COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Fokker F50 (F27 Mk 50) Series Aeroplanes AD/F50/21 Forward Fuselage - Stringers and Frame 5775 8/89 Applicability: Serial numbers 20103 through 20108 and 20110 through 20121. Requirement: Reinforce fuselage frame to stringer connection by installing connecting angles in accordance with Fokker Service Bulletin F50-53-004 dated 14 October 1988. Note: Netherlands AD 88-81 refers. Compliance: Prior to accumulating 10000 flight cycles. Background: Full scale fatigue testing has shown the possibility of cracks occurring on the subject area, due to insufficient frame to stringer connection.