Cessna 182 N3267S accident near Burns, Oregon

Probable cause

The failure of the left main landing gear due to fatigue cracking of a bolt that initiated at a corrosion pit, which formed in the absence of a required protective cadmium coating.

NTSB analysis

Upon approaching the intended landing area, the pilot performed a high reconnaissance to assess the wind conditions. The airplane touched down and, during the landing roll, the left wing dipped downward. The left wing made contact with the ground before coming to rest with the left main landing gear sheared off.

The attachment bolt that connected the left main landing gear leg bracket exhibited a fatigue crack that had progressed through about one third of the bolt cross-section. This fatigue crack initiated at multiple fatigue crack initiation sites along the thread root. This thread root exhibited corrosion pitting that had penetrated the outer cadmium plating, which had led to fatigue cracking. When the remainder of the bolt could no longer support the stress at landing, the remaining cross section fractured in overstress. As this crack had propagated through about one third of the cross section without a discernible transition or mixed fracture zone between the fatigue crack and subsequent stress, the final fracture occurred at a high load event, such as the accident landing.

The bolts had been plated with cadmium, a corrosion inhibitor. However, corrosion pitting was observed along the threads, which led to fatigue cracking in both bolts. Although the cadmium plating can provide protection to the underlying alloy steel, corrosion such as in the form of pitting can still occur. This may be due to the operational environment, including locations near saltwater, as well as from a crevice corrosion mechanism.

With the fatigued bolt having fractured, the bracket would be able to flex or slide out the side. With these components able to flex, the bottom of the support housing could not support the downward forces from the gear spring, which led to its fracture. This would allow a cantilever force on the inboard support, which would have been enough force to fracture the attachment bolt on that support and the upper support frame. Although a hard landing would have caused this bolt to fracture, the fatigue crack diminished the amount of stress it could withstand.

Text from the NTSB. Open the NTSB docket for WPR19LA020 for the full record.