The US Federal Aviation Administration has issued an emergency airworthiness directive (AD) ordering the review and removal of any Sikorsky S-76 helicopter main rotor blades that have a history of lightning strikes. This follows the 16 July fatal crash in the North Sea of a Bristow-operated S-76A which resulted from blade failure.
The preliminary findings of a UK Air Accident Investigation Branch (AAIB) inquiry into the crash points to the failure of one of the helicopter's four main rotor blades, resulting in the gearbox and rotor hub separating from the fuselage mounting. The S-76 crashed into the sea from a height of 400ft (120m) while on approach to the Santa Fe Monarch oil platform, killing all 11 passengers and crew on board the aircraft.
"At this stage there are a number of variables that are being evaluated to determine the likely origin of the failure. One variable may be related to a lightning strike suffered by the subject blade in 1999, but there are other variables under active consideration," says the AAIB.
The emergency AD issued on 26 July directs that the history of all individual blades on S-76A/B/C helicopters be reviewed and any with a past occurance of lightning strikes be removed for inspection. The AD identifies removal of a number of specific blades which are known to have sustained lightning damage. Where a blade history cannot be determined, it must be removed before any further flights are made.
An examination of the fractured blade recovered from the crash site revealed evidence of fatigue. After the lightning strike the blade had been sent to the manufacturer for evaluation before it was returned to service with Bristow.
Source: Flight International
