Accident Sequence
Moments after lift-off, the propeller separated from the aircraft. The airplane subsequently veered off the runway, contacted soft soil, and nosed over.
Investigation Findings
Examination of the propeller hub revealed that it had failed forward of the attachment flange, around the circumference of the hub. The pilot reported performing a thorough preflight inspection and engine run-up, during which no discrepancies were noted.
Metallurgical analysis of the hub identified crack arrest marks characteristic of fatigue. The cracking originated from the fourth thread root on the aft piece of the hub. The crack was estimated to have been several inches long before the final failure occurred.
Maintenance History
A review of maintenance records disclosed that McCauley Service Bulletin 179A, McCauley Service Letter 1989-5, and Airworthiness Directive (AD) 89-26-08 had not been complied with during the previous five annual inspections. These documents referenced propellers that should have incorporated an oil-filled configuration with red dye, which would become visible once a hub had begun to crack.
Airworthiness Directive Applicability
The AD in question cited McCauley "2A36C23/84B-0" Constant Speed Propellers. Both McCauley and the FAA engineering directorate, which authored the AD, reported that the intent was to cover all 2A36C23 hubs regardless of blade models. The hub installed on the accident aircraft was a McCauley 2A36C23-P-E, with blade Models S-84B-0 (rather than the blade Models 84B-0 referenced in the AD).
Mechanics and Inspection Authorizations (IAs) who had worked on the aircraft during the preceding five annual inspections unanimously responded that the AD was not applicable because the propeller was not a "2A36C23/84B-0" as stated in the directive.