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The helicopter was cruising at about 1,500 ft AGL when the pilot heard a loud bang. The helicopter yawed to the left and the pilot noticed that he no longer had pedal control. He commenced an autorotation and turned into wind. The touchdown was relatively heavy but the helicopter remained upright. After the pilot evacuated the cabin, he noticed that the auxiliary fuel tank had been damaged by the main drive shaft. He was able to remove an ELT, survival gear and some drinking water before the helicopter caught fire and was destroyed. Wreckage examination found that the main rotor drive shaft forward flexplate had failed and pieces had punctured the auxiliary fuel tank. The examination determined that the forward flex plate had failed under load as a result of metal fatigue. The fatigue cracking had initiated at an area of corrosion pitting on the forward edge of the flex plate. Areas of corrosion were also observed under sections of blistered paint at several other sites on the flexplate surface. The edges of the flexplate had been painted with a chromate primer and a protective top coat. The paint had blistered and formed sites that acted to trap moisture next to the metal. Chlorides contained in the moisture then acted as the electrolytic agent that caused the corrosion. The investigation was not able to determine the source of the chlorides. However, they could have resulted from environmental factors caused by the helicopter operation in coastal areas or over dry salt lakes. A further possibility was that 'hard' water (containing minerals) from artesian or other sources could have found its way on to the plates. The maintenance records indicate that with respect to main rotor drive shaft alignment, the helicopter was maintained in accordance with the manufacturers maintenance procedures. The maintenance documentation indicated that the failed flexplate (p/n A947-1) had been in service for a total of 1,249 hours. Crack initiation to failure occurred over 350 flights on the flex plate. Prior to this accident, a flexplate fitted to VH-HEW (BASI report 9301775) failed in similar circumstances. A similar failure has occurred in New Zealand. In the New Zealand example, the number of cycles to failure was not determined. However, the total time in service for this component was 1,486 hours. The Bureau recently obtained a flexplate which had been in service for 4,500 hours and which showed no visible evidence of cracking. The manufacturer does not specify a service life for the flexplates. Significant Factors 1. Localised corrosion pitting was created under regions of blistered and lifting paint on the edge of the flexplate. 2. Fatigue cracking had initiated from one area of pitting. 3. The flexplate failed as a result of fatigue cracking. Safety Action Based on preliminary information of this and the similar occurrence (report 9301775) the Bureau of Air Safety Investigation made the following Interim Recommendation which was responded to by the CAA: Interim Recommendation (IR 930272) The Bureau of Air Safety Investigation recommends that the Civil Aviation Authority advise all R22 operators of these failures to the flexplates, institute fleet inspection for drive shaft flexplate integrity and introduce repetitive inspections for cracking at appropriate intervals considering the crack propagation rate of those items which have been examined. Dependent on the results of fleet examination and in consultation with the manufacturer and industry, consideration should be given to the imposition of flexplate life limitations. The CAA response, in part, stated: 'Your recommendations are generally accepted, however the exact form of our requirement may vary after discussion with the manufacturer and the FAA.' Interim recommendation IR 930272 suggested that the CAA to consider, amongst other things, the imposition of a service life for flexplates. In the light of information from the manufacturer, review of other flexplates and specialist report 75-93 this part of that recommendation may no longer be appropriate. Specialist report 75-93 on the failed flexplates from these two aircraft identifies that the deteriorated paint treatment on the edge of the flexplates indirectly contributed to the corrosion and ultimately the fatigue failure. It suggests that prevention could be achieved either by maintaining an adherent paint film on the plate or, alternately, by leaving the plate uncoated, thereby eliminating the possible entrapment of corrosives. Following these two failures (and another in New Zealand) Robinson Helicopters P/L issued SB No. 73 on 15 March 1994. This SB requires an inspection of the forward flexplate Pt. No. A947-1 for corrosion and cracking and proposes an amendment to the R22 Maintenance Manual procedures in respect to this component. Although the bulletin requires an inspection within 25 hours, the status of the bulletin has not been classified mandatory. The CAA in response to IR 930272 sent a letter to all Certificate of Registration holders of R22 aircraft advising of the flexplate failures. However, there is no mandatory requirement for R22 operators to have their aircraft inspected to determine flexplate serviceability in respect to these failures or to report findings for those who do have their aircraft inspected. The fleet status therefore remains unknown. The Bureau of Air Safety Investigation therefore makes the following recommendation: R940092 1. That the Civil Aviation Authority institutes a mandatory inspection to determine the status and condition of in-service flexplates in respect to deteriorated paint finish or the onset of corrosion. The results of these inspections together with component in-service history should be reported and analysed. 2. That the Civil Aviation Authority in conjunction with the manufacturer, whilst considering the fleet status, analyse all of the information now available, review current inspection periods and formulate inspection procedures which will enable the integrity of the flexplate material to be checked.
General details
Date: 17 October 1993 Investigation status: Completed 
Time: 13:30 EST  
 Investigation type: Occurrence Investigation 
 Occurrence type: Transmission and gearbox 
Release date: 01 September 1994 Occurrence class: Technical 
Report status: Final Occurrence category: Accident 
Aircraft details
Aircraft manufacturer: Robinson Helicopter Co 
Aircraft model: R22 ALPHA 
Aircraft registration: VH-HBO 
Sector: Helicopter 
Damage to aircraft: Destroyed 
Departure point:Mareeba QLD
Destination:Karumba QLD
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Last update 24 July 2015