Heavy Damage to Alaska B738 at KSNA During Tropical Storm

[Lake1952]

An Alaska 738 made a "heavy" landing during Tropical Storm Hilary at KSNA. The left main looks like it pushed through the wing box.

https://www.radarbox.com/blog/alaska-airlines-flight-as1288-from-seattle-suffers-damage-after-heavy-landing-at-santa-ana

https://aviationsourcenews.com/incident/alaska-airlines-flight-suffers-damage-in-santa-ana/

[1southernman]

Having peered into the 37 wheel well many times that's hard to believe...Joked a few times about "at we didn't drive the wheels thru wings" but didn't think it could actually happen...In the cabin left side video the touchdown "seems" pretty benign...

[RickNRoll]

It's because someone didn't have their phone on airplane mode. :oh:

[fdr]

Not your usual scene from the window... but, not that nasty, the breaks will be interesting examinations.

The loading of the gear that is certified is a symmetric loading § 25.473 (a)(1) Landing load conditions and assumptions. § 25.479 Level landing conditions.

the main gear are assumed to touch symmetrically, the OEM goes beyond that, but a high sink rate, or with a roll rate developing, can be ugly, recall the MD11/11F's at [take your pick... but Narita is still pretty impressive]. The auxiliary spar giving up here is probably a best alternative on the day. Going to need a fair bit of bondo filler for it to RTS though.

[tupungato]

Here is the passenger video:
https://www.youtube.com/watch?v=8VpBHiQVYt4

[Lake1952]

https://avherald.com/h?article=50d52882&opt=0

[Diamond Bob]

I wonder if there was undetected preexisting damage to the wing or gear. As several have commented, the landing did not seem exceptionally hard.

[BFSGrad]

The summary at Aviation Safety Network states, “The aircraft was able to vacate the runway via taxiway E and came to a full stop on the taxiway.”

The flight crew chose to taxi an aircraft in that condition? And with a 90-degree right turn no less. Or was the aircraft towed? There is a photo of the aircraft parked on taxiway E with the tail clear of the hold bars.

[DaveReidUK]

The summary at Aviation Safety Network states, “The aircraft was able to vacate the runway via taxiway E and came to a full stop on the taxiway.”

The flight crew chose to taxi an aircraft in that condition? And with a 90-degree right turn no less. Or was the aircraft towed? There is a photo of the aircraft parked on taxiway E with the tail clear of the hold bars.

ADS-B suggests that the 737 didn't stop at any point until it had completed the 90° left turn off 20R onto Twy E and cleared the runway.

[Lake1952]

It's hard to believe that the aircraft could taxi with the left engine dragging on the pavement.

Before everyone assumes that the accident was caused by heavy landing forces alone, there has been speculation about an underlying defect in the gear, either metallurgical or related to prior maintenance procedures as a cause or a contributing cause.

Runway 20 at KSNA is only 5700 feet long. Throw in tropical storm rain and a gusty crosswind and one can easily understand a desire to get those mains on the ground. More data will be helpful.

[BFSGrad]

Perhaps similar to the December 2019 gear collapse of a UAL 737-800 at Denver. Cause was a faulty overhaul of the main gear resulting in failure of the aft trunnion pin. Aircraft was repaired and returned to service.

N87513 (https://aviation-safety.net/database/record.php?id=20191222-0)

[amsm01]

That does not look anywhere or remotely close to the kind of vertical G to punch the gear through the wing. There’s some kind of fatigue or component failure there

[Chiefttp]

