Quoting the AAIB report.....
"The AAIB and the helicopter's manufacturer are of the opinion that the electrical energy imparted by the lightning strike in 1999 exploited the anomaly that was built into the blade at manufacture and damaged the spar.
It has not yet been established whether the anomaly seen in the fractured blade went unnoticed or whether it was permitted by concession. If the anomaly was not detected during manufacture, there may be other blades where the anomaly is slightly different in nature and/or severity. The anomaly now exposed on the fractured blade would have been hidden from view during routine in-service blade inspections. Conceivably there could be another damage mechanism such as abrasion or fretting between the titanium spar and the titanium erosion strip that might induce fatigue. Consequently, there may be other S-76 blades that have a similar hidden anomaly that may be relevant to the continued airworthiness of those blades. "
The Federal Aviation Administration mandates appropriate action to ensure the continued airworthiness of Sikorsky S-76 main rotor blades which have either:
A two-piece leading edge titanium sheath (erosion strip).
or
Have suffered a lightning strike.
This safety recommendation was copied to the UK CAA."
Felt these excerpts were worth copying to the thread here.