Ongoing inspections of older Boeing 737 Next Generation (NG) structural parts have turned up cracks outside the original area targeted, prompting Boeing and the FAA to expand the checks.
Checks of frame fittings and failsafe straps, or pickle forks, turned up cracks in four aircraft near fasteners that were not covered in the original inspection order issued Oct 3. As a result, Boeing on Nov. 5 recommended that operators expand the checks to include eight fasteners, up from two in the previous order. FAA will mandate the checks in an airworthiness directive (AD) slated to be published Nov. 13 and effective immediately.
The original AD "required repetitive inspections for cracking of the left- and right-hand side outboard chords of frame fittings and failsafe straps at a certain station”—station 663.75—"around two fasteners," and repairs of any cracks, the new directive said. "From these findings, four airplanes have been identified to have cracking outside the initial inspection area. Based on these findings, the FAA has determined that the inspection area must be expanded from the area around two fasteners to the area around eight fasteners on both the left- and right-hand sides.”
The initial checks were required within 7 days for aircraft with 30,000 or more flight cycles, and within 1,000 cycles for aircraft with 22,600-29,999 cycles. Repetitive checks are required every 3,500 cycles. About 1,200 aircraft have gone through the original inspections, and cracks have been reported on about 60.
The new directive, which is expected to be adopted by other regulators, calls for aircraft in the high-cycle category to be re-inspected within 60 days. Aircraft in the lower cycle category must re-inspect the entire area within 1,000 cycles. The global NG fleet numbers about 6,300, but only about 25% have enough cycles to fall under the inspection mandates.
Boeing discovered the original problem when an 737-800 being prepared for a passenger-to-freighter conversion was inspected and a crack was found at station 663.75. Further inspection turned up cracks on both the left- and right-side fittings.
"The cracks are common to the aft fastener hole joining the frame fitting outer chord and fail-safe strap at water line 199,” Boeing explained in a Sept. 30 operators’ message. "The frame fitting cracks are estimated to be 1-inch long from the aft edge of the fitting. The cracks in the failsafe strap are also extending from the aft edge, but the length is unknown because the cracks are hidden between the fitting and the skin/ [failsafe] strap.”
The FAA used the new AD’s preamble to go into an unusual level of explanatory detail about its methodology.
"The FAA has taken all inspection findings into consideration in assessing the merits of the existing regulatory action,” the agency said. "The findings support that the initial inspection thresholds are adequate to ensure fleet safety. All airplane structure has an initial period when fatigue cracking is not anticipated. Beyond this period, structural safety can be maintained with a damage-tolerant design and inspection program.”