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Thread: Main landing gear collapse caused by too smooth landing!!!!

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    Senior Member Gabriel's Avatar
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    Default Main landing gear collapse caused by too smooth landing!!!!

    http://avherald.com/h?article=48e5e437&opt=0

    "... aircraft touched down at 139 KIAS at 2-3 degrees left bank angle and 1 degree drift angle to the right, the vertical rate of descent was 120 feet per minute and the aircraft experienced +1.1G. [...] This condition was induced with the good intention of achieving a smooth landing touchdown, but it had a negative impact on the landing gear shimmy effectiveness. According to Boeing the low sink rate during landing touchdown increases the likelihood of shimmy damper failure. [...] When prolonged flare is attempted to achieve a perfectly smooth touchdown, the aircraft is at risk of landing gear torsion link failure,"

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    Okay... most of that article makes sense in a weird sort of way (smooth landing = aircraft damage???) but these bits I find confusing:
    Quote Originally Posted by AvHerald
    A passenger reported the captain announced prior to landing that they would need to land without left main gear. The aircraft subsequently touched down at an angle and skidded along the runway with sparks spraying off the left hand side.
    Quote Originally Posted by AvHerald
    The SACAA reported the flight data recorder showed the aircraft touched down...at 2-3 degrees left bank angle...
    So it seems (assuming the passenger report is correct) the pilot thought there was a problem with the left main gear before landing, yet landed with a left bank angle? I thought SOP if a gear problem is suspected is to keep said gear off the runway as long as possible... so you'd want to be banked to the right in this case.
    Be alert! America needs more lerts.

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    Senior Member Gabriel's Avatar
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    Quote Originally Posted by elaw View Post
    Okay... most of that article makes sense in a weird sort of way (smooth landing = aircraft damage???) but these bits I find confusing:

    A passenger reported the captain announced prior to landing that they would need to land without left main gear. The aircraft subsequently touched down at an angle and skidded along the runway with sparks spraying off the left hand side.

    The SACAA reported the flight data recorder showed the aircraft touched down...at 2-3 degrees left bank angle...
    So it seems (assuming the passenger report is correct) the pilot thought there was a problem with the left main gear before landing, yet landed with a left bank angle? I thought SOP if a gear problem is suspected is to keep said gear off the runway as long as possible... so you'd want to be banked to the right in this case.
    The three paragraphs that follow the one in red are:

    The airline confirmed a landing incident just after 12:00L, all passengers disembarked safely without injuries. The airline later reported that shortly after touch down the crew noticed an unusual vibration which was shortly followed by the collapse of the left main landing gear.

    A ground witness reported it appeared the left main tyres burst on touchdown, the aircraft fell onto the wheel bogies, the left main gear strut was ripped off and left behind, the aircraft skidded on its left engine to a halt.

    South Africa's Civil Aviation Authority (SACAA) reported in their October bulletin that the crew reported they had a normal landing, but a few seconds after touchdown they felt vibrations from the undercarriage.
    This clearly means that your "assuming the passenger report is correct" is putting a bit too much faith in a pax report on versus a ground observer, the pilots, the airline, and the investigating agency.

    Th landing was just with 120 fpm (that's as smooth as it gets), normal touchdown pitch, normal airspeed (although the ground speed was higher due to the high density altitude of the airport), and 2-3 degrees of bank is nothing out of the ordinary either, and Boeing "blames" the pilot for touching down too smoothly.
    That an airplane can't handle a soft landing is off the charts.

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    Senior Member Evan's Avatar
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    Quote Originally Posted by Gabriel View Post
    That an airplane can't handle a soft landing is off the charts.
    Is it possible that the fuse pins were already damaged or weakened here? AFAIK that strut is designed and certified to withstand a complete high-speed tire failure scenario.

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    Quote Originally Posted by Gabriel View Post
    This clearly means that your "assuming the passenger report is correct" is putting a bit too much faith in a pax report on versus a ground observer, the pilots, the airline, and the investigating agency.
    You're probably right. Although... wouldn't the investigating agency have been able to verify that claim by listening to the CVR? If so, you think they would know for sure and put different verbiage in the report.

    Quote Originally Posted by Gabriel View Post
    Th landing was just with 120 fpm (that's as smooth as it gets), normal touchdown pitch, normal airspeed (although the ground speed was higher due to the high density altitude of the airport), and 2-3 degrees of bank is nothing out of the ordinary either, and Boeing "blames" the pilot for touching down too smoothly.
    That an airplane can't handle a soft landing is off the charts.
    The one thing I wonder about is whether the a/c could have been moving laterally relative to the runway. So while the vertical force imparted to the left main gear (the only gear in contact with the ground at the time) was minimal, there may have been significant horizontal force. And of course because of the length of the lever arm, the farther the gear is extended, the more force is put on the supporting components in that circumstance.
    Be alert! America needs more lerts.

    Eric Law

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    Quote Originally Posted by Evan View Post
    Is it possible that the fuse pins were already damaged or weakened here? AFAIK that strut is designed and certified to withstand a complete high-speed tire failure scenario.
    Not the fuse pins specifically, but they do allude to something of that nature in the report:
    Quote Originally Posted by Report quoted on AvHerald
    It is also possible that the torsion link had already lost its maximum strength during the cause of the life it had already spent in operation.
    Be alert! America needs more lerts.

