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/6 Blow-Off Valve Operating Limit 4/90 Applicability: Viper Mk 601-22. Requirement: Action in accordance with Rolls Royce SB Viper Mk 601-22 75-All. Compliance: Effective 19 April 1990. Background: This Directive has been issued to require a revised blow-off valve operating point. The Requirement reduces the possibility of engine surge or stall.