The Saab 340A, VH-EKT, was engaged on a scheduled passenger service from Moorabbin, Victoria to Devonport, Tasmania. During the flight, the crew experienced difficulty in controlling the right propeller RPM. When the aircraft landed at Devonport, directional control was lost. The aircraft departed the runway and ran through a ditch in soft, muddy ground. The aircraft sustained substantial damage but there were no injuries to passengers or crew. Significant factors: 1. The right propeller control unit was defective. 2. The crew did not consult the Aircraft Operations Manual, the Aircraft Flight Manual or the Checklist as a means of dealing with the malfunction. 3. Effective communication between the crew members to address possible strategies in response to the malfunction, did not occur. 4. The crew did not appreciate that the malfunction could have been dealt with using the Uncommanded Powerplant Indications procedure in the Abnormal Procedures Checklist. 5. After landing, the crew did not check that the Beta lights had illuminated before selecting reverse thrust, which resulted in the application of substantial asymmetric thrust and consequent loss of directional control. 6. The captain was improperly influenced by his understanding of a recent similar occurrence which had not resulted in a loss of directional control. 7. Although uncommanded PRPM reductions had occurred to SAAB aircraft in this operator’s fleet over a number of years, the possible consequences were never fully appreciated by the company until this accident.