Air Accidents Investigation Branch
Summary:
A search and rescue training flight encountered turbulence without any visual indication which resulted in the helicopter exceeding its maximum speed. The flight subsequently diverted to the City of Derry Airport.
An overspeed inspection of the helicopter identified three items which were rectified by the replacement of components. The helicopter returned to service two days later. The manufacturers representative believed the findings likely pre-existed the overspeed event.
Analysis of meteorological reports suggested that EI-ICU probably flew through mountain waves associated with the Sperrin Mountains, creating the turbulence which affected the helicopter. Moderate turbulence was forecast in the region and discussed in the pre-flight briefing, but analysis of subsequent meteorological information suggested that mountain wave activity was present at the time of the event.