History of Flight
On February 23, 2011, at approximately 1615 Pacific standard time, a Eurocopter AS350 B2, registration N965SD, experienced a loss of control and made a hard landing at March Air Reserve Base in Riverside, California. The helicopter was operated by the Los Angeles County Sheriff's Department (LASD) as a training flight under 14 CFR Part 91. The crew consisted of a commercial pilot who was also a certified flight instructor (CFI) and a private pilot undergoing instruction (flying pilot, FP). Both sustained minor injuries, and the helicopter sustained substantial damage. The flight departed Long Beach, California, around 1550. Visual meteorological conditions prevailed, and no flight plan was filed.
The CFI had briefed the FP to perform a maximum performance takeoff simulating a 20- to 25-foot obstacle, during which the CFI would simulate a hydraulic failure by activating the hydraulic test switch. Prior to the maneuver, the FP acknowledged that once the hydraulic failure warning activated, he would accelerate to 40 knots before activating the hydraulic isolation switch. According to the training procedure, the CFI should have reset the hydraulic test switch before the FP activated the isolation switch.
During the takeoff, at about 20-25 feet above ground level, the FP applied forward cyclic and the CFI activated the hydraulic test switch. The FP reported that immediately after activation, he was unable to apply right pedal because the control forces were "too great." As the helicopter began to yaw left, the FP activated the hydraulic isolation switch. He believed that although control forces would increase, he could overcome them. However, the hydraulic fluid had already been evacuated from the tail rotor's yaw load compensator system due to the test switch activation, so the hydraulic boost from the normally closed compensator was unavailable.
The helicopter continued yawing left and entered a spin about its vertical axis. The CFI attempted to let the FP regain control, but the FP was trying to regain control by "flying" toward the left rotation to gain forward airspeed. The CFI then applied right cyclic, counter to the FP's left cyclic input. He unsuccessfully attempted to communicate via intercom. The helicopter spun several times before impacting the runway and sliding to a stop. The crew performed emergency shutdown and exited.
Personnel Information
CFI: Held commercial pilot and flight instructor certificates with ratings for rotorcraft helicopters and airplanes. He held a first-class medical certificate issued April 19, 2010, with a limitation for near-vision glasses. He had accumulated 6,728 total flight hours, including 4,276 hours in the accident make and model, and 359 hours of instruction in that same model. His last flight review was February 19, 2010, in a Beechcraft B200.
FP: Held private pilot certificate with rotorcraft and single-engine airplane ratings. He held a first-class medical certificate issued December 23, 2010, with no limitations. He had 245 total flight hours, including 73 hours in the accident make and model. His last flight review was July 3, 2010, in a Robinson R-22.
Wreckage and Impact Information
Examination at the scene revealed a debris field about 75 feet in diameter. The first identified point of contact was a ground scar from the left skid, with the fuselage oriented at 350 degrees. Damage included a buckled tailboom, collapsed skid landing gear, deformed and partially collapsed main cabin, evidence of vertical stroking of pilot seats, intact Starflex, and impact damage to all three main rotor blades, primarily on trailing edges.
Tests and Research
On February 25, 2011, an examination was conducted at the LASD Aero Bureau Facility. The tail rotor driveshaft shear point had disconnected, and the tail rotor blades and yoke were damaged during impact; no further damage to the drive train. The hydraulic manifold, servos, and tail rotor compensator servo showed no abnormalities. Power was reapplied, and all solenoids activated and deactivated as expected. Control continuity was established for cyclic, collective, and tail rotor systems. A hydraulic mule was used to test the hydraulic system, and no abnormalities were found when switches were cycled.
Additional Information
The Eurocopter AS350 B2 is equipped with a single hydraulic system providing boost to cyclic, collective, and tail rotor controls. In case of hydraulic pressure failure, main rotor servo accumulators provide boost for about 30 seconds, allowing a landing if in hover or establishing 40-60 knots to reduce control forces. The helicopter can be flown without hydraulic pressure, requiring about 9 pounds lateral and 11 pounds forward cyclic force; forces increase and oscillate in hover. Run-on landing requires up to 37 pounds longitudinal and 33 pounds lateral force.
The hydraulic test pushbutton (HYD TEST) is on the center console. When pressed, it depressurizes the system and opens the tail rotor servo solenoid valve, depressurizing the yaw load compensator accumulator while main rotor servos remain boosted from accumulators until energy is exhausted.
Hydraulic system failure training follows Supplement 7 of the Flight Manual. The instructor depresses HYD TEST, and the student adjusts to 40-60 knots. Then the instructor resets the button, restoring system pressure and recharging accumulators. The student then sets the collective hydraulic isolation switch to off, introducing manual control loads within two seconds, but the yaw load compensator remains pressurized to assist tail rotor control. Recommended landing is a shallow approach to a flat field with a no-hover, run-on landing at 10 knots with headwind.
Eurocopter's instruction manual states that the helicopter can be controlled without servo actuators, but requires non-negligible forces that are difficult to gauge.