NTSB Identification: LAX06FA080
Nonscheduled 14 CFR Part 135: Air Taxi & Commuter
Accident occurred Thursday, January 05, 2006 in Lahaina, HI
Probable Cause Approval Date: 09/30/2009
Aircraft: EUROCOPTER EC 130B4, registration: N11QD
Injuries: 6 Uninjured.
NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this aircraft accident report.
The pilot reported that he heard the main rotor low speed warning horn, and saw that the rotor speed was decreasing. He entered into an autorotation to make a forced landing and tried to regain engine torque and rotor speed, but was unsuccessful and the low rotor horn sounded again. He lowered the collective and flared for landing. The helicopter came down in trees, with the main rotor blades contacting the treetops.
The helicopter was equipped with an on board video recording system that also recorded audio channels. Sound spectrum analysis of the audio recording confirmed the main rotor system speed decay consistent with the pilot's report. The power plant mechanical components: Hydro-Mechanical Unit (HMU), Digital Engine Control Unit (DECU), Engine Back-up Control Ancillary Unit (EBCAU), and Anticipator System Collective Potentiometer (XPC) were tested individually and no abnormalities were found within any component. Visual inspection of the DECU-XPC wiring harness showed very little external damage. Further examination and testing of the wiring harness revealed a loss of internal shielding continuity and shorts between a wire and the shielding.
No evidence of lightning strike damage was found in any of the wiring. Faults in the manufacturing assembly of one of the XPC harness backshells were also discovered. Further examination and testing revealed the insulation breakdown was a result of wire damage due to a tight bend in the harness, located about 80 and 90 cm from the rear of the connector (potentiometer side).
It was discovered that the AS350B3 and EC130B4 XPC cables (a part of the aircraft wire harness) had been manufactured with extra length to accommodate multiple installation configurations. The length of the accident helicopter's harness was longer than necessary, and it had been bent in a tight radius during aircraft production to accommodate the extra length in during installation. The wiring faults resulted in a gradual, intermittent decay of the XPC signal to the DECU. A review of the DECU design with the manufacturer confirmed that the DECU logic would not detect the XPC signal shift error, such as occurred in this event, which would cause an uncommanded and undetected loss of rotor speed. The aircraft flight manual only contains emergency procedures for a decrease in main rotor speed below 360 rpm when it corresponds with an engine failure. The aircraft flight manual does not address an intermittent XPC signal failure.
As a result of the investigation, on March 13, 2009, Eurocopter issued Alert Service Bulletin No. 76.00.18, which called for a modification to eliminate the potential for NR drop in flight - Slow drift in the XPC anticipator (collective pitch signal) on AS350B3 and EC130B4 helicopters.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: An intermittent electrical continuity failure and short of the DECU-XPC wiring harness, which resulted in an uncommanded and an initially undetected engine deceleration and a resulting loss of main rotor rpm. Also causal was the manufacturer's inadequate installation of the wiring harness at the time of manufacture. Full narrative available
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