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Thread: 737 rudder hardover

  1. #41
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by elaw View Post
    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.
    Where did I say repeated, alternating or PIO? I am talking of an engine failure where the pilot is slow to react, so the plane yaws.

    Let me ask you a question, if the nose of the plane suddenly goes way left for whatever reason (normal turbulence, wake turbulence, a lateral wing gust or wind shear, engine failure, sudden double engine failure on the 2 left engines in a 747...), what do you think that would be the natural response of a pilot? Well, the plane is not certified for that natural response.

    The requirement is, at Va (maneuver speed):
    1- Start from coordinated flight.
    2- Fully depress the rudder in one direction and keep it there to induce a sideslip.
    3- Let the sideslip reach the equilibrium sideslip but wait, keep that pedal down. The plane has inertia so it will keep going a bit more.
    4- Let the plane go past the equilibrium side slip and reach the max overswing angle, but wait, don't release the pedal, keep the pedal down trying your best to keep increasing the sideslip or at least to hold it.
    5- Since you are at the overswing angle, no matter how hard you try, the plane will yaw a bit back crossing the equilibrium angle again, and will make a few damped oscillations around that equilibrium angle finally stabilizing at the equilibrium sideslip.
    6- Remember, you are still holding the pedal down. Keep it there... keep it there... a few seconds more... AND NOW YES!
    7- Neutralize the pedal.

    For certification, you are not required to EVER use the rudder pedal to counteract yaw or sideslip. Only to increase it or let go.
    Now, what do you think a pilot will do if, for whatever reason, the plane moves as described in 2, 3 and 4?
    That is one of the most ridiculous pieces of regulation in FAR 23 and 25.

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  2. #42
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    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.
    I do not think that the A300 fin will survive a single "well perfromed" rudder reversal.
    AA was flying slower than Va and likely the pilot didn't apply full rudder exactly at max overswing the first times.

    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.
    Certification does not require full anti-yaw rudder from a single overswing sideslip, or partial anti-yaw rudder input from a moderate sideslip, or and anti-yaw reversal. It's like saying "don't use right ailerons if the plane is rolling left".

    aa587 reached 2 times the limit load at a speed quite lower than Va. Imagine how much easier would be to reach and exceed 1.5 times the limit load at Va.

    Boeing does have an internal requirement for full and rapid anti-yaw rudder from max overswing. Airbus doesn't (or didn't).

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  3. #43
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    Quote Originally Posted by Gabriel View Post
    - 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).
    That's not how I was taught, and I went to what some would say was (and is) a third-tier school...Va was never taught as a free-for-all speed.

  4. #44
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by ATLcrew View Post
    That's not how I was taught, and I went to what some would say was (and is) a third-tier school...Va was never taught as a free-for-all speed.
    To clarify, this is not what I was taught in the university. In the university I had to study from the FAR themselves. At some point I forgot this definition and re-learned the "wrong" one as a pilot.

    In fact, the definition of Va is the speed that will make your plane stall at the limit load factor (it is the C "corner" of the V-n diagram, also called envelope). Slower than that, you will stall BEFORE achieving the limit load factor. Faster than that, you will can exceed the limit load factor (and brake your wings in the process). So it is pretty much the speed where your wings will not break from aerodynamic loads for example in a steep turn or pulling up from a dive.

    The other requirements take this already defined Va and say "At Va the plane must withstand this and such control inputs".

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  5. #45
    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel View Post

    Certification does not require full anti-yaw rudder from a single overswing sideslip, or partial anti-yaw rudder input from a moderate sideslip, or and anti-yaw reversal. It's like saying "don't use right ailerons if the plane is rolling left".

    aa587 reached 2 times the limit load at a speed quite lower than Va. Imagine how much easier would be to reach and exceed 1.5 times the limit load at Va.

    Boeing does have an internal requirement for full and rapid anti-yaw rudder from max overswing. Airbus doesn't (or didn't).
    Hold on... The FARs require the fin to withstand a full rudder deflection in one direction, followed by a return to neutral, followed by a full deflection to the other direction, x1.5 overload. What do you mean by "anti-yaw rudder" if not this? From max overswing (assuming you got there from an engine failure with a neutral rudder) you can still apply the maximum amount of rudder available to correct for that yaw and remove it as needed to cancel out the yaw effect of the thrust asymmetry. Why would you do this to the opposite overswing?

