What happened
On 26 September 2012, a Sikorsky S-76C, registration G-CGOU, was performing a commercial passenger flight from Humberside Airport to a platform in the Ravenspurn Gas Field. During the initial cruise climb, the aircraft began to exhibit an uncommanded nose-up pitch and a roll to the right. Simultaneously, the helicopter began yawing in an uncommanded "fishtailing" motion while in stability augmentation system mode.
Upon noticing these flight disturbances, the commander disengaged the autopilot to regain straight and level flight. The crew noted a strong smell of smoke within the cockpit and cabin, though no visible smoke was present. After issuing a ‘PAN PAN’ call to Air Traffic Control, the commander opted to perform a precautionary landing at a private coastal airfield located less than two miles from the aircraft's position. There were no injuries to the two crew members or the eight passengers on board. Following the landing, several passengers confirmed they had also detected the smell of smoke and felt the erratic motion of the aircraft.
The investigation
Investigators examined the aircraft and found that the circuit breakers for the number 2 cyclic control trim and the collective control trim had tripped. An engineering analysis determined that an electrical short had occurred within a wiring loom located above the forward left area of the cabin.
Further examination revealed that the wiring bundles were likely disturbed during the installation of a customer option. The investigation also noted that this specific area of the aircraft is vulnerable to chafing.