What happened
On 19 June 2013, a Eurocopter AS3/350B3e Squirrel, registration G-ECUK, was performing a private flight near an Oxford landing site. The aircraft, carrying a pilot and one passenger, was executing a landing approach to a grass field adjacent to a private pad. The weather was clear with calm winds.
As the pilot initiated a slight flare and a left pedal input to correct the aircraft's track, the helicopter experienced a sudden jolt and an immediate, violent yaw to the left. The pilot attempted to maintain directional control using pedal inputs, but the required control forces became extreme. During the subsequent rotations, the right skid of the AS350B3e struck the ground, causing the main rotor blades to impact the earth. The pilot sustained serious injuries, specifically three broken vertebrae, while the passenger was uninjured. The aircraft sustained substantial damage to its airframe, rotor blades, and dynamic components.
The investigation
Investigators examined the aircraft and identified evidence of a hydraulic leak at the base of the tail boom. They discovered that hydraulic fluid had been escaping from two unions that secured hydraulic pipes to the tail rotor hydraulic servo and load compensator. These unions were found to be loose enough to be rotated by finger pressure.
Testing revealed that the leak rate from these loosened unions could deplete the hydraulic system's usable fluid within 30 minutes. The investigation also looked into the maintenance history, noting that the aircraft had recently undergone a modification to fit a load compensator to the tail rotor servo. A review of the manufacturer's maintenance task cards revealed that the process allowed for the load compensator and unions to be positioned in a way that placed the components under mechanical load, which could lead to the unions loosening during operation.
Findings
- The primary cause of the accident was the loss of hydraulic system pressure resulting from a leak at two hydraulic pipe unions.
- The loose unions allowed hydraulic fluid to escape, which led to a loss of assistance for the tail rotor yaw controls.
- The sudden increase in required control forces likely led the pilot to believe a total tail rotor failure had occurred.
- The pilot's attention was likely consumed by the immediate need to manage the unexpected yaw at a low altitude, which may have prevented the perception of cockpit hydraulic warnings.
- The orientation of the load compensator during previous maintenance work likely placed undue stress on the hydraulic unions, causing them to loosen during flight.