The pilot was carrying out a series of aerobatic manoeuvres when the propeller separated from the airframe. A forced landing was carried out in a suitable paddock. The pilot then noted that one wing had sustained damage from the departing propeller. The evidence available indicates that the propeller fell from the aircraft when the remaining five of the six propeller retaining bolts failed as a result of fatigue cracking and overload. The sixth bolt was not found but lack of marks on the crankshaft flange indicate that it had fallen out prior to failure of the other five bolts. All bolts were in place and secured with lock wire prior to take off. CAA Airworthiness Directive AD/PEP/1 AMDT 2 addresses a known problem which relates to propeller wood shrinkage and loosening of the retaining bolts. The directive recommends special checks on the propeller bolt torque in certain circumstances. The propeller was fitted to the aircraft in Victoria, in a cool moist environment, and the aircraft was then transferred to Perth and operated in a warm dry environment. This change in environment is one of the circumstances set out in the airworthiness directive. No evidence was found to indicate recent compliance with the directive. It is probable that shrinkage of the propeller wood caused loosening of the propeller retaining bolts. The loose bolts were than able to "work" eventually resulting in the failure of the lock wire and loss of one bolt and the failure of the other five bolts.
