Summary
We propose to adopt a new airworthiness directive (AD) for all The Boeing Company Model 737 series airplanes. This proposed AD was prompted by a report that structural fatigue cracks can develop in certain aluminum pressure module check valves prior to the design limit. This proposed AD would require an inspection to determine the part numbers of the four hydraulic systems A and B pressure module check valves and applicable on-condition actions. We are proposing this AD to address the unsafe condition on these products.