What happened
On November 4, 2002, a Hughes Helicopters 369 D helicopter, registration EC-DSJ, was conducting instructional flight maneuvers at the La Cañada ultralight airfield in Madrid. The flight, which involved an instructor, a student, and a passenger, was intended for practicing autorotation techniques as part of a type rating course.
During one of the autorotation maneuvers, the aircraft made contact with the ground with more force than intended. The instructor, remaining inside the cockpit, performed a visual inspection of the tail section. Finding no apparent damage, the crew proceeded with the next maneuver. However, as the pilot applied power to initiate a climb, the rear third of the tail cone suffered a structural failure. The tail rotor assembly and associated components became partially detached, held only by electrical wiring. There were no fatalities or injuries to the crew or passengers, and no damage was caused to persons or property on the ground.
The investigation
The investigation focused on the structural integrity of the tail boom and the sequence of events leading to the impact. Investigators examined the tail cone, which consisted of two semicircular panels joined by rivets. They found that the upper rivet rows had been sheared, and the paint had been stripped from the area.
Physical evidence showed that the main rotor blades had struck the upper portion of the tail cone. Traces of aircraft paint were found on the underside of the blade tips, and progressive impact marks were visible on the blades, corresponding to the sequence of contact. The investigation also revealed that the tail rotor drive shaft had failed due to torsion, and the secondary shaft had failed due to bending. Metal shavings were found inside the tail boom, indicating that the drive shaft had been rubbing against its supports.
Findings
- The primary cause of the accident was an excessive pull on the collective pitch control while the helicopter was at a very low altitude.
- This action caused the main rotor RPM to drop below the critical minimum required for safe flight.
- The resulting loss of lift caused the main rotor blades to descend and strike the upper part of the tail cone.
- The impact damaged the riveted joint of the tail boom structure, leading to a loss of structural rigidity.
- The weakened structure could not withstand the forces exerted by the tail rotor, resulting in the total fracture of the tail cone and the subsequent failure of the drive shaft.