Only aircraft that have flown more than a certain number of flights as at 10 April 1989 need to be inspected. This threshold number of flights is dependent upon the maximum cabin pressure differential for the aircraft. The threshold is 30,000 flights if the aircraft may have at any time operated at a cabin pressure differential of 5.5 psi, and 50,000 flights if the aircraft has always operated at a maximum cabin pressure differential of 4.16 psi. The compliance date for inspection is not later than 1 June 1989.