Riverside, CA, USA
N965SD
EUROCOPTER AS 350 B2
The flight instructor had briefed the flying pilot that he wanted him to perform a maximum performance takeoff simulating a 20- to 25-foot obstacle and that he was going to simulate a hydraulic failure during the maneuver by activating the hydraulic test switch. Prior to performing the maneuver, the pilot confirmed with the instructor that once the hydraulic failure warning activated he would accelerate to 40 knots before activating the hydraulic isolation switch. The instructor would need to reset the hydraulic test switch prior to the pilot activating the hydraulic isolation switch. After the briefing, the pilot initiated the maximum performance takeoff, and as the helicopter attained 20-25 feet, the instructor activated the hydraulic test switch. The pilot stated that as soon as the test switch was activated, he could no longer input right pedal because the control forces were too great. The helicopter began to yaw to the left, and the pilot then activated the hydraulic isolation switch because he thought the helicopter would be more controllable. The pilot did not consider that the hydraulic fluid had been evacuated from both the tail rotor accumulator and the yaw load compensator actuator due to the activation of the hydraulic test switch by the instructor, and the test switch had not yet been reset. As a result, the hydraulic boost from the normally closed yaw load compensator system was unavailable. The helicopter continued to yaw to the left and developed into a spin about its vertical axis. The instructor told the pilot to “fly the helicopter” and was hoping the pilot would recover from the loss of control condition. The pilot attempted to recover by following the direction of the nose of the helicopter to try to gain airspeed. The instructor attempted to intercede by inputting right cyclic, but since the pilot was applying left cyclic, the cyclic inputs were ineffective. The helicopter spun around several times before impacting the taxiway. According to the training procedure for a loss of hydraulic pressure, the instructor should have reset the hydraulic test switch prior to the pilot activating the hydraulic isolation switch. This flight manual specific sequence of switch selections and corresponding actions when simulating a hydraulic system failure were not followed by the pilots. Postaccident examination of the helicopter systems, including the hydraulic system, revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation.
HISTORY OF FLIGHT On February 23, 2011, about 1615 Pacific standard time (PST), a Eurocopter AS 350 B2, N965SD, made a hard landing following a loss of control at March Air Reserve Base (RIV), Riverside, California. The Los Angeles County Sheriff’s Department (LASD) was operating the helicopter as a training flight under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The commercial pilot, who was a certified flight instructor (CFI), along with the private pilot undergoing instruction (Flying Pilot-FP), sustained minor injuries; the helicopter sustained substantial damage from impact forces. The local instructional flight departed Long Beach, California, about 1550. Visual meteorological conditions prevailed, and no flight plan had been filed. The certified flight instructor (CFI) had briefed the flying pilot (FP) that he was to perform a maximum performance takeoff simulating a 20- to 25-foot obstacle, and that the CFI was going to simulate a hydraulic failure during the maneuver by activating the hydraulic test switch. Prior to performing the maneuver the FP acknowledged to the CFI that once the hydraulic failure warning activated he would accelerate to 40 knots before the FP would activate the hydraulic isolation switch. According to the training procedure for a loss of hydraulic pressure, the CFI should have reset the hydraulic test switch prior to the FP activating the hydraulic isolation switch. During the takeoff, when they had reached an altitude of about 20-25 feet above ground level (agl), as the FP applied forward cyclic, the CFI activated the hydraulic test switch to simulate a hydraulic failure. The FP stated that as soon as the hydraulic test switch was activated, he was unable to apply right pedal because the control forces were “too great.” As the helicopter began to yaw to the left, the FP stated that he activated the hydraulic isolation switch. He thought that although the control forces would be greater, he believed that he would be able to overcome them. The FP did not consider that the hydraulic fluid had already been evacuated from the tail rotor's yaw load compensator system when the CFI activated the hydraulic test circuit. As a result, the hydraulic boost from the normally closed tail rotor compensator system was not available. The helicopter continued to yaw to the left and developed a spin about the helicopter’s vertical axis. The CFI attempted to allow the FP to regain control of the helicopter, but the FP was trying to regain control by “flying” toward the left rotation in an attempt to gain forward airspeed. The CFI then attempted to gain control of the helicopter with the use of right cyclic control, which was counter to the left cyclic the FP was applying. The CFI attempted unsuccessfully to communicate with the FP via the intercom. The helicopter spun several times before it impacted the runway surface and slid to a stop. The FP and CFI performed the emergency shutdown procedures and exited the helicopter. PERSONNEL INFORMATION CFI (Left Seat) A review of the Federal Aviation Administration (FAA) airman records revealed that the CFI held a commercial pilot certificate with ratings for rotorcraft helicopters, single engine land, and multi-engine land airplanes, and instrument airplane. He also held a flight instructor certificate for rotorcraft helicopters, single engine land and multi-engine land airplanes, and instrument airplane. The pilot's most recent first-class medical certificate was issued on April 19, 2010, with a limitation that he must have available glasses for near vision. According to the CFI, at the time of the accident he had accumulated a total of 6,728 hours of flight time. He had a total of 4,276 hours in the same make and model as the accident helicopter, as well as 359 hours of instruction in the accident helicopter make and model. The CFI’s last biannual flight review was completed on February 19, 2010, in a Beechcraft B200. FP (Right Seat) A review of FAA airman records revealed that the FP held a private pilot certificate with ratings for rotorcraft helicopters and single engine land airplanes. The pilot's most recent first-class medical certificate was issued on December 23, 2010, with no limitations. According to the FP, at the time of the accident he had accumulated a total flight time of 245 hours. He had logged a total of 73 hours in the same make and