COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Bell Helicopter Textron 412 Series Helicopters AD/BELL 412/13 Tail Rotor Trunnion Bearing Housing 9/86 DM Applicability: All model 412 S/N 33001 to 33118, 33120 and 33121. Requirement: Action in accordance with Bell Alert Service Bulletin No. 412-86-25, Revision A. Compliance: Part I: At the next 20 hour inspection and thereafter at each 20 hour inspection until Part II is complied with. Part II: Within 150 hours time in service or 60 days, whichever occurs first after 31 July 1986. Background: A trunnion bearing housing end web has been found below specified dimensions. Failure of the housing caused excessive tail rotor vibration and loss of control.