Summary
We propose to adopt a new airworthiness directive (AD) for all The Boeing Company Model 707 airplanes and Model 720 and 720B series airplanes. This proposed AD was prompted by a determination that undetected web fatigue cracking caused by oil canning may exist in the station 1440 aft pressure bulkhead web. This proposed AD would require repetitive detailed inspections for any oil canning or cracking of the station 1440 aft pressure bulkhead web, and related corrective actions if necessary. We are proposing this AD to address the unsafe condition on these products.