COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Boeing 737 Series Aeroplanes AD/B737/21 Generator Control Unit (GCU) Filter Modules 7/89 Applicability: All model 737 series aircraft. Requirement: Replace OR modify the GCU filter modules in accordance with Westinghouse SB 87- 101 OR 87-102. Note: FAA AD 89-07-13 refers. Compliance: Prior to 13 December 1990. Background: Advice has been received that GCU filter failures have occurred which have resulted in smoke in the cockpit. This condition if not corrected could lead to additional GCU failures producing smoke in the cockpit and partial loss of electrical power.