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Thread: 737 rudder hardover

  1. #21
    Senior Member Gabriel's Avatar
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    Ok, let me clarify that post because you got it wrong

    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?

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

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  2. #22
    Senior Member 3WE's Avatar
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    Quote 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...

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  3. #23
    Senior Member 3WE's Avatar
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    Quote 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?
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  4. #24
    Senior Member Evan's Avatar
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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.
    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:

    https://www.ntsb.gov/investigations/...ts/AAR9901.pdf

    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.

  5. #25
    Senior Member BoeingBobby's Avatar
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    Quote Originally Posted by LH-B744 View Post
    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"?

  6. #26
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    @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.

  7. #27
    Senior Member 3WE's Avatar
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    AA-587 Busted Rudder from control pedal inputs
    Quote 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.
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  8. #28
    Senior Member Evan's Avatar
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    Quote 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)

  9. #29
    Senior Member 3WE's Avatar
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    Quote 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...

    https://www.youtube.com/watch?v=W_ErZ1nakd0

    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...
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  10. #30
    Senior Member Gabriel's Avatar
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    Quote 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.

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  11. #31
    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel 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. (aka the limit load) plus a factor of 1.5 (aka the ultimate load)
    just for complete completeness.

    The forces applied to the rudder of aa587 were about 2 times the limit load and well beyond the ultimate load because certification does not require alternating from overswing sideslip angles.

    Because why should it?

  12. #32
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    he 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 (aka the limit load) without permanent deformation plus a factor of 1.5 (aka the ultimate load) in a condition that would allow the plane to keep flying and land perhaps for the last time before becoming soda cans.
    just for total complete completeness.

    ...certification does not require alternating from overswing sideslip angles.

    Because why should it?
    Because...
    - It would be good to have a speed where the pilot knows that he will not break the plane from aerodynamic forces no matter what he does with the primary flight control surfaces (that was believed by many pilots to be the defnition of Va, including myself even when studied the correct definition in the university).
    - It can be done
    - The rudder is precisely intended to be used against the side slip.
    - There are mistakes that can reasonable happen that would induce the pilot to do it (AA was not one of them). Imagine a left engine failure at high thrust (like CLB) and the pilot by mistake steps on the left pedal (this sounds like a ridiculous mistake because it should be so obvious and intuitive, but it happens, to the point that pilots invented a mnemonic rule: dead engine - dead leg). So the plane yaws to the left because of the asymmetric thrust + the wrong pedal depression and pretty quickly (more or less by when the plane is overswinging) the pilot reacts to that yaw by applying corrective rudder input.
    - Because Boeing does require that feature. With a margin of 1.2 instead of 1.5, but still.

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  13. #33
    Senior Member 3WE's Avatar
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    Quote Originally Posted by Gabriel View Post
    - The rudder is precisely intended to be used against the side slip.
    What?
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  14. #34
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    Quote Originally Posted by Gabriel View Post
    - The rudder is precisely intended to be used against the side slip.
    I also take issue with this, in one specific sense. When pilots are trained to use the rudder, the circumstances under which they use it are more or less steady-state: countering torque etc. on takeoff, compensating for adverse yaw, sideslipping the aircraft, and straightening the aircraft when landing in a crosswind.

    I can't think of a situation where pilots are trained on repeated alternating application of rudder. I think the most likely need for that in day-to-day operations is in a wake turbulence encounter, but pilots don't get much training on that except that they're told to avoid it.

    The key point (IMHO) being that repeated alternating rudder inputs can easily turn into PIO which is another thing pilots don't get much training on. And PIO is a situation where it's very easy to overstress things.
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  15. #35
    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel View Post
    just for total complete completeness.
    I'm ok with that.