    Aircraft aren't required to do this cyclically without the neutral centering between cycles, because the only reason that would ever happen is due to very very bad, very very wrong piloting and a completely flawed understanding of the role of rudder.

    Now there is still the phenomena of aircraft-pilot coupling that needs to be addressed. Should it be done by pilot training or by making the airframes more robust?

  6. #46
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    Hold on... The FARs require... From max overswing (assuming you got there from an engine failure with a neutral rudder) you can still apply the maximum amount of rudder available to correct for that yaw and remove it as needed to cancel out the yaw effect of the thrust asymmetry.
    You are wrong. The FARs don't have provsion for anti-yaw / anti-sideslip rudder input, and it doesn't allow a single rudder reversal. Not at overswing, not at equilibrium sideslip, not at... well: NOT. (and the little point is a period)

    Let me go again:
    1- You start from zero sideslip
    2- Suddenly apply rudder to the stop (but don't need to exceed 300 lb of force on the pedal)
    3- Keep full rudder (stop or 300 lb) until plane achieves max overswing.
    4- Keep full rudder until the plane stabilizes at the equilibrium sideslip. (note that from point 1 to 4 you have held constant full rudder deflection in the same direction).
    5- From the equilibrium sideslip, suddenly neutralize the rudder.

    Done. Test finished. Passed. Congrats. Let's go test something else now.

    Not only anti-sideslip rudder deflection is required at overswing, it is not even required from equilibrium sideslip.
    In fact, it is not required that the rudder/fin must withstand ANY corrective rudder input. If the plane yaws to the left for whatever reason (even pilot inadvertently stepping a bot on the left rudder pedal) and you apply any little bit of right rudder to correct, you are doing something that the certification doesn't require.

    Anti-yaw / anti-sideslip means that you apply rudder in a way that the forces generated by the rudder deflection would cause a yaw moment contrary to the direction of the yaw / sideslip. So it is a corrective input. Which is a very reasonable rudder input if you ask me. And one the FAR doesn't require that the fin withstands.

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  7. #47
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    Now there is still the phenomena of aircraft-pilot coupling that needs to be addressed. Should it be done by pilot training or by making the airframes more robust?
    I would start for designing a control system that is less susceptible to PIO.
    (Hint: that just a couple of pounds over the friction-break force and just a couple of inches is all it takes for full pedal deflection, goes in the wrong direction)

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  8. #48
    Senior Member Evan's Avatar
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    Do I have to post the Airbus/Boeing pedal travel/pedal force tables again? You're going to indict a lot of Boeing aircraft into this "danger" zone...

    But the elephant-in-the-cockpit question is:

    If the plane yaws to the left for whatever reason (even pilot inadvertently stepping a bot on the left rudder pedal) and you apply any little bit of right rudder to correct,
    ...why can't pilots learn rudder? You don't apply a little left rudder pedal (not that that would overstress the airframe) , you remove the rudder pedal you indvertently stepped on. It's not a bicycle, it's a rudder.

  9. #49
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    Do I have to post the Airbus/Boeing pedal travel/pedal force tables again? You're going to indict a lot of Boeing aircraft into this "danger" zone...
    Whomever. I don't care. I am not in an A vs B war business.

    ...why can't pilots learn rudder? You don't apply a little left rudder pedal (not that that would overstress the airframe) , you remove the rudder pedal you inadvertently stepped on. It's not a bicycle, it's a rudder.
    I said for whatever reason for God's sake!!
    Inadvertent rudder input
    Engine failure
    Lateral gust
    Lateral windshare
    Natural turbulence
    Wake turbulence

    Do you really expect the pilot NOT to use the ruder to correct unwanted sideslip? (and no, I don't consider "step off the rudder" as a form of "use the rudder").

