COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES DHC-6 (Twin Otter) Series Aeroplanes AD/DHC-6/48 Introduction of Beta Range Test Switch 2/80 Applicability: DHC-6 Aircraft S/No. 1 thru 450. Document: 1. De Havilland Aircraft of Canada Ltd., SB No. 6/309, Rev. C; or 2. Transport Canada Airworthiness Directive CF-79-24 De Havilland Canada. Compliance: Within next 500 hours time in service after 31 March 1980. Background: Investigations of two accidents overseas where inadvertent reverse thrust was suspected, drew attention to the need to ensure that serviceability of the beta back-up system could always be checked before flight. Incorporation of the beta range test switch ensures that such a check can be accomplished.