What happened
On August 21, 2022, a Cessna 172N, registration EC-ETC, operated by Real Aeroclub de Baleares, was conducting a flight training session at Son Bonet Aerodrome in Mallorca, Spain. The flight involved an instructor and a student pilot practicing emergency procedures, specifically a simulated engine failure following takeoff.
During the maneuver, the instructor reduced power to idle at 100 feet to maintain a target speed of 65 knots. Due to a high rate of descent, the aircraft experienced a hard landing. Upon touchdown, a loud cracking sound was heard, and the aircraft began to tilt to the right. The instructor noted that the right main landing gear wheel had detached from the aircraft.
Despite the loss of the wheel, the instructor applied power to keep the aircraft level and continued along the runway. However, as the aircraft slowed to 40 knots, the right side settled onto the spring assembly of the gear. The loss of the wheel had also caused a loss of hydraulic fluid, rendering the brakes inoperable. The aircraft eventually exited the runway at taxiway S3 and came to a stop on the grass. Both occupants were uninjured.
The investigation
Investigators examined the fractured right main landing gear leg and performed metallurgical analysis in a laboratory. The study focused on the material composition and the nature of the fracture. /nTechnical analysis of the fracture surface revealed that the failure began with a brittle mechanism, followed by a mixed ductile-brittle progression. The investigation also included a review of the aircraft's maintenance records, which showed no documented repairs to the right main gear leg. Non-destructive testing (NDT) was performed on the left main gear leg, which showed no signs of cracks or corrosion.
Findings
- The investigation established that the material fragility in a critical zone of the right main landing gear leg was the primary cause of the accident.
- This fragility was the result of a microstructural transformation caused by a significant thermal input of unknown origin, which raised the material temperature above 800°C.
- This intense, localized heating caused the steel to undergo a martensitic transformation, making the component excessively brittle and prone to intercrystalline cracking.
- A high rate of descent during the simulated engine failure maneuver contributed to the hard landing that triggered the failure.
- The exact source of the heat—whether from a previous undocumented repair, a manufacturing defect, or proximity to brakes during a fire—could not be determined.