COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Boeing 767 Series Aeroplanes AD/B767/74 Hydraulic Power - Ram Air Turbine Wiring 4/94 Applicability: Model 767 series aeroplanes listed in Boeing Service Bulletin 767-29-0021 Revision 4 dated 12 May 1988. Requirement: Unless previously accomplished; replace the Ram Air Turbine (RAT) airspeed switch; cap and stow the wire between the RAT airspeed switch and the air/ground relay and then perform a functional test of the RAT deployment system; in accordance with Boeing Service Bulletin 767-29-0021 Revision 4 dated 12 May 1988. Note: FAA AD 94-01-02 Amdt 39-8784 refers. Compliance: Prior to 30 June 1995 or within 4000 hours time in service after 31 March 1994, whichever occurs later. Background: The aircraft manufacturer has received reports of the RAT airspeed switch relay chattering during take-off and landing. Although the relay is designed and tested to cycle in excess of one million times, it was only expected to cycle once each flight, therefore the switch may fail due to excessive cycling. If the failed switch remained in the open position the RAT may not automatically deploy, leading to a possible loss of hydraulic power. Compliance with this Directive is intended to prevent the inability to maintain hydraulic system power in the event that power is lost in both engines.