Investigation into fatal accident in June reveals fatigue failure of rotor blade root fitting and disbonding of adhesive

The Australian Transport Safety Bureau (ATSB) has recommended that the US Federal Aviation Administration and Robinson Helicopter examine main rotor blade root fittings on R-22 helicopters to establish the integrity of the adhesive bond in the spar to root fitting joint. The recommendation follows the ATSB's ongoing investigation of the fatal crash of an R-22 near Sydney in June involving the loss of a main rotor blade.

The ATSB says the FAA's review should establish the extent of the loss of adhesion and the extent to which corrosion has penetrated around the inboard bolt hole of the blade root fitting. When disbonding is discovered, the operating history and in-service flight spectrum of the helicopter, along with the environmental conditions under which it operated, should be assessed, it adds.

During its investigation of the crash, which killed two people, the ATSB found signs of a fatigue failure of the main rotor blade root fitting, in the counterbore of the bolt hole. The ATSB says that failures in this area have been responsible for two earlier in-flight rotor blade failures on R-22s in Australia and had been identified by Robinson as the critical location for fatigue cracking during testing. The bureau found evidence of corrosion where the fatigue crack initiated, along with "significant" areas of breakdown of the adhesive bonding between the skin, the end of the spar and the rotor blade root fitting. There was no cracking on the upper or lower rotor blade skin, but similar disbonding was found on the other blade. The total time in service of the helicopter was 2,009.5h, according to the ATSB.

The ATSB examined six other R-22 main rotor blades which all exhibited similar disbonding of the adhesive in the area of the end of the spar and the rotor blade root fitting. Disbonding allows moisture, salts and contaminants to enter the area, with corrosion accelerating fatigue cracking, says the ATSB.

The ATSB plans to publish in November a supplementary report into the May 2000 crash of a Whyalla Airlines Piper Navajo Chieftain. It will address "significant evidence" uncovered since the original report issued in 2001, says Kym Bills, executive director. The new report follows rejection of a number of the ATSB's investigation conclusions and disagreements over crash causes by the South Australian coroner in July (Flight International, 29 July-4 August).

Source: Flight International