What happened
On 5 February 2023, a Sikorsky S-92A, registration EI-ICU, was conducting a search and rescue training exercise. The flight departed from Sligo, Ireland, with a crew of four, intending to rendezvous with a lifeboat near Portrush. While cruising at 2,000 ft amsl, the helicopter encountered unexpected severe turbulence approximately 5 nm east-nertheast of City of Derry Airport.
During this encounter, the aircraft experienced pitch oscillations and a sudden acceleration. The airspeed reached 175 KIAS, which surpassed the aircraft's maximum operating speed (VNE) of 165 kt, triggering an aural warning. The pilot responded by increasing the pitch attitude and reducing the collective to slow the aircraft to approximately 100 KIAS. Following the event, the crew noted an increase in airframe vibration and elected to divert to City of Derry Airport, where they landed without further incident.
The investigation
The investigation examined flight data recorder (FDR) and Health Usage Monitoring System (HUMS) data, alongside meteorological reports. FDR analysis revealed that the aircraft experienced pitch variations and a significant change in wind speed during the event. While the FMS recorded a tailwind, the investigation noted that the FMS wind calculation algorithm includes a time lag, meaning the environmental change likely occurred simultaneously with the initial oscillations.
Following the flight, an overspeed inspection was performed. This identified three specific issues: the need to replace the stabilizer strut, the aft tail drive shaft bearing support, and the forward tail drive bearing support. A manufacturer's representative suggested these defects may have been pre-existing rather than caused by the overspeed event. The aircraft was returned to service two days after the inspection and repairs were completed.
Findings
- The helicopter encountered mountain wave activity associated with the Sperrin Mountains, which generated the severe turbulence.
- Although moderate turbulence was noted in the pre-flight briefing, subsequent meteorological analysis indicated that mountain wave activity was a specific possibility in the region at the time of the flight.
- The airspeed exceedance was driven by a sudden change in wind conditions while the aircraft was flying in the lee of the hills.