What happened
On 19 May 2006, an MBB BO 105 CBS-4 helicopter, registration PH-RPX, was conducting a scheduled test flight following a 50-hour inspection. The flight, which included a pilot and an aircraft engineer, departed from Amsterdam Schiphol Airport at 20:35 hours.
Approximately six minutes into the flight, while at an altitude of 800 feet, the pilot began preparations for an engine power check. As the torque on both engines was reduced to 40%, the crew heard a loud bang. This was immediately followed by a total loss of tail rotor control. The crew responded by selecting idle power and successfully executed an autorotation landing in a field. There were no injuries to the occupants, though the aircraft sustained serious damage.
The investigation
Investigators from the National Aerospace Laboratory (NLR) and Eurocopter examined the wreckage to determine why the tail rotor unit and gearbox had separated from the vertical fin. The investigation focused on the maintenance performed prior to the flight, specifically the installation of the vertical fin cowling. This composite cowling is secured with twenty-two quick-access fasteners that require a screwdriver to lock.
While the engineer stated all fasteners were secured, the investigation revealed that the first seven fasteners on the right-hand side of the cowling were either unsecured or unlocked. The investigation also noted that the upper fasteners are difficult to reach without a platform and that the dark blue color of both the cowling and the fasteners made it difficult to visually distinguish between open and closed positions under hangar lighting.
Findings
Technical analysis established that the separation was triggered by an excessive unbalance in the tail rotor. The sequence of events likely began when the unsecured right-hand fasteners allowed the cowling to lift due to aerodynamic pressure and vibrations during cruise. During the power reduction phase, changes in airflow caused the cowling to fail along its trailing edge.
This failure led to the rotating tail rotor blades making contact with the cowling. This contact caused the tip of a rotor blade to deform and a portion of the blade tip to break off. The loss of mass from the blade created a severe imbalance, generating massive forces that ultimately ripped the entire tail rotor assembly away from the aircraft's structure.
Safety action
To improve the detectability of unsecured fasteners, the area underneath the fastener lips on the cowling was painted orange as a precautionary measure following the accident.