From another website,

Narrative:
• Flight departure was delayed about 45 minutes out of SEA. Crew called Dispatch to see about an extension for the curfew at SNA and it was extended until 2330 PDT.
• Ran SNA Rwy 20R landing performance Flaps 40, AutoBrakes 3.
• Aircraft flew ILS to Rwy 20R. Stable approach, aircraft fully configured prior to FAF. Before Landing Checklist Completed. Aircraft broke out about 800’.
• SNA ATCT was closed.
• CA flying, touched down firmly on purpose just pass the 1000’ marker but prior to LTP. CA and FO both stated it was a firm landing, but nothing that alarmed them.
• CA felt what he described as "the tire popping” and noted the green light for the Left MLG was gone but no red light. Other two LG lights were still showing green.
• Aircraft pulled a bit to the left, but CA got it back to center line with rudder.
• Turned left off Rwy 20R onto Taxiway E and stopped. It was then that the airplane really started listing. After landing flow done.
• CA opened window and looked out and saw left wing resting on the nacelle and a wheel.
• Electrical source was transferred to APU and shut down engines. Did not pull fire switches.
• Called SNA Ops to send maintenance out.
• SNA Airport CFR heard the flight advise CTAF that they thought they had a gear collapse. SNA CFR came out and assessed the situation. Advised no visible fuel leak and that airplane was basically resting on the left engine cowling and one of the tires. Wing tip not touching ground.
• Ground crew initially planned air stairs on left side of airplane for deplaning through L1 door. Mechanics came on the intercom and suggested deplaning from R1 door to avoid further collapse and instability. Crew agreed and passengers deplaned from R1 door.
• Passengers were pretty calm. Took about an hour to get all the passengers off and to the terminal. Had passengers stay seated until everything was set for them to deplane. It was 30-45 minutes until they started deplaning and 60 minutes or so until everyone was off the airplane.

I’m also hearing that the plane went thru some maintenance on the gear and they suspect that some bolts weren’t put back in.

[1southernman]

That makes sense that it's probably mechanical given the robustness of Boeing gear....Not nearly as serious but had a tiller not steer (737-300) on landing roll out...Cleared the runway but had to get towed to the gate...Mech took a peek and said a linkage bolt was missing...Maintenance had been done on it at our dep arprt...Obviously fell out sometime after I stopped using the tiller on dep...

[kenish]

KSNA is my local airport. I will have to drive over there and look for the plane- it's not going anywhere in the next few days!

[Asturias56]

certainly didn't look like they "dropped" it from the utube vid. Not as hard as some of the carrier style landings at the old Singapore airport to avoid the bounce on the runway ridge

[jetpig32]

We had one at United a few years back. First gear overhaul by vendor was determined unsat and lead to trunnion pin failure.

[hunbet]

This type of failure has happened multiple times and is usually due to improper maintenance during overhaul. Here is one from 2015 that clearly shows a sheared trunnion pin. Repairs are not difficult and the plane usually returns to service.

​​​​​​http://www.b737.org.uk/incident_vt-jga.htm

[Lake1952]

AvHerald is reporting that a failure of the aft trunnion shear pin cause the failure, not a hard landing.

Accident: Alaska B738 at Santa Ana on Aug 20th 2023, gear punched through wing (http://avherald.com/h?article=50d52882&opt=0)

[procede]

Shear pins generally do not break on soft landings, but the landing was probably well within limits.

[thcrozier]

Another point of view from some kids in row 2. They’re pretty entertaining :D

https://www.youtube.com/shorts/V0RxSc-NwXw

[DaveReidUK]

Shear pins generally do not break on soft landings

Shear Landing gear trunnion pins generally do not break.

[FlexibleResponse]

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.

[DaveReidUK]

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.

Correct.

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. :O

The "trunnion shear pin" appears to have been an invention of Avherald, subsequently parroted verbatim by several of the enthusiast sites.

Mea culpa. :\

[FlexibleResponse]

For the sake of humour, I was taking a cheap shot and consequently disrespectful and irreverent...please accept my apologies!

The MLG aft trunnion failed in shear - but that doesn't make it a shear pin. https://www.pprune.org/images/smilies/embarass.gif

The "trunnion shear pin" appears to have been an invention of Avherald, subsequently parroted verbatim by several of the enthusiast sites.


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.

[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.

[megan]

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


https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/470x482/ab266_bc0fccc71e694e9fec68b96fb8bf9f6097eefc7d.png

[FlexibleResponse]

megan

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,

Thank you megan, that makes a lot more sense for the design failure case when described in that way.


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.

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.

[hunbet]

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.

You can't see it without removing the landing gear.

[procede]

You can't see it without removing the landing gear.

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.

[BFSGrad]

You can't see it without removing the landing gear.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.

[thcrozier]

https://www.youtube.com/shorts/qw98xuxu1-w

[BFSGrad]

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 (https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/192903/pdf)

[BFSGrad]

N516AS flew from KSNA to KOKC on 12/20/23 (likely non-revenue) and returned to service on 12/22/23.