WASHINGTON, PA, USA
N101UC
Aerospatiale AS-355F
A mechanic changed one of the helicopter's lateral cyclic servos. The helicopter was test flown by the pilot with the mechanic onboard. After a successful test flight, the mechanic exited the helicopter and the pilot was to reposition the helicopter to the hospital helipad. When the pilot raised the collective, the helicopter lifted about 2 to 4 feet above the ground, and then began to roll left. The pilot attempted to correct for the left roll; however, the helicopter did not respond and the main rotor blades struck the ground. Examination of the helicopter revealed control continuity, and that the hydraulic servos were installed correctly. No preimpact failure of the airframe was observed. Examination of the servos revealed that two of the units tested to manufacturer's parameters. The master servo unit failed the bench test, had the appearance that it had been worked on, and did not meet manufacturer's specifications. The helicopter manufacturer stated that this would not induce a 'hard over.'
On March 5, 1997, at 1215 eastern standard time, an Aerospatiale AS-355F, N101UC, was substantially damaged when it rolled over after lifting to a hover at the Washington County Airport (AFJ), Washington, Pennsylvania. The certificated commercial pilot received minor injuries. Visual meteorological conditions prevailed for the positioning flight that was to depart AFJ, for the Washington County hospital helipad. No flight plan had been filed for the flight conducted under 14 CFR Part 91. In the pilot's written statement, he said that the mechanic had just changed the cyclic lateral servo, and the helicopter required a maintenance test flight. The pilot conducted a hover check with the mechanic, then flew the helicopter "to perform forward flight checks." The pilot remained in the airport's traffic pattern, and landed back on the taxiway. He stated, "Aircraft performed as advertised." The mechanic exited the helicopter and checked it for leaks, while the pilot kept the engines running. The mechanic departed the helicopter to allow the pilot to takeoff and reposition the helicopter to the hospital helipad. The pilot stated that when he raised the collective, the helicopter lifted from the ground. The pilot further stated: "At 3 - 4 feet hover above the ground, I began to again match the torques prior to transition to forward flight. Aircraft began to roll to left. I attempted to correct for what I thought was a crosswind. The aircraft did not respond normally. Left roll continued and aircraft impacted ground on its left side..." According to a helicopter mechanic who witnessed the takeoff, he stated: "...I heard the STAT Helicopter begin to apply pitch in the rotor system so I decided to watch it takeoff. I saw the rotor disc start to cone upward and the skids left the ground. As the skids left the ground (approx. 1 1/2 to 2 feet), the aircraft rolled sharply to the left. As the aircraft rolled sharply left, it also drifted left at which time the rotor blades struck the PSP matting [ground]..." According to a Federal Aviation Administration (FAA) Inspector, examination of the helicopter revealed that the hydraulic servos were installed correctly, control continuity was confirmed, and no preimpact failure of the airframe was observed. The hydraulic servos were removed for further examination. The three servos were examined at the Hawker Pacific Facility, Sun Valley, California, under the supervision of a FAA Inspector of the Van Nuys Flight Standards District Office. The FAA Inspector's report stated: "...At your request, these were bench-tested to manufacturer's specifications. Units 1 & 2 met manufacturer's parameters...The master servo, Unit 3, failed the bench test...It was found that the cams at the input lever were out of adjustment; with the rig pin in place, the input was not "O" or nulled. With the pin in, system 1 was not in sync with system 2..." the report also stated: "...NOTE: Unit 3, S/N 450, appearances were that someone had worked on this unit and that it did not meet manufacturer's specifications. The covers were not painted; there was no torque seal on any of the safety wires; and the manifold bolts were not lockwired as required by the manual..." A letter from American Eurocopter Corp., stated that: "Even with extreme de-synchronization, 60% authority will remain with the servo. This type of error can not induce a hard-over..."
The malfunction of a cyclic servo which resulted in a cyclic hard over at a hover.
Source: NTSB Aviation Accident Database
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