The FAA has issued an airworthiness directive (AD) requiring earlier and more frequent fuselage inspections of a specific area on some Boeing 737-300, -400 and -500 series aircraft after a report of two similarly situated cracks that went through the frame and fail-safe chord.

“We are issuing this AD to detect and correct cracking in the fuselage frames and frame reinforcements, which could reduce the structural capability of the frames to sustain limit loads and result in cracking in the fuselage skin and subsequent rapid depressurization of the airplane,” the FAA says in AD 2011-23-05, issued on Nov. 1.

The directive affects 605 U.S.-registered aircraft and is effective as of Nov. 16, the FAA says.

The new AD supersedes AD 2009-02-06 RI, which requires repetitive inspections for cracking of the 1.05-inch nominal diameter wire penetration hole in the frame and in the frame reinforcement, between stringers S-20 and S-21, on both the left and right sides of the airplanes. The 2009 directive resulted from reports of cracking in the frame, or the frame and frame reinforcement, common to the penetration hole intended for wire routing, and followed a December 2007 Boeing Alert Service Bulletin on the same issue.

Since that 2009 directive, the FAA says, it has received a report of four adjacent cracked frames in the forward cargo compartment between S-20L and S-21L on a 737-300. Two of the cracks went completely through the frame and fail-safe chord.

The cracks were discovered when the aircraft had accumulated 44,535 flight cycles and 44,876 flight hours—before the compliance time required by the earlier directive. That discovery led Boeing in September to revise its service bulletin to shorten the compliance time for the initial inspection to 30,000 cycles and to reduce the repetitive interval from 14,000 cycles to 4,500.

The directive adopts the initial inspection revision. It also requires reinspections within 4,500 flight cycles, with a grace period of 90 days. Airlines have up to 90 days to inspect aircraft that already have accumulated more than 40,000 cycles.