What happened
On March 8, 2018, a Piper 3/8 (registration F-GDFQ) was performing local training circuits at Nancy Essey aerodrome. During the second circuit, the pilot was configured for landing with flaps extended at a speed of 75 knots. An AFIS agent reported winds from 230 degrees at 14 knots.
Upon touchdown, the aircraft encountered a gust, causing it to bounce and lift approximately two meters off the runway. While the pilot maintained control, the aircraft subsequently made a second, heavy contact with the ground. As the pilot applied braking and attempted to steer the aircraft off the runway using the right rudder, vibrations were felt, and the left side of the aircraft collapsed. This caused the aircraft to rotate approximately 60 degrees.
The investigation
The investigation focused on the structural integrity of the landing gear and the maintenance history of the aircraft. Investigators examined the landing gear components in a laboratory setting and reviewed the aircraft's maintenance logs.
Technical analysis of the left main landing gear revealed that the attachment bolts and washers showed advanced corrosion and wear. The bolts were found to be bent, and the bracket was deformed, likely due to the heavy loads experienced during the hard landing. Furthermore, the bolt heads were not drilled, which prevented the use of safety wiring to secure them.
Findings
- The primary cause of the landing gear failure was the failure of the left main landing gear bolt due to a combination of fatigue and corrosion.
- Laboratory analysis determined the failure mode was mixed: fatigue cracks initiated on both sides of the bolt diameter due to alternating bilateral bending stresses. As these cracks propagated, the remaining cross-section of the bolt became insufficient to withstand the load of the hard landing.
- While a previous airworthiness directive had mandated the replacement of original bolts with higher-strength versions, the specific inspection and replacement intervals outlined in the manufacturer's Service Bulletin (SB1200) were not mandatory under a formal airworthiness directive.
- Consequently, the degradation of the bolts went undetected, as the aircraft's maintenance records did not show evidence of the specific inspections required by SB1200.