    Because...
    - It would be good to have a speed where the pilot knows that he will not break the plane from aerodynamic forces no matter what he does with the primary flight control surfaces (that was believed by many pilots to be the defnition of Va, including myself even when studied the correct definition in the university).
    - It can be done
    - The rudder is precisely intended to be used against the side slip.
    - There are mistakes that can reasonable happen that would induce the pilot to do it (AA was not one of them). Imagine a left engine failure at high thrust (like CLB) and the pilot by mistake steps on the left pedal (this sounds like a ridiculous mistake because it should be so obvious and intuitive, but it happens, to the point that pilots invented a mnemonic rule: dead engine - dead leg). So the plane yaws to the left because of the asymmetric thrust + the wrong pedal depression and pretty quickly (more or less by when the plane is overswinging) the pilot reacts to that yaw by applying corrective rudder input.
    - Because Boeing does require that feature. With a margin of 1.2 instead of 1.5, but still.
    Airbus jets will stand up to a full reversal (aa587 did) but not repeated, cyclic reversals. Certainly I would like aircraft to be able to withstand all kinds of pilot abuse with an overload factor of 10, but I also want them to perform as efficiently as possible. I think airframers have found that balance (while still leaning towards safety). But according to the reports of previous senior pilots, the pilot of aa587 perceived the rudder as a very active flight control and used it aggressively where it wasn't even needed. The solution to that is not to build the planes stronger.

  16. #36
    Senior Member 3WE's Avatar
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    Quote Originally Posted by elaw View Post
    ***more or less steady-state:***
    Oh hell no!

    Do you ever sit in the back of the plane in turbulence?

    Have you ever sat anywhere on a plane during a crosswind takeoff?

    I ROUTINELY feel liberal rudder input adjustments and reversals.

    Did you look at my youtube where we see very significant elevator and aileron reversals?

    Ever look out the window and see aileron reversals?

    Rudder reversals occur!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! (Even yaw-damper-generated rudder reversals).

    Regarding AA-587, we will never know what the pilot was thinking...

    Did he fear total wake turbulence upsets and was he doing something akin to AA upset recovery procedures?

    Was he just trying to be a good stick and rudder pilot, and suddenly the low-displacement/low-force mode of the airbus pedals had him slamming the rudder totally back and forth, when he thought he was just doing slight deflections?

    Was he a total dumbass, wantonly slamming it back and forth for no good reason as some people seem to think?
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  17. #37
    Senior Member Evan's Avatar
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    Quote Originally Posted by 3WE View Post
    Regarding AA-587, we parlour-talkers who cannot be bothered to read the very exhaustive final report will never know what the pilot was thinking...
    The NTSB knows what the pilot was thinking. He was thinking that was how you recover an A300 from roll upset, with the rudder .

  18. #38
    Senior Member 3WE's Avatar
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    Quote Originally Posted by Evan View Post
    The NTSB knows what the pilot was thinking. He was thinking that was how you recover an A300 from roll upset, with the rudder .
    Accept my apologies. I must have glossed over and misread. I THOUGHT the report said that the data from the cockpit thought recorder was damaged and unrecoverable. My bad.

    Edit: Actually, I just checked the report. It's pretty clear at the top of page 9 that the both of the cockpit thought recorders were damaged to the point of being unable to recover data...although I understand those things may be prone to bias- particularly Evan's.
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  19. #39
    Senior Member Evan's Avatar
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    Quote Originally Posted by 3WE View Post
    Accept my apologies. I must have glossed over and misread. I THOUGHT the report said that the data from the cockpit thought recorder was damaged and unrecoverable. My bad.
    Yes, unfortunately we don't have the recording of the F/O stepping on both rudder pedals while saying, "Hey Ed, ya know what I'm thinking?"

    How about reading all the stuff in there relating his training for wake turbulence, his piloting issues, etc...

  20. #40
    Senior Member 3WE's Avatar
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    Quote Originally Posted by Evan View Post
    Yes, unfortunately we don't have the recording of the F/O stepping on both rudder pedals while saying, "Hey Ed, ya know what I'm thinking?"

    How about reading all the stuff in there relating his training for wake turbulence, his piloting issues, etc...
    So, riddle me a few quick questions:

    Were rudder reversals part of AA upset training?

    Are small, light rudder pedal inputs an oft-used input for the sake of yaw correction?

    Now, tell me how that extrapolates into such high confidence that he was reversing the rudder to fight off an upset.
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