Incident
The helicopter was operating at a hover between 1,000 and 1,200 feet during an electronic news gathering flight when it lost hydraulic system power. The pilot later stated he had no specific recollection of the beginning of the accident sequence, but felt the helicopter was not responding to his control inputs as expected. He did not recall reporting any specific problems over the radio; however, review of communication recordings revealed that he had stated experiencing a "tail rotor failure."
Another helicopter captured the accident sequence on video. The footage showed the accident helicopter entering a steep descent and quickly leveling off. After briefly maintaining a fairly constant level attitude and heading, the helicopter entered an unstable hover and began an approximately two-revolution left rotation about its vertical axis. During the rotation, the helicopter descended and struck the roof parapet of a four-story building before impacting and coming to rest on the roof of an adjacent two-story building.
Investigation
Examination of the helicopter's hydraulic system revealed that the hydraulic pump drive belt had been installed inside-out and had failed due to overload, leading to a complete hydraulic system failure. The belt had accumulated about 440 hours of service, with a service life limit of 600 hours. The hydraulic system toggle switch, which controls hydraulic pressure, was found in the "on" position.
On May 11, 2004, Eurocopter issued a service bulletin recommending the installation of a "Poly-V" hydraulic pump belt. This improved belt featured multiple longitudinal V-shaped ridges on its inner surface that mated with corresponding surfaces on the hydraulic pump pulley and had a service life limit of 1,500 hours. Eurocopter recommended that all flat drive belts be replaced with Poly-V belts.
Emergency Procedures
According to Eurocopter's emergency procedures for the helicopter, the pilot action following a hydraulic system failure should be to "calmly reduce collective pitch and adjust the airspeed to between 40 and 60 knots in level flight" and then cut off the hydraulic pressure by moving the toggle switch on the collective pitch lever to off. The procedures state that when the accumulators are exhausted, control forces become significantly higher but "not unmanageable." The helicopter could be controlled without the main and tail rotor servo actuators being hydraulically powered, but this would require the pilot to apply non-negligible forces that are substantially different from those required with the hydraulic system operating properly.