COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Cessna 550 (Citation II) Series Aeroplanes AD/CESSNA 550/1 Wing Main Spar - Inspection and Modification Applicability: Model 550 series aeroplanes with S/Nos. as referenced in SL No. 550-57-1. Requirement: 1. Inspect the wing spar upper and lower cap in the area of wing station 37 in 2/82 accordance with Cessna SL No. 550-57-1 Rev. 1 dated 1 May 1979. FAA AD 79-12-06 also refers. 2. Inspect and modify in accordance with Cessna SB No. 550-57-3 Rev. 1 dated 28 March 1980. 3. Visually inspect for further cracks the wing main spar upper and lower caps of aircraft modified in accordance with SB No. 550-57-3 Rev. 1 in the area of the WS 37 modification. Compliance: For Para. 1 - As detailed in the requirement document. For Para. 2 - In accordance with the requirement document when any crack monitored under the provisions of Para. 1 reaches or exceeds 0.30 inches in length. Note: The modification may be carried out at any time (i.e. not necessarily at crack detection) and thus exempt the aircraft from the inspections under Para. 1. For Para. 3 - At or prior to exceeding 1200 hours after modification and thereafter at intervals not exceeding 1200 hours or 1200 landings whichever occurs first. Background: The results of full-scale fatigue testing and the inspection of several high time Citation aircraft have revealed small fatigue cracks in the main spar upper and lower caps at WS 37. In service cracks were found in 8 instances in the upper cap and in 1 instance in the lower cap.