Incident Overview
On June 26, 2014, at 1502 central daylight time, a Eurocopter AS350B3 helicopter, registration N808LF, was departing from Draughon-Miller Central Texas Regional Airport (TPL) in Temple, Texas. The helicopter was operated by Air Methods under 14 Code of Federal Regulations Part 91 as a positioning flight, with three occupants on board—a pilot and two flight crew members. Visual meteorological conditions prevailed, and the flight operated on a company flight plan, destined for Hamilton, Texas.
After completing an uneventful air medical flight, the pilot landed at TPL at 1437 for refueling. Preflight checks were normal. Following a courtesy radio call to Temple UNICOM, the pilot initiated the takeoff. Immediately after lifting off, the helicopter began a counter-clockwise yaw. The pilot reported that the anti-torque pedals were locked in the neutral position and felt jammed. Despite attempts to correct the rotation, the pilot was unable. After the helicopter rotated several times, the pilot reduced altitude from approximately 15 feet to 4 to 5 feet and executed a precautionary hovering autorotation. The helicopter contacted the ground on its right front skid and slid upright to a stop. The pilot then shut down the helicopter, and all occupants exited uninjured.
Investigation
A post-incident examination of the helicopter and its hydraulic system was conducted on July 15, 2014, in Grand Prairie, Texas. The helicopter was equipped with a dual-hydraulic system consisting of two independent circuits: an upper and a lower circuit. Each circuit had separate reservoirs, pumps, and filters. The upper circuit powered independent servos on the three main rotor servos (fore-aft, right roll, and left roll). The lower circuit powered the tail rotor servo and the yaw load compensator, which included an accumulator with a rubber bladder charged with nitrogen gas to approximately 15 bars.
Investigators examined and documented the condition of the lower hydraulic system, tail rotor flight controls, and cockpit switches and fuses. Visual inspection revealed no damage or anomalies. Fluid levels in both reservoirs were sufficient for functional testing. The nitrogen charge in the yaw load compensator accumulator measured about 15 bars, consistent with no hydraulic pressure in that component.
Control continuity from the pedals to the tail rotor was confirmed. The pedals were too stiff to move by hand but could be operated with foot pressure. The yaw servo isolate switch on the collective stick was observed in the "off" position (pointed aft, away from the switch guard). The cockpit electrical fuses related to the hydraulic system were in the "in" positions.
Hydraulic System Functional Tests
A functional test of the lower hydraulic system was performed. The yaw servo isolation valve, mounted on the main transmission deck, is activated when the yaw servo isolate switch is in the "off" position, which electrically powers a solenoid that closes the valve. The valve is open when no electrical power is applied (switch in the "on" position). During the test, the switch remained in the "off" position with electrical power applied. Movement of the valve was confirmed by physical touch, and magnetization of the solenoid was confirmed with a steel tool.
A hydraulic mule providing about 30 bars of hydraulic pressure was connected to the lower hydraulic circuit. The "HYD", "SERVO", and "LIMIT" lights on the caution-warning panel illuminated as expected. The cyclic and collective sticks were manipulated by hand with forces consistent with hydraulic assistance. The pedals were manipulated by foot; with the switch off, increased resistance was felt (forces consistent with no hydraulic assistance). When the yaw servo isolate switch was placed in the "on" position, the pedals moved with forces consistent with normal hydraulic assistance.
The helicopter's start-up hydraulic checks from the flight manual were performed, including a test of the yaw servo isolation valve and a check of the yaw load compensator accumulator by depressing the "ACCU TEST" button. No anomalous results were noted. After the mule was powered off, the right pedal moved forward due to residual pressure from the compensator, as expected during shutdown.
To simulate the pilot's shutdown procedure, the switch was placed in the "off" position and the mule powered off; the right pedal again moved forward. Another test simulated the start-up checks but omitted returning the switch to the "on" position: after charging the lower circuit, the switch was placed in the "off" position, the accumulator discharged and then closed, and the mule powered off while the switch remained off. The pedals remained stationary with forces consistent with no hydraulic assistance.
Post-Test Examination
After functional tests, the hydraulic systems were visually examined for leaks; none were found. When a gauge was attached to the yaw load compensator accumulator, some nitrogen pressure was lost, measuring about 7 bars (normal is 15 bars). The accumulator was re-pressurized to 15 bars, which it held, indicating no significant leakage. The mule was powered on, and the gauge read about 30 bars, consistent with the applied hydraulic pressure.
The hydraulic filter clogging indicator remained in the "in" position, indicating no clog. The filter element was removed and examined; no debris was found on the element, but very fine debris was observed in the bottom of the filter bowl.
The hydraulic pump drive belt from the lower circuit was removed to verify spline coupling engagement, which was confirmed. The tail rotor pitch change links were disconnected, and full pedal movement with corresponding pitch change was verified.
Safety Notice
According to the helicopter manufacturer, if the pilot fails to restore the yaw servo hydraulic switch to the "on" position before takeoff during the hydraulic system check, there will be a lack of hydraulic boost to the tail rotor system. On August 21, 2014, Airbus Helicopters issued Safety Information Notice 2776-S-29, reminding pilots of procedural differences between dual and single hydraulic systems and warning of the consequences of not restoring the switch. The notice also described improvements to the dual hydraulic circuit, including dedicated amber HYDR caution lights for each circuit and a flashing mode on HYDR2 when the yaw servo hydraulic switch is off. These changes were incorporated on new production helicopters and planned for retrofit via a Service Bulletin. Additionally, a future modification was announced to replace the two-position ACCU TEST push button with a stable/momentary design to mitigate the possibility of leaving it in the "on" position.