What happened
On July 22, 2018, a Cessna 3/10R, registration PT-GMA, was performing a private passenger transport flight from Cipó, Bahia, to Morro de São Paulo, Bahia. During the landing roll at the Morro de São Paulo Aerodrome (SDGX), the left main landing gear unexpectedly retracted. This caused the left propeller blades to strike the ground, leading the aircraft to veer off the runway and come to a stop approximately 90 degrees from the runway centerline. The aircraft sustained light damage, but the pilot and the single passenger were uninjured.
The investigation
CENIPA investigators examined the landing gear mechanism, which is electrically actuated via a reduction box and a push-pull tube system. The investigation focused on the downlock mechanism, specifically the side links and the bellcrank assembly. Investigators discovered a fracture in the bolt support that secures the bellcrank to the strut assembly, as well as a rupture in the downlock link adjusting screw.
Analysis of the fractured adjusting screw revealed significant signs of corrosion. This corrosion led to the failure of the screw, which caused the downlock link to lose its effectiveness. Consequently, the side links moved out of the overcenter position, releasing the mechanical lock and allowing the left gear to retract under the stresses of the landing roll. The investigation also noted that the aircraft was based in Salvador, a coastal city with high humidity and salinity, which likely accelerated the corrosive process.
Findings
- The primary cause of the gear retraction was the rupture of the downlock link adjusting screw due to corrosion.
- The corrosion was not identified during the last annual maintenance inspection or during pre-flight checks.
- Maintenance procedures for the Cessna 310R recommend increasing inspection frequencies for wear and corrosion when operating in tropical or highly humid environments, a practice that was not implemented for this aircraft.
- The failure of the adjusting screw caused a mechanical discontinuity that allowed the gear to unlock and retract.
Safety action
CENIPA issued recommendations to the National Civil Aviation Agency (ANAC) to ensure that maintenance providers for the Cessna 310R model are performing inspections strictly according to maintenance manuals, particularly regarding corrosion identification in landing gear systems. Additionally, operators and maintainers are encouraged to increase inspection intervals for components subject to corrosion when operating in high-salinity environments.