Helicopter crash blamed on rotor hit by lightning

A helicopter that crashed into the North Sea last month killing all 11 people on board had been critically weakened by a lightning strike three years ago, an official report said yesterday.

The strike in 1999 caused severe damage to one rotor blade of the Sikorsky, worsening an existing "manufacturing anomaly", the Air Accidents Investigation Branch (AAIB) said.

The investigators said neither the damage nor the anomaly had been detected during an inspection by Sikorsky after the lightning strike, and the helicopter, an S-76A, was returned to service.

The aircraft, owned and operated by Bristows, was transporting workers between gas platforms in the Shell field when it crashed 28 miles from Cromer on 16 July, killing both pilots and nine passengers. The pilots had been on the fifth and penultimate flight of the day, transferring one passenger between the installations before returning the remaining passengers to Norwich.

With the helicopter at a height of about 320ft, workers on a drilling rig heard a loud bang and the aircraft was seen diving steeply into the sea.

The AAIB said the crash had been caused by the outer section of one of the helicopter's four rotor blades snapping off.

A "black box" audio recording recovered from the sea-bed showed that about four and a half minutes into the flight, the crew discussed an increase in vibration but had not been unduly concerned.

The recording indicated an increase in noise associated with "main rotor vibration" towards the end of the flight.

The AAIB added: "The audio recording ends abruptly with these unusual, probably structure-borne, sounds."

Analysis of the wreckage showed three of the rotor blades had suffered only superficial damage but the outer section of the fourth blade was missing. The gearbox and rotor head had also sheared off.

Laboratory tests revealed the fourth blade's inner section had "clear evidence of fatigue".

Investigators also found evidence of an "anomaly" in the blade when it was manufactured 21 years ago. The AAIB, part of the Department for Transport, said it believed "the electrical energy imparted by the lightning strike exploited the anomaly that was built into the blade at manufacture".

The AAIB warned there was a possibility that there were other helicopters still in service that had the the same anomaly. But it pointed out that within 10 days of the accident, Sikorsky urged operators to check any rotor blade hit by lightning and to see whether other blades on helicopters in service had the anomaly.