Above picture credit to Airguide.info
On 21-Aug an Alaska Airlines landing at Santa Ana CA. Airport (also know as John Wayne) from Seattle WA. suffered damage to the left main landing gear.
As the landing was made in turbulent conditions during the aftermath of an hurricane, many people, including from the Aviation community jumped to conclusions that the damage was a result of a hard landing and placed blame squarely on the crew.
The landing was recorded on video by a passenger, showing the trail of sparks caused by the engine dragging after the gear collapsed.
To us, the landing seemed rough but not excessive, given the fact that 737's are generally robust aircraft and are able to take a good beating without damage.
There is some interesting data available!
The NTSB produced their preliminary report. Downloadable below.
It appeared that the aircraft recorded 1.7G as touchdown acceleration, which is not smooth but not excessive either.
It appeared that the left main gear aft trunnion pin fractured, which leaves the gear leg supported by two points; the side strut, and forward trunnion pin. This is statically undetermined, hence unstable. As a result the gear leg pivoted backward, pushing the aft trunnion and walking beam through the secondary (non load bearing) upper wing skin, causing the aircraft to list left.
Below the Wing Rear Spar and Wing landing gear beam with the gear assy omitted for clarity;
It is obvious that the trunnion pins which are inserted in the gear assemblies and bearings in the forward and aft trunnion fittings on the Wing Rear Spar and Main Landing Gear Beam respectively, are highly loaded parts, and consequently made from high strength material, usually a high strength steel alloy.
Anyone familiar with the basics of metallurgy, knows that high strength come with a price and a drawback usually, being fatigue sensitive and prone to corrosion. This is why these parts are, among others life limited. In other words, irrespective of the condition of the part, it must be discarded after a certain number of service cycles.
In addition, 737 Main Gear Trunnion Pins have been subject to service publications; to name a few;
Service Bulletin 737-32-1376; Forward Main Gear Trunnion Pin Inspection and Replacement Effective for line numbers 533 thru 1422
Service Bulletin 737-32-1402; Forward Main Gear Trunnion Pin Inspection and Replacement
Effective for line numbers 1423 thru 3526 (essentially an effectivity extension of 32-1376).
The above SB's only affect Forward Trunnion Pins
Service Bulletin 737-32-1448; Forward and Aft Trunnion Pins inspection and replacement.
Effective for Line Numbers 1 thru 3526.
This SB is related to FAA AD 2019-01-03 (superseding AD 2016-18-01). It's a relatively complex AD that has compliance divided in four parts;
Par (g) determination of applicable part numbers in the landing gear assemblies. This can be done by research of maintenance records
Par (h) repetitive lubrication program of the FWD and AFT reunion assemblies
Par (i) repetitive inspections of the FWD and AFT trunnion pin assemblies
Par (j) Modification of affected parts
The accident aircraft is Line number 2751, manufactured in December 2008, which is not to say that it is directly affected as the landing gears have most likely already been replaced due to the overhaul interval of 10 years.
This SB addresses detection of corrosion damage in the trunnion pins and stipulates mitigation of fracture. See below for general impression.
Needless to say that research to be conducted whether or not the broken trunnion pin has been in compliance with above Service Bulletin takes an extensive review of technical records of both the landing gear unit and the airframe records.
It will be interesting to see what the conclusions in the final report will be