COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 106 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Rolls Royce Turbine Engines - Viper Series AD/VIPER/3 Blow Off Valve Operation Limit 3/90 Applicability: Viper Mk. 521, 522 engines. Requirement: 1. Action in accordance with Rolls Royce Service Bulletin No. 75-A32, or 2. CAO 106 AD/RRT-V/13. Compliance: Effective 22 March 1990. Background: This Directive has been reissued to require a revised blow-off valve operating point to reduce the possibility of engine surge or stall.