What happened
On 23 May 2013, a Sikorsky S-7CO (registration G-XXEB) was performing a private flight when the crew encountered unexpected behavior in the aircraft's yaw control system. During the descent and approach to Denham Aerodrome, with the autopilot disengaged, the pilot observed the helicopter yawing to the right.
Upon attempting to correct the deviation with left pedal input, the pilot encountered a heavy, restricted sensation in the pedals. The crew noted that the right pedal would move toward its forward limit without input whenever pressure on the left pedal was released. The co-pilot confirmed these symptoms were present in their controls as well. After attempting various system checks and finding the yaw trim switches ineffective, the pilot declared a PAN. The crew briefed the six passengers on the situation and prepared for an immediate engine torque reduction if directional control was lost. The aircraft completed an uneventful running landing and taxied to dispersal, where the yaw abnormality persisted until shutdown.
The investigation
The investigation focused on the Pedal Damper Trim Actuator (PDTA), which is responsible for providing yaw trim, damping, and responding to the Automatic Flight Control System. The faulty component had been installed approximately 73 flying hours prior to the incident following a recent overhaul.
In coordination with the NTSB, investigators performed a Computerised Tomography (CT) scan of the PDTA. The scan revealed that a small steel sealing ball within the solenoid valve had split in half. Functional tests conducted by the manufacturer on a hydraulic test bench replicated the exact symptoms experienced by the pilot during the flight.
Findings
- The internal fracture of the steel ball allowed hydraulic pressure to leak past the solenoid valve.
- This leakage caused the PDTA to remain in a continuous trim mode.
- The leaking pressure also interfered with the damping circuit, creating the heavy and restricted pedal feel reported by the crew.
- The most likely cause of the ball fracture was an anomaly during the heat treatment process during manufacturing.
Safety action
- The helicopter manufacturer is issuing a Safety Advisory and a revision to the Rotorcraft Flight Manual to assist operators in identifying PDTA faults and implementing necessary crew actions.