What happened
On 10 January 2022, a PS-28 Cruiser, registration EC-NKP, operated by Flyschool Air Academy, was performing a dual-command instruction flight at Casarrubios del Monte Aerodrome in Toledo, Spain. During the takeoff run on runway 26, a few seconds after applying full power and reaching approximately 15-20 knots, the crew noticed the right wing dropping and observed the right main wheel overtaking the aircraft.
The pilot immediately interrupted the takeoff by reducing power. The aircraft traveled approximately 70 meters along the runway before coming to a halt, supported by the remaining landing gear and the brake disc of the right main gear. The two occupants, an instructor and a student, were uninjured.
The investigation
The CIAIAC investigation focused on the mechanical failure of the right main landing gear wheel assembly. Laboratory analysis of the fractured bolts revealed a specific sequence of failure: bolt 3 fractured first due to flex fatigue, followed by bolt 2, which suffered from unilateral flex fatigue and significant ovalization of its housing. Finally, bolt 1 failed due to pure tensile stress once the other two bolts had ruptured.
Investigators examined the maintenance history of the aircraft and found that the right tire had been replaced three times since the aircraft's manufacture in 2020. The investigation also reviewed the manufacturer's Maintenance Manual (procedure 8.5.1 Tire Change) and found several discrepancies, including the omission of specific instructions for the assembly of the hub and rims, and the lack of a requirement to discard self-locking nuts.
Findings
- The primary cause of the accident was the deterior of the threaded joint between the wheel hub and the inner and outer rims, caused by a loss of tightening torque.
- This deterioration was driven by the reuse of self-locking nuts that had lost their locking capability, leading to relative movement between the hub and the rims.
- The fatigue in the bolts was exacerbated by the fact that the Maintenance Manual did not explicitly prohibit the reuse of these nuts, nor did it specify the necessary torque for this specific assembly.
- The investigation confirmed that the failure of the bolts was a result of the cumulative effects of vibration and improper maintenance procedures following multiple tire changes.