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  10. #50
    Senior Member 3WE's Avatar
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    Quote Originally Posted by Gabriel View Post
    I said for whatever reason for God's sake!!
    Inadvertent rudder input
    Engine failure
    Lateral gust
    Lateral windshare
    Natural turbulence
    Wake turbulence

    Do you really expect the pilot NOT to use the ruder to correct unwanted sideslip? (and no, I don't consider "step off the rudder" as a form of "use the rudder").
    Yes, he really expects the pilots to not use the rudder pedals.

    Evan is extremely binary... never touch the pedals in flight (perhaps we need an automatic pedal lock out)... maybe it's ok for crosswind landings, but we need to incorporate a built in system to pause the rudder at neutral and training to only make steady-state inputs.
    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

  11. #51
    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel View Post
    Do you really expect the pilot NOT to use the ruder to correct unwanted sideslip? (and no, I don't consider "step off the rudder" as a form of "use the rudder").
    Of course I expect them to, carefully, in a single direction, if the disturbance is not quickly corrected by the aircraft's natural directional stability. I don't want pilots getting into PIO overcontrol situations that might exceed structural design loads however. I want pilots to be fully educated on what the rudder is there for and how it should be used. If they learned it wrong bck in their aggie days, I want them to relearn it before becoming CTPL holders.

    Alternating full inputs on the control wheel and rudder pedals, such as those made by the first officer, should not be necessary to control a transport-category airplane under any circumstance.
    There is a procedure that advises the use of deliberate alternating sideslips to force gear downlocks in an unsafe gear situation. Even that procedure requires the aircrft to be stabilized between sideslips:

    The sideslip should be initiated using the rudder on the same side of the aircraft as the unsafe gear indication, i.e., if the right main landing gear is unlocked, slowly apply right rudder up to full deflection if necessary while maintaining wings level to generate sideslip. If the gear still fails to lock, then slowly return the rudder to neutral, allow the airplane to stabilize, and then slowly apply opposite rudder.
    The structural issue again is specific to rapid rudder reversals, for which there is no reason or need.

    From a structural capability standpoint, the pilot does not have to be concerned about how fast or how hard to push the rudder pedal in one direction (from zero to full available pedal deflection) throughout the normal flight envelope. However, it is important to emphasize that limiters do not protect against the structural loads or excessive sideslip angles that can be generated from rapid full deflection flight control reversals.
    However I do agree with you that CFR Part 25 should be modified to include design requirements that provide protection against this level of pilot error and aircraft-pilot-coupling (APC). So does the NTSB:

    Rudder control systems with a variable ratio rudder travel limiter may provide better protection against high loads from sustained rudder pedal inputs at high airspeeds than systems with a variable stop rudder travel limiter because variable ratio rudder travel limiter systems require more physical effort from a pilot (in terms of force and displacement) to produce cyclic full rudder inputs. For airplanes with variable stop rudder travel limiter systems, protection from dangerous structural loads resulting from sustained alternating large rudder pedal inputs can be achieved by reducing the sensitivity of the rudder control system (for example, by increasing the pedal forces), which would make it harder for pilots to quickly perform alternating full rudder inputs.

    The Safety Board concludes that certification standards are needed to ensure that future airplane designs minimize the potential for APC susceptibility and to better protect against high loads in the event of large rudder inputs. Accordingly, the Safety Board believes that the FAA should modify 14 CFR Part 25 to include a certification standard that will ensure safe handling qualities in the yaw axis throughout the flight envelope, including limits for rudder pedal sensitivity.
    It's just that, as your mantra goes, airmanship should come first.

  12. #52
    Senior Member Gabriel's Avatar
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    Evan, I am not arguing against the fact that the way the rudder was used in that accident was way off. In fact, it all started with a bad understanding of upset recovery principles, so yes, the simple fact that he used rudder was wrong to begin with, not to mention the repeated use of full rudder in opposite directions. I agree with you and the NTSB regarding the need of a better prescription of yaw handling qualities in the FAR.

    But in my last posts I am not arguing that. Please answer without mentioning repeated alternate rudder reversals. Because that is NOT what I am talking about.

    Do you agree that there is a big gap in the FAR by not prescribing any structural requirement whatsoever related with any a single rudder input in the direction opposite to any sideslip?

