On 20 June 2003 at approximately 0840, a Robinson Helicopter Company Model R22 helicopter, registered VH-OHA (OHA), was being used to conduct flying training in the Bankstown training area with an experienced flight instructor and student pilot. The helicopter was observed and heard flying in a normal manner. Witnesses reported subsequently hearing a number of loud bangs and one witness observed what appeared to be a main rotor blade separating from the helicopter. The helicopter descended to the ground in an inverted attitude and both occupants were fatally injured.
Examination of the accident site and helicopter wreckage confirmed that one main rotor blade had failed in-flight. Examination of the helicopter and its systems did not reveal any other abnormality that would have contributed to the loss of the main rotor blade.
The helicopter had recently re-entered service following maintenance which included the fitting of an overhauled engine and the completion of a 100-hourly inspection. The helicopter also underwent maintenance action to rectify a main rotor blade vibration. This maintenance action involved a number of experienced R22 helicopter engineers being consulted about the possible reasons for the main rotor blade vibration. Rectification action was completed in accordance with normal maintenance practices and the manufacturer’s maintenance manual. Subsequent examination of the maintenance manual for the R22 helicopter revealed that it did not contain any information in the tracking and balancing section that indicated that a vibration may be the result of a crack in the main rotor blade. The manufacturer had produced other documentation containing this information, but these documents did not formally form part of the maintenance manual.
The helicopter had been manufactured in 1991 and had been imported into Australia in 1996. In the time prior to the accident it had been owned and operated by a number of organisations and individuals, and was operated both commercially and privately.
Following the accident, industry suggestions about the possible under-recording of time in service on the helicopter led the Australian Transport Safety Bureau to concentrate part of the investigation to the recording of time in service of the helicopter. Coincident with this investigation, a separate investigation of the recording of time in service on the helicopter was conducted by the Australian Civil Aviation Safety Authority (CASA).
Both investigations examined a wide range of documentation and records from numerous sources. The conclusion of both investigations was that the helicopter had not exceeded the mandatory time in service life of 2,200 hours, nor had it exceeded the mandatory calendar time in service life of 12 years. The final time in service of the helicopter was calculated to be 2,055.6 hours and the calendar time in service was 11 years and 8 months.
An examination of the main rotor blade in the ATSB laboratories revealed that it had failed as a result of fatigue crack growth in the blade root fitting at rotor station 10.35. The fatigue crack initiated as a result of localised pitting corrosion in the counterbore of the inboard bolthole. The examination also revealed that while the fatigue failure was in a similar position to two previous main rotor blade failure accidents in Australia, in OHA’s case, there was an area of adhesive disbonding between the main rotor blade skin and blade root fitting. This adhesive disbonding meant that the crack in the blade root fitting did not propagate into the blade skins and so was undetectable using visual means. The two previous failures were linked to under-recording of hours.
The material failure analysis found that the disbonding present on the failed main rotor blade was also present in a number of other main rotor blades that were examined. As a result, the ATSB issued a safety recommendation to the United States Federal Aviation Administration (FAA) and to the Robinson Helicopter Company, seeking that they conduct further testing on main rotor blade root fittings to evaluate the extent of adhesive disbonding in the blade root fitting. This examination was conducted on a total of 51 main rotor blades that had between zero and 2,200 hours time in service. Results of the examination revealed that adhesive disbonding between the spar and root fitting was present in all blades and that the extent of the disbonding was variable.
Subsequent to this accident there was another in-flight failure of a main rotor blade. In February 2004, an R22 helicopter being operated in Israel sustained an in-flight failure of a main rotor blade. This blade had failed as a result of fatigue in the same location as the failure in the Australian accident. The Israeli failure exhibited a similar loss of adhesion and corrosion. Both blades had failed before their mandatory time in service retirement lives and represented a failure of the fatigue fracture control plan. A third failure occurred in New Zealand in November 2004. Preliminary investigations have revealed that the failure may be the result of loadings on the blade that may have exceeded those intended by the manufacturer. The investigation of that accident is continuing.
The manufacturer has issued a safety letter and a service bulletin relating to revised retirement lives for main rotor blades, and has introduced a redesigned main rotor blade into service. The manufacturer indicated that it intends to publish safety alerts and notices on its Internet website as an additional means of bringing safety related information to the notice of owners, operators and maintenance organisations.
The R22 maintenance manual has also been amended by the manufacturer as a result of this investigation. The main rotor blade tracking and balancing section now contains information, which alerts maintenance personnel to the fact that a main rotor blade vibration may be the result of a developing crack.
Safety action taken by the CASA as a result of this accident was to amend an existing airworthiness directive to take into account the findings from the examination of the blade and to introduce additional amendments to the directive, when updated information became available from the manufacturer. They also introduced a discussion paper on the installation of mandatory time in service recorders for helicopters. As at October 2005, CASA was still evaluating the public comments on the discussion paper.
In addition, CASA has drafted a Notice of Proposed Rulemaking (NPRM 0503CS) in which it is proposed to require the retirement of similar main rotor blades by 1 March 2006 on Australian registered Robinson R22 helicopters.
The United States FAA issued a special airworthiness information bulletin and an emergency airworthiness directive.
As a result of several accidents involving main rotor blade failures, the European Aviation Safety Agency, issued an airworthiness directive on 5 July 2005 mandating compliance with the Robinson service bulletin.
The ATSB has contracted research to assess the validity of the usage spectrum assumptions that were used for certification of the Robinson R22 helicopter. A research investigation report on the project is planned to be released in 2006.