Beech 99A MLG Collapse (N326CA, Alpine Air)
On 19 January 2017 Beech 99A N326CA of Alpine Air experienced a landing gear collapse on landing at the Billings Logan International Airport (BIL), Montana while making a Part 135 cargo flight. The pilot was uninjured.
According to the US National Transportation Safety Board (NTSB) safety investigation report (released 13 April 2020), after take off from BIL the pilot retracted the landing gear and heard a ‘thud’. The Main Landing Gear (MLG) ‘unsafe’ light illuminated, and the left MLG light remained green, indicating that it was still extended.
The pilot returned to BIL and just before touch down, feathered the propellers and decreased the airspeed. During touchdown, the right MLG collapsed. This caused substantial damage to the right aileron and right wing spar.
The NTSB say:
During the post-accident examination of the landing gear, the left MLG actuator’s piston rod was found fractured.
Failed MLG Actuator of Alpine Air Beech 99 N326CA (Credit: NTSB)
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.
Surprisingly:
It could not be determined if the manufacturer required a specific torque to be applied to the nut during assembly.
The 1970 made aircraft had accumulated 46,416.3 flying hours. The left actuator was last overhauled in November 2001, at which time the piston rod was replaced. The part had 7,445.2 hours and 8,267 cycles since overhaul. The landing gear hydraulic actuators need to be overhauled or replaced every 10,000 hours.
The NTSB concluded:
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.
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