    Again, your left engine fails and your plane yaws strongly to the left and you are a little bit sloppy and let the sideslip increase a bit too much. What do you do now? Idle the right engine and let the natural directional stability handle it? Because if you apply right rudder at that point, the rudder or fin can legally fail, since there is no requirement in the FAR that it should withstand such an obvious and rational rudder input.

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  13. #53
    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel View Post
    Do you agree that there is a big gap in the FAR by not prescribing any structural requirement whatsoever related with any a single rudder input in the direction opposite to any sideslip?
    Yes, I think I just said that, although I don't know how big of a gap that really represents. When you take in both the design requirements and the certification tests, I don't see a structure that cannot withstand the corrective inputs you describe. (the A300 certification tests showed the attachments failed around 2x the load limits, well beyond certification requirements.)

    Nor, despite all such corrective inputs performed over the years, is there evidence of this:

    The FAA airframe engineer stated that , since the 1953 implementation of Civil Aeronautics Regulation 4B (the predecessor to the FAR that described the maneuvering conditions for the design of the vertical stabilizer), no historical evidence would lead the FAA to believe that the design loads envelope for the vertical stabilizer was inadequate.
    But on November 12th, 2001, fin separation occurred. So where does the problem lay:

    The airframe engineer stated that a maneuver with alternating rudder inputs was an extreme maneuver and that, if the maneuver were performed, loads would build that would exceed the current requirements. He further stated that, if two sets of alternating rudder inputs were performed, a series of dynamic maneuvers would start that could be benign or “could lead the airplane into a severe dynamic situation where, at the proper frequency, this continued application of this surface would allow the motion of the airplane to build up to the point where the sideslip would become excessive and overload the airplane.”
    That is, a repeated, cyclic abuse of rudder, amplified by aircraft-pilot-coupling due in part to phase lag, resulting in a PIO building to the point of overload... caused the fin to separate. Nothing short of that caused the failure.

    Conclusion: a repeated, cyclic abuse of rudder can cause the fin to separate. There is a structural danger there. There is no evidence to conclude that a similar structural danger exists in the conditions you describe.

    But I agree out of an abundance of caution that it should be added to the certification testing requirements. The NTSB does as well. (and if this is added tomorrow, it will affect the aircraft introduced into service around 2030. In the meantime, it is far more important that pilots are made to understand the non-aerobatic nature of rudder on transport aircraft before they are certified to fly them.) Agreed?

  14. #54
    Senior Member Evan's Avatar
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    Quote Originally Posted by FAA FCHWG Recommendation Report November 2013
    There are four known notable rudder reversal events in commercial aviation history, two of which occurred as a result of a wake encounter, and the commercial airplane fleet has more than a combined 500 million flight hours. Assuming an equal probability across all airplanes in the commercial fleet, the probability of a rudder reversal that reaches or exceeds design limit loading is approximately 10-8 per airplane flight hour. For this reason, it was determined that use of an ultimate load condition (factor of safety = 1.00) was deemed appropriate.
    However that is being reconsidered. Gabriel, read this:

    https://www.faa.gov/regulations_poli...rr-3282011.pdf

    Read especially: Attachment B – Proposed New Regulation 25.353 Section 25.353 Rudder control reversal conditions (Version 1 – Single Doublet Condition). I think it is wht you are proposing.

    Regarding the potential danger this represents:

    Quote Originally Posted by FAA FCHWG Recommendation Report November 2013
    The work of Table 1 was repeated, but this time with the pedal reversal coincident with the peak overswing sideslip. Results are presented in Table 2 and show the sensitivity to timing of the rudder pedal reversal input. It should be noted that for most aircraft types there was no more than 2% difference.
    Quote Originally Posted by FAA FCHWG Recommendation Report November 2013
    The results from Tables 1 and 2 show that the ultimate load single pedal doublet condition defined herein does not generate higher airframe loading than the current FAR design ultimate load level.

  15. #55
    Senior Member Gabriel's Avatar
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    Quote Originally Posted by Evan View Post
    However that is being reconsidered. Gabriel, read this:

    https://www.faa.gov/regulations_poli...rr-3282011.pdf

    Read especially: Attachment B – Proposed New Regulation 25.353 Section 25.353 Rudder control reversal conditions (Version 1 – Single Doublet Condition). I think it is what you are proposing.

