75-09-03 BOEING: Amendment 39-2175. Applies to all Boeing Model 707-100 and 720 series airplanes noted in Boeing Alert Service Bulletin 3220 certificated in all categories. Compliance required as indicated. \n\tA.\tTo detect wing center section lower skin cracks, within the next 100 flights after the effective date of this AD, for airplanes with 20,000 or more flights, visually inspect the wing center section lower skin for cracks at the air conditioning duct anchor channel just outboard of RBL 12.78 beam on stringer 17 and at the air cycle machine support clip just outboard of LBL 12.78 beam on stringer 18. Particular attention should be given to the wing skin at the forward and aft edges of the two support fittings. \n\tB.\tSkins found cracking are to be repaired in accordance with the Boeing structural repair manual or in a manner approved by the Chief, Engineering and Manufacturing Branch, FAA Northwest Region. \n\tC.\tThis directive will be amended to require repetitive inspections when such have been determined. \n\tD.\tFor the purpose of complying with the Airworthiness Directive, subject to acceptance by the assigned FAA maintenance inspector, the number of flights may be determined by dividing each airplane's hours' time in-service by the operator's fleet average time from takeoff to landing for the airplane type. One flight is defined as one takeoff and landing. \n\tThe manufacturer's specifications and procedures identified and described in this directive are incorporated herein and made a part hereof pursuant to 5 U.S.C. 552(a)(1). \n\tAll persons affected by this directive who have not already received these documents from the manufacturer may obtain copies upon request to Boeing Commercial Airplane Company, P.O. Box 3707, Seattle, Washington 98124. The documents may also be examined at FAA Northwest Region, 9010 East Marginal Way South, Seattle, Washington. \n\n\tThis amendment becomes effective April 16, 1975.