What happened
On 1 February 2018, a Sikorsky S-61N helicopter, registration G-ATBJ, was preparing for a flight from Marchwood, Hampshire. The aircraft had recently been transported by sea from the Falkland Islands to the UK for maintenance. During the takeoff sequence, as the pilot raised the collective to lift the aircraft off its wheels, the helicopter began to move forward and subsequently pitched nose-down.
Despite the commander applying significant aft cyclic input, the aircraft failed to respond effectively and continued to pitch forward. To prevent further damage, the pilot promptly lowered the collective, causing the helicopter to strike the ground with its nose. The impact resulted in the aircraft being damaged beyond economical repair, though there were no injuries to the two crew members on board.
The investigation
Investigators examined the flight control systems and the aircraft's recent history. It was noted that during the sea voyage, the rotor head and transmission had been left unprotected from the elements. The investigation focused on the flight control servo system checks performed prior to the flight. The crew had performed these checks but did not move the cyclic to its full and free extremes, primarily due to time pressures and the need to conserve battery power.
Detailed examination of the swashplate assembly revealed that the spherical bearing had seized. Analysis of the grease and the bearing surfaces showed evidence of corrosion pitting and the presence of metallic deposits. The investigation also reviewed maintenance records, finding that the required Safety Inspection, which included a check for binding in the ball-ring socket, had not been performed following the aircraft's period of inactivity.
Findings
- The primary cause of the nose-down pitch was a seizure of the spherical bearing within the swashplate assembly.
- This seizure was the result of corrosion build-up, likely caused by saltwater exposure and moisture ingress while the rotor head was unprotected during sea transit.
- Pre-flight control checks were insufficient because the cyclic movement was not tested to the full limits of the control envelope.
- The crew's perception that the controls were functioning correctly was due to the fact that the guide tube was flexing under load, allowing for a limited but deceptive amount of blade pitch change.
- The required Safety Inspection for the aircraft had not been conducted following its long-term storage and transit.