    Eric Law

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    Senior Member 3WE's Avatar
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    FASCINATING! My one personal near-total-disaster event was a shimmy damper failure on a 737...and now I wish I remembered more.

    In our case the plane did vibrate in a very impressive manner- and I think we lost a cheap composite interior trim panel or two. We stopped on the runway (literally) and were towed in (I guess there was a some risk of gear collapse and they didn't want to scrunch whirling turbine blades if that happened).

    Our landing occurred during a very good gusty day (maybe 20 to 25? kts from the SW and landing NW???)- I recall the guys retracting the flaps a tad as we got on final approach. My recollection (subject to eyewitness imagination) was that we landed left wing low and really compressed the left MLG (not a rough landing, but it seemed the spoilers popped and the weight sat down on it?...but maybe the right hand gear touched down gently?...and I think it was the right side that did the shimmy?

    At first it seemed like no big deal, but as we slowed it amplified and was not unlike an old 172 with a ragged out nose gear (or a late 1970's 4WD Chevrolet truck with a worn out tie rod damper)...It did seem that things were rocking and rolling and flexing big time and I could believe the strut giving way if it had gotten much worse or lasted longer.

    I recall that the pilots switched to rather gentle braking and wound up rolling out to the very very end of a 9000 ft runway...coming to a very gentle stop as we just began to turn off.

    We sat there for a few minutes while the fire crews inspected stuff...and then for about an hour while they pried a tug away from their regular operations. Some sarcasm here- they also seemed to have broke something with the air conditioning or APU (it did actually fail- as to whether that was the shake down or just another random event, who knows.).
    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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    Some interesting comments from AVHerald:

    Earlier 737s
    By Michael on Wednesday, Mar 15th 2017 08:18Z

    You'd think an extra smooth landing would be ideal, but Boeing recommends otherwise for the earlier 737's due to this exact problem -- it's a known issue.

    Combine low sink rate, slightly worn (but within-spec) components, the high altitude airport with faster landing speeds, and the likelihood of a failure increases. For those interested, search for Boeing AERO magazine 2013q3 -- it has an article on shimmy events.
    Torque Link / Shimmy Damper pt 1
    By Jack Johnson on Wednesday, Mar 15th 2017 13:33Z

    So in a nutshell, this is how the torque link and shimmy damper works.

    The sliding piston (the lower part of the landing gear) is essentially free to rotate in the static piston (which is attached to the aircraft). The torque link connects the two pistons with a two-part mechanism (the torque link), that transmits torsion load from the wheels to the airframe.

    With the gear extended in flight, the apex of the torque link is closer to the centre-line of the shock strut. Hence, the ability of the shimmy damper to dampen shimmy is reduced (force x distance).

    (continued)

    Torque Link / Shimmy Damper pt 2
    By Jack JOhnson on Wednesday, Mar 15th 2017 13:34Z

    (continued)

    As weight is put on the wheels, the shock absorber closes, putting the Torque Link apex further from the shock strut centreline. At this point the torque link and shimmy damper becomes more effective at preventing shimmy.

    The shimmy damper is basically a big baffles with oil, where load is dissipated (like in a shock absorber) mechanically. The force with which the oil is forced through the shimmy damper depends on the distance from the Torque Link apex to the shock strut centreline.

    So, a ultra soft float landing could contribute to failures due to shimmy.

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    Senior Member Gabriel's Avatar
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    Quote Originally Posted by elaw View Post
    You're probably right. Although... wouldn't the investigating agency have been able to verify that claim by listening to the CVR? If so, you think they would know for sure and put different verbiage in the report.
    I doubt that this was even a "claim" for the investigating agency. No investigating agency will assign resources to confirm or deny every thing reported by the press.

    The one thing I wonder about is whether the a/c could have been moving laterally relative to the runway. So while the vertical force imparted to the left main gear (the only gear in contact with the ground at the time) was minimal, there may have been significant horizontal force.
    It is in the quote in the opening post: 1 degree drift to the right. That is nothing.

    And of course because of the length of the lever arm, the farther the gear is extended, the more force is put on the supporting components in that circumstance.
    The less strut compression the longer the lever arm and also the torque generated by a given force on that lever arm.
    But at the same time the less strut compression the lighter compression force and the contact force between the wheel and the pavement (i.e. the "normal" force also known as WOW or Weight On Wheel), and hence the lower the friction (which has a maximum of "friction coefficient"*WoW), so the lower the torque for a given lever arm. Long explanation to say I don't think that that would be an issue.

    However, the less strut compression the more stretched the torque link is, and hence the lower the torsion arm and the greater amplification of any free-play, which could then increase the risk and severity of oscillatory torsion movements (i.e. shimmy),

    --- Judge what is said by the merits of what is said, not by the credentials of who said it. ---
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    Senior Member 3WE's Avatar
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    This video is greatly off-topic.

    However, around 16:00 on the video, I think we see a 737 with a failed shimmy damper on the right main gear.

    For some reason, my experience with this is that it's 'always' the right hand gear. (Personal, this thread, and the video)

    Grossly, unsubstantiated speculation...but...

    https://www.youtube.com/watch?v=KlpJTGAv2Oc
    Les règles de l'aviation de base découragent de longues périodes de dur tirer vers le haut.

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