What happened
On February 11, 2018, an experimental, amateur-built Cirrus VK-30, registration N52TH, crashed near Agua Dulce, California. The flight originated from Henderson Executive Airport in Las Vegas, Nevada, with the intention of landing at Van Nuys Airport.
At approximately 1059 PST, the aircraft impacted terrain between rolling hills and power transmission lines. The pilot and three passengers sustained 4 fatal injuries.
A witness observing the aircraft from a nearby park reported seeing the plane at an altitude of 2,000 to 3,000 feet. The witness noted that the right wing appeared to lift upward as the aircraft approached the mountains, followed by an increase in engine noise and a sudden vertical descent into the ground.
The investigation
The investigation focused on the aircraft's unique engine configuration, which utilized a "pusher" setup. In this system, the engine is mounted mid-fuselage, and power is transferred to a rear-mounted propeller via a torque tube (driveshaft) and elastomeric couplings.
Investigators discovered that the driveshaft had fractured approximately 2 feet forward of the propeller. Metallurgical analysis determined the fracture was caused by overstress loads in torsion and rotational bending. Examination of the hardware revealed that one bolt on the forward coupling and one bolt on the aft coupling had backed off.
Maintenance records showed that the flywheel and coupling bolts had been retorqued approximately five flight hours before the accident, though the records did not specify the torque values used. The investigation concluded that the bolts were likely under-torqued, leading to vibrations that caused the nuts to back off, resulting in the driveshaft separation.
Following the failure, the engine likely overspeeded. The broken end of the driveshaft likely struck the horizontal stabilizer fairing, causing it to separate in flight and inducing a loss of control. Additionally, evidence suggested a propeller blade struck the left elevator, causing it to separate from the empennage.
Findings
- The driveshaft fractured due to overstress loading and rotational bending.
- The failure was initiated by under-torqued nuts on the elastomeric couplings backing off during flight.
- The separation of the horizontal fairing due to contact with the fractured driveshaft led to an in-flight upset and loss of control.
- Inadequate maintenance contributed to the accident.