabriel View Post
    Because the vulnerability was discovered only after a plane survived the incident. The previous cases had been closed without identifying what went wrong, there were suspicions that a wrong pilot reaction to an upset generated by wake turbulence could be the cause.
    No, I mean the design using a single rudder actuator with a single servo. That obviously lacked redundancy. That doesn't mean that is was obviously prone to reversal. But my feeling is that Boeing chose not to revise the design after the initial investigation because doing so would appear to be an admission of design weakness and thus open them up to liability. So the 737 fleet remained in operation with this weakness until 2002.

    Anyway, you know how I feel. The 737 should have been retired by now and Beoing should have a 21st-century single-aisle airframe in production.

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    Quote 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?
    Yes, indeed. In a 747 I know (at least) three different axes, in order of appearance, elevator, aileron, rudder, and in my eyes, the throttle quadrant is the fourth axis.

    Thus, I immediately had this one in mind:
    16 years ago, a long haul airbus has lost her rudder inflight.

    You can say that I probably watch to many TV broadcasts with Greg Feith in it. But everytime I see this man, I again learn something! Yes, documentaries.

    If we ask Flight Captain Chesley Sullenberger III, surely we both seem to be laymen, me probably even more than you. That's why I stumbled across the word PCU. I'd call that a rudder servo.

    If that's the thing we are talking about, then I mentioned the wrong flight #.. Back then in November 2001, 265 people died not due to a frozen rudder servo. The complete tailfin broke into pieces "due to unnecessary rudder inputs", as Greg Feith would've written it.

    Where do pilots learn to excessively use the pedals?! Since I know how to use a Baron 58 propeller simulator, I always use aileron to manually ensure LNAV (sorry, Boeing language, but we are on topic).
    The cockpit changes (a little bit) but the procedure does not change if you sit in a 747.

    In case of a doubt we should ask
    a) a B748 test pilot
    or
    b) a B747 flight instructor
    or
    c) the LH B747 fleet chief.

    But I don't think that I have learned something wrong.
    LH also has a intercontinental history, the Hamburg - Düsseldorf - Shannon - NYC route, open since June 1st, 1955.
    A/C type: Lockheed Super Constellation.

    Aviation enthusiast since more than 30 years. Almost a decade here on this platform.

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    Quote Originally Posted by LH-B744 View Post
    Thus, I immediately had this one in mind:
    16 years ago, a long haul airbus has lost her rudder inflight.
    That accident bothers me. If you can break the rudder by manual input, to me that's a design flaw. I don't care who's piloting the airplane.

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    Quote Originally Posted by Nirwanda View Post
    That accident bothers me. If you can break the rudder by manual input, to me that's a design flaw. I don't care who's piloting the airplane.
    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.

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    Quote Originally Posted by Evan View Post
    No, I mean the design using a single rudder actuator with a single servo. That obviously lacked redundancy. That doesn't mean that is was obviously prone to reversal. But my feeling is that Boeing chose not to revise the design after the initial investigation because doing so would appear to be an admission of design weakness and thus open them up to liability. So the 737 fleet remained in operation with this weakness until 2002.

    Anyway, you know how I feel. The 737 should have been retired by now and Beoing should have a 21st-century single-aisle airframe in production.
    Wow. A (not yet) online friend who also thinks that a PCU is a servo? And I swear, I didn't read your #10 entry here, before I wrote my #11. But that's not the reason... Am I allowed to quote you twice?
    The 737 should have been retired by now and Beoing should have a 21st-century single-aisle airframe in production.
    Wow. You know which airline is mentioned in the B737 en wiki as the 737 launch customer? I wasn't aware of that. First flight of a Boeing 737: and that's again a remarkable day, February 10th 1968. The airline? Well, that must've been the day when the LH B737 fleet chief began his work... But "we" are not able to celebrate these golden jubilee in February. Spohr today does not own one single B737. Founding member of the 737 club, but we left the club in .... ? Which in my eyes is a shame!

    If you ask me and/or if you've read my forum entry in the Delta 747 topic, then you can see the successor for the DL 747. Imho the DL-A359. And for the LH 737?
    LH-A320 and LH-A321.

    LH-A320 can't be a bad a/c. Spohr flies it. But is an Airbus able to be the legal successor of a Boeing 737, which since half a century is everywhere on the planet?

    You ask me? - No.

    Greetings and a Happy New Year!
    LH also has a intercontinental history, the Hamburg - Düsseldorf - Shannon - NYC route, open since June 1st, 1955.
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    Quote 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 I meant that accident, and yes it was the whole fin which makes it even worse.

    How else can you structurally break a fully functioning modern aircraft by input (aside from overspeed).
    i'm genuinly curious

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    Quote 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!
    LH also has a intercontinental history, the Hamburg - Düsseldorf - Shannon - NYC route, open since June 1st, 1955.
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    Quote 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?

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    Quote 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.

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    Quote 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.

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    Quote 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.

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