model as the accident helicopter. His last biannual flight review was completed on July 3, 2010, in a Robinson R-22. WRECKAGE AND IMPACT INFORMATION Investigators examined the wreckage at the accident scene. The debris field was about 75 feet in diameter with a ground scar from the left skid as the first identified point of contact (FIPC.) The orientation of the fuselage was 350 degrees. Damage to the helicopter included the following: the tailboom was buckled; the skid landing gear had collapsed; the main cabin was deformed and partially collapsed; both pilot seats had evidence of vertical stroking of the seats during the accident sequence; the Starflex remained intact with no obvious fractures; and all three main rotor blades sustained impact damage with the majority of the damage to the trailing edges. TESTS AND RESEARCH An examination was conducted on February 25, 2011, at the LASD Aero Bureau Facility located at Brackett Airport (POC), La Verne, California. Examination of the tail rotor drive train revealed that the driveshaft shear point had disconnected, and the tail rotor blades and yoke had been damaged during the impact sequence. There was no further damage to the tail rotor drive train. The hydraulic manifold, the right and left lateral servos, the longitudinal servo, and the tail rotor compensator servo were examined with no damage or abnormalities noted. Power was applied to the helicopter by reconnecting the battery. The servos, hydraulic manifold, and the compensator solenoids were tested by activating the system switches. All solenoids activated and deactivated as expected. The cyclic, collective, and tail rotor pedals were examined and photographed prior to conducting the hydraulic system tests. Control continuity was established between the cyclic, collective, and tail rotor systems. To facilitate testing of the hydraulic system, a hydraulic mule was attached to the helicopter. The hydraulic system was energized, and the systems were activated and cycled on and off using the hydraulic test switches and the hydraulic isolation switch. No abnormalities were noted. ADDITIONAL INFORMATION AS350 B2 HYDRAULIC SYSTEM The Eurocopter AS350 B2 is equipped with a single hydraulic system, which provides hydraulic boost to the cyclic, collective, and tail rotor controls. The main rotor control system consists of a series of rigid rods interconnected by bell cranks and reversing levers. The respective control linkages interface with the swash plate through three hydraulic servo actuators, which are designed to exert the necessary control force. A mixing unit is located aft of the cockpit and utilized as an interface for the cyclic and collective pitch controls. The unit enables each control to operate independently without mutually coupling. The helicopter is designed so that in the event of a hydraulic pressure failure, main rotor servo accumulators provide boost (over a duration of about 30 seconds), enabling the pilot to land if the helicopter is configured in a hover, or establish the recommended airspeed (40 to 60 knots) to lessen control forces in forward flight. According to Eurocopter, the helicopter can be flown without hydraulic pressure, which will require a lateral control force of about 9 pounds and a forward cyclic control force of about 11 pounds. If the pilot attempts to hover without hydraulic assistance, the control forces intensify and oscillate in direction. Lateral and longitudinal forces can be upward of 12 pounds, which can change in direction. During the recommended run-on landing (performed at a minimum of 10 knots), a longitudinal force of up to 37 pounds may be required, with 33 pounds necessary for the right or left lateral control. HYDRAULIC TEST BUTTON The hydraulic test pushbutton (HYD TEST) is mounted on the center console between the two forward seats and consists of two positions. The TEST position (button pushed in) initiates the test function, and normal operation is restored when the button is released out. The primary function of the HYD TEST pushbutton is to enable the pilot to verify the functionality of the servo accumulators prior to flight. Additionally, the HYD TEST pushbutton is used to simulate the onset of a hydraulic failure during training. When the TEST position is selected, it results in the solenoid valve opening on the regulator unit, immediately depressurizing the hydraulic system. It additionally opens the tail rotor servo solenoid valve, depressurizing the accumulator of the yaw load compensator, but allowing the main rotor servos to remain boosted by their respective accumulators until the exhaustion of their stored energy. HYDRAULIC SYSTEM FAILURE TRAINING Hydraulic system failure is simulated by carrying out a specific sequence of switch selections and corresponding actions, which are documented in the helicopter's Flight Manual within Supplement 7. Practice 'hydraulics OFF' approaches are conducted in two phases: The transition to a recommended speed range; and a transition to landing. The instructor is to depress the HYD TEST pushbutton to the TEST position, and the student should respond by adjusting the helicopter's airspeed to between 40 and 60 knots. When in steady flight conditions, the instructor is then required to reset the HYD TEST pushbutton, which restores the system pressure and recharges the main and tail rotor accumulators. The student then positions the collective hydraulic isolation switch off, which, within two seconds, introduces the main rotor manual control loads. The yaw load compensator remains pressurized to assist the tail rotor servo with tail rotor control. This switch configuration ensures that if hydraulic power is required, turning the collective hydraulic isolation switch on will immediately reinstate the powered controls. The recommended procedure for landing is to make a shallow final approach to a flat field in an effort to minimize operation of the collective lever. The manual states that the pilot should perform a no-hover, run-on landing with the helicopter configured at 10 knots and a headwind. Specifically, the helicopter should not be hovered or taxied without hydraulic pressure assistance. EUROCOPTER LITERATURE The Eurocopter AS350 Instruction Manual states that the "helicopter can be controlled without servo actuators, but this requires the pilot to apply non-negligible forces that are difficult to gauge," and that the aforementioned control loads are "absorbed by hydraulic servo actuators so that the pilot can fly the helicopter PRECISELY and EFFORTLESSLY [Emphasis in original]."
The failure of both pilots to follow proper procedures during a simulated hydraulic emergency that resulted in a loss of helicopter control. Contributing to the accident was the flight instructor’s delayed remedial action.
Source: NTSB Aviation Accident Database
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