LAX06FA080

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.

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.

HISTORY OF FLIGHT On January 5, 2006, about 0945 Hawaiian standard time, a Eurocopter EC130B4 (ECO-star), N11QD, experienced an engine deceleration and loss of main rotor rpm, and made a hard forced landing in Honokohau Valley, near Lahaina, Hawaii. Blue Hawaiian was operating the helicopter with the call sign “Blue 21” under the provisions of 14 Code of Federal Regulations (CFR) Part 135 as a nonscheduled, on-demand tour flight. The helicopter sustained substantial damage. The pilot and five passengers were not injured. Visual meteorological conditions prevailed, and a company visual flight rules (VFR) flight plan had been filed. The local area flight departed Kahului Airport (OGG), Kahului, Hawaii, about 0930. According to the operator, the flight was on a "complete" island tour, which is 60 minutes in length. When Blue 21 did not return to Kahului Airport at its designated time, Blue Hawaiian personnel attempted to locate the flight. At 1117, they were notified that the subject helicopter had been located below the Jurassic Falls. In the pilot's written statement to the National Transportation Safety Board, he reported that as he entered Honokohau Valley, he slowed to 30 knots heading upstream to show the passengers the falls. He heard the main rotor warning, checked the rotor tachometer, and saw that it was decreasing. The pilot entered into an autorotation to make a forced landing. He also noted that the rotor rpm (revolutions per minute) was in the "green." However, because there were no available landing sites in his current direction of travel (upstream), he did a 180-degree right turn. He tried to reapply power, but the low rotor horn sounded again. He lowered the collective and "removed twist grip from the flight gate." The pilot reported that he flared for landing. The helicopter came down in trees, with the main rotor blades contacting the treetops. PERSONNEL INFORMATION The operator reported that the 56-year-old pilot held an airline transport pilot certificate with ratings for helicopter and airplane single-engine land, multi-engine land, and instrument airplane. A second-class medical certificate was issued on August 31, 2005, with a limitation that the pilot must possess corrective glasses for near vision. The operator reported that the pilot had accumulated a total flight time in all aircraft of 13,750 hours. Approximately 200 hours were logged in the last 90 days, and 75 hours in the last 30 days. An estimated 1,703 hours had been accumulated in the make and model helicopter involved in the accident, with a total rotorcraft time of 11,670 hours. A biennial flight review was completed on December 16, 2005. AIRCRAFT INFORMATION The helicopter was a Eurocopter EC130B4, serial number 3363. The operator reported that the helicopter had a total airframe time of 4,836 hours at the last 100-hour inspection. The engine was a Turbomeca Arriel 2B1, serial number 23017. Total time recorded on the engine at the last 100-hour inspection was 3,700 hours, and time since major overhaul was 700 hours. Fueling records at OGG established that the helicopter was last fueled on January 5, 2006, with the addition of 72 gallons of Jet-A fuel. The operator reported that there were no unresolved maintenance discrepancies against the helicopter prior to departure. It is noteworthy that this helicopter was reportedly the first EC130B4 delivered to a customer by Eurocopter. Also, the helicopter sustained a lightning strike in August 2004 while being operated by a lessee in Linden, New Jersey. A review of maintenance records from this event revealed that a functional check was performed on the electrical system. ENGINE SYSTEMS: ENGINE FUEL CONTROL: The fuel control, as it relates to the free turbine rpm N2 control, is as follows: when the load varies, the Digital Engine Control Unit (DECU) uses the input parameters (N2, P0, T1, N1, P3, Torque) and the anticipators (collective pitch and yaw) to compute a new setting for the metering unit via the stepper motor in order to bring N2 back to the set point value. The helicopter is also equipped with an Engine Back-up Control Ancillary Unit (EBCAU) board and associated back-up fuel metering unit electrical actuator. In case of a failure of both DECU channels, the EBCAU is designed to automatically govern the engine. The electronic board is installed on ASU No. 1 and No. 2 boards, and is electrically activated by the DECU. It carries out the simplified back-up governing function using the N2 value. ENGINE OPERATING CONTROLS: The engine is controlled via a selector on the instrument panel, a twist grip on the collective pitch lever, and an automatic back-up system: EBCAU (Engine Back-up Ancillary Control Unit). • Starting selector: - OFF: engine shut down. The guard is raised. - ON: the DECU runs the automatic starting sequence. This is the normal setting in flight. In the ON position, the rating of the engine depends on the position of the twist grip (IDLE or FLIGHT). • EBCAU test selector: -The "EBCAU TEST" control button is used to switch to the back-up mode to test the engine back-up control system on the ground. When the "EBCAU TEST" button is pressed, the position of the main control actuator controlled by the DECU is frozen. The fuel flow is then monitored by the EBCAU, which controls the back-up control valve. The amber light of the "EBCAU TEST" button comes on and the red "GOV" light illuminates on the warning and caution panel. • "Forced idle" Microswitch: In autorotation training, the pilot twists the grip to move it out of the "FLT" detent to the "IDLE" setting and to activate a microswitch. The DECU then adjusts the engine to the idle rpm. Moving the grip back to the "FLT" detent resets the DECU in "flight" mode. • Description of warning lights: - Red "GOV" light, indicating a major engine fuel control system failure with seizure of the metering unit or the EBCAU on ground test. - Amber "GOV" light, indicating a minor failure resulting in degraded engine fuel control. When flashing, the light indicates a failure not affecting the engine fuel control system, such as loss of redundancy. - Red "TWT GRIP" light, indicating the twist grip is no longer in the "FLT" detent. COLLECTIVE ANTICIPATOR SYSTEM: The controlling function adjusts the gas generator speed to balance the power supplied with the power needed and in order to maintain the constant N2 (or NR) speed. In addition, the collective anticipator system supplies an immediate load variation signal from the collective potentiometer (XPC) to the DECU allowing it to react more rapidly to demand changes. The DECU uses a voltage signal from the collective anticipator (or collective potentiometer (XPC)) to anticipate power requirements induced by pitch changes. The DECU supplies the XPC potentiometer with a 10V power supply. The XPC potentiometer then returns a modified voltage to the DECU which is proportional to the collective pitch position. If there is a collective anticipator failure, the DECU is designed to switch to the proportional/integral mode to maintain an NR speed equal to the N2 set point whatever the pitch requirement. This should result in the illumination of the amber GOV light on the caution warning panel and a VEMD Test Code 122. There are three conditions designed to trigger a collective-pitch anticipator failure (VEMD code 122) in the event of XPC signal drift: • XPC signal value in flight decrease below 5% (out of range) resulting NR decrease below 360 RPM • XPC signal value increase above 95% (out of range) • XPC gradient test (350°/s = 7% / 20ms) If XPC signal remains within range (5%; 95%) and gradient is lower than 350°/s, no failure will be detected. METEOROLOGICAL INFORMATION The closest official weather observation station was Kahului Airport (OGG), which was located 10 nautical m