Aerospatiale AS350BA N357NT

The helicopter landed hard following a loss of directional control during a precautionary landing after a hydraulic system failure, with the main rotor blades cutting off the tail boom. This was a tour flight that was en route back to the departure airport when a complete hydraulic system failure occurred and the pilot decided to make a precautionary landing at a nearby airport. He had planned on conducting a run-on landing; however, during the approach the airspeed dropped to 30 knots. The helicopter's nose dropped and it started to spin to the left. The pilot pulled back on the cyclic enough to get the nose up; however, forward airspeed continued to decay and the rotation rate increased. The pilot said that the flight controls were not responding, and he believed the only way to stop or slow the rotation was to shutdown the engine. The helicopter completed two revolutions before it impacted the ground in a nose level attitude, 180 degrees from its original direction of travel. A witness to the accident reported the altitude of the helicopter as it began to spin was about 30-50 feet above the ground and he saw the main rotor blades cone upward as it fell to the ground. For a hydraulics failure, Federal Aviation Administration approved flight manual required the pilot to reduce the collective pitch and adjust the airspeed between 40 to 60 knots in level flight. The pilot was then instructed to cut off the hydraulic pressure by activating the collective lever pushbutton, and to make a flat approach over a clear landing area and land with forward speed. During examination of the hydraulic pump, investigators noted that the coupling sleeve splines were worn beyond serviceable limits. They also noted the general condition as rust colored, and no lubrication was found inside the pump housing or coupling sleeve splines. A company maintenance work order contained a serviceable tag for the hydraulic pump assembly. The serviceable tag that the operator's maintenance personnel wrote said that the hydraulic pump was "inspected, cleaned filter, new O rings installed on connections, inspected coupling, new pump assembled." The maintenance records also revealed that the assembled hydraulic pump was installed 15 days prior to the accident and had been in service for 74.6 hours when it failed. A metallurgical examination of the hydraulic pump revealed that it had failed due to the wearing away of the splines on the coupling sleeves. The wear was due to insufficient lubrication and soft splines on both coupling sleeves. The failure mode was assisted by an increase in internal rotational resistance at operating temperatures. The hardness testing indicated that the case hardened layer on the splines was not deep enough and the hardness, within the layer, was slightly below the specified range. The inner surfaces of the gear bearings displayed rubbed areas, consistent with hard contact, and areas of bluing, consistent with elevated temperatures. Examination of the coupling sleeves revealed a total loss of inwardly protruding spline material and hardness below the design requirements. The manufacturer's maintenance manual requires that the pump drive shaft, coupling sleeves, and bearing be packed with "abundant" grease during assembly. The examination found only a trace amount of grease in the coupling sleeves that did not satisfy the "abundant" requirement specified in the manufacturer's maintenance manual. There was rust on the front retaining rings and bearings, which indicated that there was insufficient grease in the splines to retain it or lubricate the splines.

Quoted verbatim from the NTSB record.