Tail Rotor Bearing Separation Causes Heavy Vibration in S-92 Helicopter

Casualties unknown • Approximately 65 nm north-east of Aberdeen, GB

A Sikorsky S-92A experienced severe in-flight vibrations due to the detachment of a tail rotor pivot bearing while operating over the North Sea.

What happened

On 23 April 2007, a Sikorsky S-92A, registration G-CHCK, was performing a commercial passenger flight to offshore platforms in the North Sea. While approximately 65 nm north-east of Aberdeen, the crew experienced a sudden and heavy vibration throughout the aircraft. The commander immediately took control and initiated a return to the coast, descending to a lower altitude to prepare for a potential ditching.

During the descent, the crew notified Air Traffic Control of a technical problem and requested a landing at Longside. The aircraft successfully completed a run-on landing at the airfield. There were no fatalities and no injuries to the 15 passengers or 2 crew members on board.

The investigation

The investigation focused on the source of the intense vibration and the failure of the flight recorder to capture relevant data. Examination of the tail rotor blade revealed that a large portion of the outboard pivot bearing had detached. This detachment was caused by the disbond of the bearing retainer from the flexible spar of the blade. The force of centrifugal loading caused the detached components to migrate inside the torque tube.

Investigators also found that the Multi Purpose Flight Recorder (MPFR) had failed to record flight data correctly. This was due to a configuration mismatch: the replacement MPFR was set to a data rate of 128 words per second, while the Data Acquisition Unit was transmitting at 256 words per second. This mismatch caused the recorder to misinterpret the incoming data stream.

Findings

  • The sudden vibration was caused by the in-flight separation of a tail rotor pivot bearing.
  • The separation originated from the disbond of the bearing retainer from the flexible spar.
  • Once the retainer disbonded, the inner end of the bearing became unsupported, leading to excessive strain and eventual separation.
  • The flight recorder failed to capture data due to an incorrect data rate configuration following a recent hardware replacement.
  • Previous maintenance instructions for inspecting the pivot bearings were found to be insufficient and difficult to execute due to limited access within the blade.

Probable cause

The primary cause of the vibration was the detachment of the outboard pivot bearing, which occurred after the bearing retainer disbonded from the flexible spar. This was likely due to inadequate inspection procedures and difficulty accessing the internal components of the tail rotor blade.

Frequently asked questions

What happened in the 2007-04-23 Sikorsky S-92A accident near Approximately 65 nm north-east of Aberdeen, GB?

A Sikorsky S-92A experienced severe in-flight vibrations due to the detachment of a tail rotor pivot bearing while operating over the North Sea.

What aircraft was involved and where did it happen?

The accident on 2007-04-23 involved a Sikorsky S-92A, registration G-CHCK, at Approximately 65 nm north-east of Aberdeen, GB.

What was the probable cause of the accident?

The primary cause of the vibration was the detachment of the outboard pivot bearing, which occurred after the bearing retainer disbonded from the flexible spar. This was likely due to inadequate inspection procedures and difficulty accessing the internal components of the tail rotor blade.

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