What happened
On December 20, 2021, a Thrush S2R-T34, registration PR-GAT, was performing an agricultural application flight in the municipality of Pimenta Bueno, Rondônia. The aircraft departed from an unpaved landing strip at Fazenda Figueira. During the takeoff roll, the right main landing gear failed. In response to the failure, the pilot jettisoned the aircraft's load and continued the takeoff attempt.
The pilot subsequently landed at an unregistered landing site known as Base Operacional Cone Sul Aeroagrícola, in Chupinguaia, RO. During the landing roll, the aircraft lost directional control and exited the right side of the landing area, colliding with a fence and uneven terrain. The aircraft sustained substantial damage, but the pilot remained uninjured.
The investigation
CENIPA investigators examined the aircraft's maintenance history and the physical components of the landing gear. The investigation focused on a specific bolt (PN NAS6209-74D) used to secure the landing gear assembly to the bottom of the shock absorber. Laboratory analysis of this bolt revealed multiple fracture planes and "beach marks," which are definitive indicators of fatigue failure.
While maintenance records indicated that the landing gear bolts had been inspected, lubricated, and reinstalled in July 202 and that this specific bolt had been replaced in September 2020, investigators found significant corrosion and machining marks on the failed bolt. These findings raised doubts regarding the actual origin of the part and the effectiveness of the maintenance and supervisory processes, as the level of corrosion was inconsistent with a part that had supposedly been serviced and lubricated less than 30 days prior to the accident.
Findings
- The failure of the right main landing gear was caused by the fatigue-induced fracture of bolt PN NAS6209-74D.
- The bolt showed extensive corrosion and signs of heavy use, suggesting it may not have been the new part documented in the maintenance logs.
- The aircraft was likely subjected to excessive loads due to the nature of agricultural operations, which frequently utilize unprepared landing strips.
- Inadequate maintenance and lack of effective managerial supervision contributed to the failure to identify and replace the fatigued component.