Operator and crew in 2002 accident could not have known of danger, says UK report
A rotor blade crack that could not have been detected by the flightcrew or the operator led to blade separation followed by the breakaway of the rotor head and gearbox of an oil support helicopter, according to the UK Air Accidents Investigation Branch (AAIB).
Recommendations to European and US safety regulators include the fitting of an automatic on-board blade crack detection system to future blade designs employing hollow metal spars.
Both pilots and all nine passengers on board the Bristow Helicopters Sikorsky S-76A were killed on impact with the North Sea en route from Norwich, eastern England, to several oil rigs on 16 July 2002.
The AAIB report says the outer two-thirds of one blade had separated, and the centrifugal imbalance "tore the main gearbox from the fuselage". Although the crew had noticed an increase in vibration a few minutes before the blade failure, there was no way they could have realised what it meant, says the AAIB, and the helicopter's health and usage monitoring system "did not provide sufficient warning of impending blade failure in time to avoid the accident".
Crucial factors in the propagation of the chord-wise crack in the blade's titanium main spar included an undetected manufacturing fault in the blade's erosion cover combined with lightning damage. The fault "had no effect on the structural integrity of the blade until it was exploited by a lightning strike", says the report, explaining that the strike caused an electrical discharge to heat a part of the spar, changing "its material properties".
The change in the titanium spar's strength was "not detectable" by the manufacturer or the operator's maintenance organisation, the report notes.
After the strike the blade was inspected by Sikorsky and repaired, says the AAIB. During repair, an opaque patch applied to the erosion cover in the area where the crack eventually began to propagate "hid exterior symptoms of the developing spar crack that appeared before the accident".
AAIB recommendations to the European Aviation Safety Agency and the US Federal Aviation Administration specify improvements that would make cracking easier to detect, including transparent erosion strip repair patches, and the suggestion that 50h inspection procedures be amended to include searching the upper and lower blade surfaces for cracks.
For new-design blades using hollow metal spars, the AAIB recommends that they incorporate automatic on-board crack detection systems. Soon after the accident the AAIB had recommended that existing blades should be taken out of service permanently after a lightning strike, and the UK Civil Aviation Authority made that mandatory.
DAVID LEARMOUNT / LONDON
Source: Flight International
