Heavy Damage to Alaska B738 at KSNA During Tropical Storm
Shear pins are designed to shear!
And for very good reasons.
But you would like to have visible evidence when they do.
It would seem that evidence is gathering that the landing loads on this accident should not have caused failure...that points to previous shear pin (partial?) failure.
Years ago the shear pins on B747 engine pods were shearing and not being noticed.
Shear pin failure was then made detectable by painting witness stripes to detect pod droop after failure.
Shear pin failure that remains undetectable before further flight is an engineering design failure.
And for very good reasons.
But you would like to have visible evidence when they do.
It would seem that evidence is gathering that the landing loads on this accident should not have caused failure...that points to previous shear pin (partial?) failure.
Years ago the shear pins on B747 engine pods were shearing and not being noticed.
Shear pin failure was then made detectable by painting witness stripes to detect pod droop after failure.
Shear pin failure that remains undetectable before further flight is an engineering design failure.
Shear pins are designed to shear!
And for very good reasons.
But you would like to have visible evidence when they do.
It would seem that evidence is gathering that the landing loads on this accident should not have caused failure...that points to previous shear pin (partial?) failure.
Years ago the shear pins on B747 engine pods were shearing and not being noticed.
Shear pin failure was then made detectable by painting witness stripes to detect pod droop after failure.
Shear pin failure that remains undetectable before further flight is an engineering design failure.
And for very good reasons.
But you would like to have visible evidence when they do.
It would seem that evidence is gathering that the landing loads on this accident should not have caused failure...that points to previous shear pin (partial?) failure.
Years ago the shear pins on B747 engine pods were shearing and not being noticed.
Shear pin failure was then made detectable by painting witness stripes to detect pod droop after failure.
Shear pin failure that remains undetectable before further flight is an engineering design failure.
I (and the poster I was responding to) were guilty of imprecise language.
The MLG aft trunnion failed in shear - but that doesn't make it a shear pin.
The "trunnion shear pin" appears to have been an invention of Avherald, subsequently parroted verbatim by several of the enthusiast sites.
Mea culpa.
For the sake of humour, I was taking a cheap shot and consequently disrespectful and irreverent...please accept my apologies!
I was wondering about the engineering design concepts that would justify the use of shear pins in MLG attachments. I believe you are correct on the whole shear pin nonsense.
The MLG aft trunnion failed in shear - but that doesn't make it a shear pin.
The "trunnion shear pin" appears to have been an invention of Avherald, subsequently parroted verbatim by several of the enthusiast sites.
The "trunnion shear pin" appears to have been an invention of Avherald, subsequently parroted verbatim by several of the enthusiast sites.
Join Date: Jan 2008
Location: Netherlands
Age: 46
Posts: 343
Likes: 0
Received 0 Likes
on
0 Posts
The landing gear is meant to go through the wing at an excessively hard (crash) landing so the structure can absorb the energy by deforming without rupturing the fuel tanks or other lethal failures.
It now happened at a landing which wasn't exactly smooth, but definitely not excessively hard either. Most likely the pin was already defective before the landing. The most likely questions are how it got damaged and why it hadn't been found during inspection.
It now happened at a landing which wasn't exactly smooth, but definitely not excessively hard either. Most likely the pin was already defective before the landing. The most likely questions are how it got damaged and why it hadn't been found during inspection.
The "shear pin" comment may be referring to what Boeing call the "fuse pin" in the gear architecture, the gear is designed to depart the structure in a controlled manner when subjected to overload. From a Boeing Aero magazine re 737,
Travel onto surfaces with depressions or obstructions will generally require close inspection of all fuse pins during the two-phased inspection process outlined in the AMM. The gear then may be removed for closer inspection depending on flight crew judgment, FDR/QAR data review, consultation between the operator and technical experts, or the discovery of any structural anomalies. In addition to fuse pin deformations, axle and truck deformations may be discovered during close inspection of the gear
megan
Thank you megan, that makes a lot more sense for the design failure case when described in that way.
procede
procede makes a very good analysis of what apparently happened in this accident and raises the question of why previous damage wasn't discovered before the accident flight.
The "shear pin" comment may be referring to what Boeing call the "fuse pin" in the gear architecture, the gear is designed to depart the structure in a controlled manner when subjected to overload. From a Boeing Aero magazine re 737,
procede
The landing gear is meant to go through the wing at an excessively hard (crash) landing so the structure can absorb the energy by deforming without rupturing the fuel tanks or other lethal failures.
It now happened at a landing which wasn't exactly smooth, but definitely not excessively hard either. Most likely the pin was already defective before the landing. The most likely questions are how it got damaged and why it hadn't been found during inspection.
It now happened at a landing which wasn't exactly smooth, but definitely not excessively hard either. Most likely the pin was already defective before the landing. The most likely questions are how it got damaged and why it hadn't been found during inspection.
Join Date: Jan 2008
Location: Hawaii
Age: 76
Posts: 89
Likes: 0
Received 0 Likes
on
0 Posts
Join Date: Jan 2008
Location: Netherlands
Age: 46
Posts: 343
Likes: 0
Received 0 Likes
on
0 Posts
That seems a bit excessive. It looks like you should be able to remove the bolt by taking the weight of the gear (by using a stand) and fixing the gear in place. Something you could easily do in the hangar at every A check, or after a hard landing.
But as noted in the 2019 UAL Denver accident, even with the landing gear removed for overhaul, if the procedure is improperly performed, the trunnion pin can be damaged and placed back into service. The NTSB report is vague as to whether the 2017 overhaul of the landing gear was properly performed; i.e., whether the damage introduced during the 2008 overhaul by Goodrich should have been detected during the 2017 overhaul by UAL (the fatigue crack was not detected).
Several “expert” YouTubers have cast doubt upon the possibility of the ASA1288 accident being caused by a faulty component rather than hard landing due to the top-tier maintenance that 121 aircraft undergo. Seems that the history of 737 trunnion pin failures coupled with Alaska’s troubled maintenance history would suggest otherwise.
Several “expert” YouTubers have cast doubt upon the possibility of the ASA1288 accident being caused by a faulty component rather than hard landing due to the top-tier maintenance that 121 aircraft undergo. Seems that the history of 737 trunnion pin failures coupled with Alaska’s troubled maintenance history would suggest otherwise.
The DFDR was downloaded on and a review of the preliminary data indicates that the aircraft touched down about 23:15 PDT with a maximum vertical acceleration of 1.71g. The value is below the hard landing threshold of 2.2G per Alaska Airlines aircraft maintenance manual (AMM) Chapter 5 limitations.
NTSB Preliminary Report ASA1288
NTSB Preliminary Report ASA1288
