COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Bombardier (Boeing Canada/De Havilland) DHC-8 Series Aeroplanes AD/DHC-8/68 Beta Backup Test Switches 1/2000 TX Applicability: DHC-8 aeroplanes Models 102, 103, 106, 201, 202, 301, 311, 314 and 315 Serial Numbers 003 up to and including 538. Requirement: 1. Inspect the two beta back-up test switches in accordance with Bombardier Alert Service Bulletin A8-61-30 Revision B dated 6 December 1999 to ensure switches are Part Number (P/N) MS27407-6. 2. If, during the Requirement 1 inspection, a switch with other than P/N MS27407-6 is found to be fitted, replace the switch with a P/N MS27407-6 switch. Note: Transport Canada Emergency AD CF-99-30 refers. Compliance: 1. Within 50 hours time in service from the effective date of this Directive. 2. Before further flight. This Airworthiness Directive becomes effective on 17 December 1999. Background: An incorrect switch (P/N MS24524-27) is wrongly identified in the wiring diagram manuals as an allowable replacement for the Beta back-up test switch. This switch is used elsewhere on the aircraft and is physically interchangeable with the correct switch (P/N MS27407-6). However, the internal contacts are different, preventing the Beta back-up system from functioning correctly. Mechanical failure of the propeller control system with incorrect switches installed may result in the loss of automatic overspeed protection, allowing the propeller to go into an overspeed condition, resulting in possible engine and propeller damage. This Directive requires that all Beta back-up switches be inspected to ensure that correct switches are fitted.