AD NUMBER: PL2005-04-51
MANUFACTURER: Boeing
SUBJECT: Airworthiness Directive PL2005-04-51
SUMMARY: On May 7, 1991, we issued AD 91-11-01, amendment 39-6887 (56 FR 22306, May 15, 1991), for certain Boeing Model 747 series airplanes. That AD requires repetitive inspections for cracks of the frame structure and skin in the fuselage section 41, and repair if necessary. That AD also provides for an optional terminating action for the repetitive inspections. That AD was prompted by recommendations of the FAA-sponsored Boeing Model 747 Structures Working Group. We issued that AD to prevent sudden decompression of the fuselage. Since the issuance of AD 91-11-01, we have received several reports of large fatigue cracks common to fuselage frames in the upper deck area on Boeing Model 747-200C, -200F, and -300 series airplanes. Most of these airplanes had been inspected in accordance with AD 91-11-01. Many fatigue cracks occurred near stringers S-10 and S-10A, but other cracks were also reported. The cracking is due to cyclic pressurization of the airplanes. We also have received two recent reports of severed or nearly severed adjacent frames at body station (BS) 420 and BS 440 near stringer S-10A on Boeing Model 747-300 series airplanes. Both airplanes had been inspected in accordance with AD 91-11-01. In both reports, missing fasteners common to the skin at frame shear tie flanges were detected in the vicinity of cracks. In one case, eight fasteners were missing from the body skin at the severed frame at BS 440. One airplane had accumulated 11,641 total flight cycles; the other airplane had accumulated 11,880 total flight cycles. In light of these reports, we have determined that, for certain Boeing Model 747-100B SUD, -200C, -200F, and -300 series airplanes; and certain Boeing Model 747-200B series airplanes retrofitted with a SUD; the inspections required by AD 91-11-01 do not adequately detect fatigue cracks at BS 420, 440, and 460 between stringers S-8 and S-12 inclusive. Such fatigue cracking, if not detected and corrected in a timely manner, could lead to severed frames, and consequent rapid decompression and loss of the structural integrity of the airplane.

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