    Regarding the potential danger this represents:
    Yes, that condition is what I am proposing, and what Boeing has been proactively using with a safety factor of 1.2 (ultimate load = 1.2 * limit load).

    Now, this work of the FAA is a piece of... ok, I will skip the adjectives.
    It is designed to yield the desired result: that the current FARs are ok and that no change is needed.

    How do they do that?
    Well, first, disregarding any other abnormal condition simultaneously with the rudder doublet.
    - Weight, CG and flight envelope conditions are not most critical, but "typical" (meaning that about 50% of the time it will be worse than that).
    - Aircraft with FBW was considered to be in normal law, and yaw damper was considered on if that was the expected condition as per the AFM. Didn't it ever cross their mind that a deviation from these typical conditions may cause a change in the airplane handling characteristics that can be a factor in how the pilot reacts and uses the rudder in the first place?
    - Now, the cherry of the cake...

    The results from Tables 1 and 2 show that the ultimate load single pedal doublet condition defined herein does not generate higher airframe loading than the current FAR design ultimate load level.

    Combined with this:
    For this reason, it was determined that use of an ultimate load condition (factor of safety = 1.00) was deemed appropriate.

    Do you get it? They are comparing the ultimate load that results from the current certification standards that includes a safety factor of 1.5 over the actual loads produced by the maneuver in the certification standard (design load) with the ultimate that results of the new proposed maneuver that includes a safety factor of 1.0 (i.e, no factor of safety) over (or below, or neither over nor below) the design loads produced by the new proposed maneuver.

    Let me be more clear: The design load is 66% of the ultimate load. Whenever in the table below you see a number bigger than 0.66, the design load was exceeded:

    Aircraft Type Doublet/Ultimate
    Small/Medium Business Jet 0.40 up to 1.0 (approx)
    Medium/Large Business Jet 0.34 up to 0.75
    Fuselage-Mounted Engine Regional Jet 0.42 up to 0.79
    Wing-Mounted Engine Regional Jet up to 0.80
    Single Aisle Passenger Jet 0.61 up to 0.73
    Widebody Passenger Jet 0.50 up to 0.81

    Do you see a number greater than 0.66 for any row? Because if you do, then know that the design loads were exceeded.

    By the way: The AA A300 was subject to loads 2 times the limit load. That is 33% above the ultimate load.

    And then we have this:

    Both airplanes survived these events due to having strength in excess of that required by the current standards

    This means that by the current standards these 2 fins were legal to fail and all on board (plus maybe some on the ground) legal to die.

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  16. #56
    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel View Post
    Now, this work of the FAA is a piece of... ok, I will skip the adjectives.
    It is designed to yield the desired result: that the current FARs are ok and that no change is needed.
    I don't think you're being entirely fair. Did you read through that document? I think they are trying to find a balance between structural requirement and practicality.

    So you think the proposed ultimate loads (safety factor of 1.0) should be the limit loads (safety factor of 1.5). The practical argument is as follows:

    These conditions are anticipated to occur very rarely, and so these are considered ultimate load conditions, and no additional safety factor is applied.
    Because:

    For some types of aircraft, overly severe criteria, including multiple full-stroke doublets, would lead to structural strengthening, a weight penalty, and/or system changes that could be detrimental to normal operations.
    Especially efficiency, which is both a business concern and an environmental one.

    Whereas:

    Enhanced training (as recommended by FCHWG) is the single most effective countermeasure to inappropriate rudder control reversals.
    The most effective (and the only effective measure in the short-term) also comes without a penalty for efficiency. Although I would replace the word 'enhanced' with the word 'fundamental'.

    Meanwhile, aircraft certified under this new requirement are all within the ultimate load envelope in these very rare instances you described (and most likely with some additional safety factor provided by the airframer).

    Gabriel, I would LOVE to see pilot-proof planes that can withstand anything thrown at them, but I do recognize the point where providing for extreme situations becomes detrimental to progress.

    I think the industry sees this as well.

    Teach pilots how to use rudder.

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