COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES SAAB SF340 Series Aeroplanes AD/SF340/25 Amdt 1 Vertical Stabilizer Top Closure Rib 12/91 Applicability: All model SF340 with Serial Nos SF340A-031 through 159 and Serial Nos SF340B- 160 through 194. Requirement: Inspect and modify the vertical stabilizer top closure rib in accordance with the Technical Requirements of SAAB Scania AB Service Bulletins, SAAB 340-55-022 Revision 1 dated 27 February 1990, and SAAB 340-55-023 dated 1 October 1990. Note: Swedish Airworthiness Directive No 1-036, Revision A. refers. Compliance: Inspect upon achieving 500 hours in service after 16 May 1991, as required by the previous issue of this Directive; and then at intervals not exceeding those in SAAB Service Bulletins 340-55-022 Revision 1. Modify in accordance with SAAB Service Bulletin 340-55-023 before 1 August 1992. Background: Fatigue cracks in the vertical stabilizer top closure rib could lead to friction and ultimate jamming between the fin and rudder. This amendment calls up Revision A of the Requirement Document, which makes terminating action mandatory.