Beech 99 N326CA accident near Billings, Montana

Probable cause

The fatigue failure of the left main landing gear (MLG) actuator piston rod, which prevented the hydraulic system from producing pressure to control the MLG’s position and led to the right MLG collapsing during landing.

NTSB analysis

The commercial pilot reported that, after departing for the cross-country, cargo flight, he retracted the landing gear and heard a "thud" sound. Subsequently, the main landing gear (MLG) "unsafe" light illuminated, and the left MLG light remained illuminated green, indicating that it was still extended despite the landing gear handle being in the retracted position. The pilot returned to the departure airport, and during touchdown, the right MLG collapsed, which resulted in substantial damage to the right aileron and right wing spar.

During the postaccident examination of the landing gear, the left MLG actuator's piston rod was found fractured. Metallurgical examination of the piston rod revealed that the rod had failed in its threaded section due to fatigue cracking over about 60% of the cross-section. Contact wear was observed on the faying surface of the retaining nut and piston head and the shoulder of the piston rod. These signatures were consistent with the nut not being tight enough to mitigate sliding of the piston head relative to the piston rod and retaining nut. It could not be determined if the manufacturer required a specific torque to be applied to the nut during assembly. Based on the evidence, it is likely that inadequate torque on the nut reduced the preload on the threaded section of the piston rod and contributed to premature fatigue crack propagation in the rod and its eventual failure. The failure of the piston rod allowed hydraulic fluid to pass freely from the down-side to the up-side of the piston, which prevented the hydraulic system from producing pressure to control the MLG's position.

Text from the NTSB. Open the NTSB docket for WPR17LA056 for the full record.