BECKWOURTH, CA, USA
N49511
AEROSPATIALE SA-315
THE PILOT WAS CONDUCTING AN EXTERNAL HELICOPTER EXTERNAL LOAD LOGGING OPERATION. AFTER RAISING TWO LOGS, THE PILOT HEARD A LOUD 'THUNK' NOISE FOLLOWED BY AN UNCONTROLLED TURN TO THE RIGHT. THE PILOT WAS UNABLE TO MAINTAIN ADEQUATE ROTOR RPM BECAUSE HE COULD NOT LOWER THE COLLECTIVE. EXAMINATION OF THE COLLECTIVE SERVO REVEALED IT TO HAVE INTERNAL LEAKAGE WHICH CAUSED A HYDRAULIC LOCK IN THE SERVO. THE SERVO EXTENSION AND RETRACTION TIME EXCEEDED THE MANUFACTURER'S SPECIFICATIONS.
On October 18, 1993, at 1430 hours Pacific daylight time, an Aerospatiale SA-315, N49511, collided with the terrain while conducting a visual flight rules logging operation at Beckwourth, California. The helicopter, operated by Pacific Western Helicopters, sustained substantial damage. The certificated commercial pilot, the sole occupant, sustained serious injuries. Visual meteorological conditions prevailed. The flight originated at Nervino airport, Beckwourth, California, at 1345 hours. The operator reported in a telephone interview conducted on October 19, 1993, that the preliminary examination of the wreckage showed signatures that the clutch slipped and the helicopter lost power to the rotor blades. He also reported that the engine was still operating at impact. The pilot reported in the National Transportation Safety Board Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1/2, that he was conducting an external logging operation. He dropped some logging equipment during this cycle of operation and then flew to the accident site area. The ground person hooked up two logs to the external line and then cleared the pilot to begin the lift operation. The pilot raised the logs with the helicopter's nose turned to the right between 20 and 30 degrees. The helicopter and engine were operating normally as he raised the logs, and then he heard a "thunk" noise and the engine sound changed. The pilot could not recall if the sound change was a high pitch sound or lower sound. The helicopter's nose pitched up and momentarily yawed (turned about its vertical axis) to the left. The pilot then felt the helicopter was going to the right. The pilot could not recall when he released the remote hook and that he could not push the collective down. The helicopter settled into the trees. After the helicopter came to rest on the ground, he heard the engine noise. Despite his injuries, the pilot turned off the fuel boost pump and start switch. The on-scene investigation was conducted by Mr. Donald F. Morgan, Aviation Safety Inspector, Federal Aviation Administration (FAA), Reno [Nevada] Flight Standards District Office. Inspector Morgan reported that two of the three main rotor blades struck the trees and were destroyed; the remaining blade was not damaged. Inspector Morgan said that his inspection of the tail rotor drive shaft revealed no rotational damage at the tail boom impact separation area. The tail rotor blades and hub assembly were not damaged. At the request of the Safety Board, Inspector Morgan sent the engine, clutch, mast assembly, rotor hub, and main transmission to Heli-Support, Ft. Collins, Colorado, for disassembly and inspection. He also sent the collective servo to Hawker Pacific, a maintenance facility, Sun Valley, California. The referenced components' inspection was conducted under the supervision of the local FAA, Flight Standards District Office personnel. Heli-Support maintenance personnel examined the rotor system components on December 15 and 16, 1993, and the engine on January 12, 1994. Heli-Support maintenance personnel reported no evidence of any preexisting malfunctions or failures on the engine, clutch, mast assembly, rotor hub, and main transmission. Mr. Keith Youngblood, Hawker Pacific, conducted the collective servo examination on November 29, 1993. The component had accrued 1,250.5 hours since overhaul. Mr. Youngblood reported: ...Test shows servo slow to extend and retract. Extend time was 4.1 inches/second and retract time was 4.0 inches/second, [the] minimum [time] is 4.3 inches/second. All other tests, except internal leakage are O.K. Internal leakage was 150 cc/minute in the neutral pos[ition] (suspected cause for slow ext & ret times). [The] teardown insp[ection] shows [the] body, PN [Part number] 74777 [is] oversized. The body I/D [inner diameter] was 1.28295 [inches] and [the] piston, PN 75321 O/D [outer diameter] was 1.27895 [inches]. The clearance is 0.004 [inches, the] max[imum] is 0.0013 inches. A side load condition was also noted on I/D of [the] body resulting in internal leakage between piston and body I/D. All seals require replacement and body and piston require rework. Safety Board investigators sent the fuel control unit, serial number (s/n) 2439 (AW) and fuel pump, s/n F270B, to TurboMeca Engine Corporation, Cedex, France, for examination. Mr. Cretin, DGAC (France's counterpart of the Safety Board), supervised the examination. Messrs. S. Garcia and A. Cirendini conducted the examination of the components. They concluded: Fuel Control Unit: Good general operation fuel pump/FCU [fuel control unit] assembly; the small flow diminution reported [315 litres/hour (l/h)] is due to the diameter of the by-pass jet (01.1), installed by the operator; with the original by-pass jet (0.95) the flow is within the limits. However, note that, even with the 01.l by-pass jet, the fuel pump flow of 315 litres noted on the test bed is widely higher to the max flow necessary at 440 kw max power which is, in corrected value at 15cH an 1013 number, 246 l/h - cH = 250 l/h. Fuel Pump: Very good operation of the fuel pump; all the parameters are within the tolerances of TURBOMECA work specification, slight decrease of the flow at max power due to exchange of the by-pass by the operator.
was the worn collective servo causing a hydraulic lock and not allowing the pilot to lower the collective. This condition caused the main rotor blades rpm to decay.
Source: NTSB Aviation Accident Database
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