What happened
On May 11, 1995, at approximately 10:22 local time, the Cessna RA F177RG HB-CXU departed from Grenchen Airport for a private transition flight. The aircraft was carrying a pilot and an instructor. After retracting the landing gear, the crew flew in the Willisau-Kirchberg area to perform maneuvers with various bank angles. They then simulated approach procedures.
During the extension of the landing gear, the crew heard a dull thud that repeated when the gear was retracted. Suspecting a mechanical issue, they decided to return to their base. Near Langenthal, they prepared for the approach and attempted to lower the gear both electrically and manually. Only the nose gear locked correctly; the main gear failed to extend.
Over Grenchen, the crew tried additional methods, including positive and negative acceleration maneuvers, and followed advice from an aircraft mechanic via radio. Ultimately, they decided to perform a belly landing with the engine stopped but the propeller windmilling. The landing was executed smoothly. No one was injured, but the aircraft sustained damage to the lower fuselage, propeller, and engine.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined the technical condition of the aircraft. The investigation revealed that the connecting piece of the drive rod for the hydraulic actuator cylinder of the main landing gear had broken.
The broken component was sent to the Federal Materials Testing and Research Institute (EMPA) for analysis. The EMPA report indicated that the cadmium-plated housing, made of high-strength alloy steel, suffered from fatigue fracture. The edge zone of the fatigue crack exhibited a brittle intercrystalline character up to a depth of approximately 0.4 mm, suggesting prior damage due to hydrogen embrittlement.
The analysis concluded that the stress concentration at the oil nipple hole initiated the failure. The manufacturer had supplied a replacement part with a different design, specifically omitting the oil nipple, which addressed the material stress issue inherent in the original design.
Findings
- HB-CXU suffered a structural failure of the main landing gear drive mechanism due to a construction defect.
- The fracture was caused by fatigue cracking initiated at the oil nipple hole, exacerbated by hydrogen embrittlement in high-strength steel.
- The crew's decision to perform a belly landing was correct and executed safely, preventing injuries.
- Neither the manufacturer nor US authorities had issued airworthiness directives or manufacturer instructions regarding this specific design vulnerability at the time.
Safety action
The manufacturer modified the design of the connecting piece for repairs, removing the oil nipple to reduce stress concentrations. However, no formal airworthiness directive or manufacturer service bulletin was published by the manufacturer or US regulatory authorities to address this latent defect in other aircraft of the same type.