NYC04FA117 · Eurocopter AS-350BA

The helicopter was hovering at 1,000 to 1,200 feet, during an electronic news gathering flight, when it experienced a loss of hydraulic system power. The pilot stated he did not have any specific recollection regarding the beginning of the accident sequence. He said that he felt the helicopter was not responding to his control inputs, "the way it should be." He also did not recall reporting any specific problems over the radio; however, review of communication recordings revealed that the pilot stated he experienced a "tail rotor failure." Another helicopter captured the accident sequence from its onboard video camera. The accident helicopter was observed entering a steep descent and quickly leveling off. A fairly constant, level attitude and heading were briefly maintained before the helicopter came to an unstable hover and began an approximate two revolution left rotation about its vertical axis. During the rotation, the helicopter descended and struck the roof parapet of a 4-story building, before it impacted and came to rest on the roof of an adjacent 2-story building. Examination of the helicopter's hydraulic system revealed that the hydraulic pump drive belt had been installed inside-out and failed due to an overload, leading to a full hydraulic system failure. According to Eurocopter's emergency procedures for the helicopter, the pilot action following a hydraulic system failure would 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 located on the collective pitch lever to off. The procedures further state that when the accumulators are exhausted, the 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 than the forces required with the hydraulic system operating properly. The hydraulic system toggle switch was found in the "on" position. The "flat" hydraulic pump drive belt had accumulated about 440 hours, and had a service life limit of 600 hours. On May 11, 2004, Eurocopter issued a service bulletin which recommended the installation of a"Poly-V" hydraulic pump belt. The improved belt comprised multiple longitudinal "V" shapes on its inner surface that mated with similar 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 the "Poly-V" belts.

Verbatim NTSB analysis and probable cause from the NTSB dataset