What happened
On May 1, 2004, at 17:50, a Bell 47 G 2 helicopter, registration F-GJBK, was performing an instructional flight at Meaux aerodrome. As the flight was concluding, the instructor decided to demonstrate an autorotation maneuver. Following local aerodrome procedures for the use of runway 07, the engine power was reduced as the aircraft approached the threshold on a heading of 180 degrees.
Both the instructor and the student pilot reported that the autorotation proceeded normally, resulting in a soft touchdown followed by a slight skid. Upon completing the deceleration, the instructor moved the collective pitch lever downward and transitioned the cyclic stick from a forward position back toward the neutral position. At that moment, a brief metallic noise was heard by the crew.
As the crew attempted to transition into a hover to return to the parking area, the instructor noted that the rudder pedals had become ineffective. Following a signal from another pilot on the ground to shut down the engine, an inspection was conducted.
The investigation
The investigation focused on the mechanical failure that occurred during the transition from autorotation to ground operations. Examination of the Bell 47 G 2 revealed that the main rotor blades had physically severed the tail rotor drive shaft. Additionally, three tubes within the tail boom structure sustained damage.
Findings
- The primary cause of the incident was an inappropriate action on the cyclic stick.
- During the low rotor RPM phase of the autorotation landing, moving the cyclic stick too far aft can cause the main rotor blades to make contact with the tail rotor drive shaft.
- The structural integrity of the tail boom was compromised by the impact of the blades.