Boeing and Southwest Airlines have asked the FAA to delay mandating inspections on Boeing 717 fuselage frames, so the original equipment manufacturer (OEM) can stock necessary spares and assess new data that indicate fatigue cracking in overwing frames is occurring sooner than expected.

The problem has been found on more than 40 MD-80s—each with at least 19,600 cycles—and one MD-90, and this has already prompted the FAA to issue airworthiness directives (ADs) in 2008 and 2010 addressing those models. In October, Boeing issued an alert service bulletin detailing the first reported finding on a 717.

All three models share a similar over-wing frame design.

The FAA’s proposed 717 AD, issued May 9, mandated inspections for cracked frames and, if cracks were found, either ordered blend-out repairs or, for large cracks (defined as 0.125 in. or longer), frame replacements. Initial checks would be required at 20,000 total cycles or within 8,275 cycles after the directive’s effective date, whichever occurs later.

Boeing’s October bulletin said the 717 with the cracked frame had 14,542 cycles. In March, Boeing received a report of a second 717 with a crack large enough to need a frame replacement, the OEM revealed in comments filed late last month on the FAA’s draft rule.

The second 717 had 11,991 cycles.

“With this recent information, Boeing is seeking additional time to conduct a review, assessment and the possible impact to the approved [service bulletin] based on this new 717 report with this airplane having accumulated flight cycles lower than the first 717 report or on the MD-80 fleet that was used to establish the initial inspection threshold and repetitive inspections for the 717,” Boeing said in its comments.

The OEM asked the FAA to extend the directive’s comment period, which closed June 25.

Southwest, which operates 86 of the 156 717s in service, pointed out another possible compliance challenge: an apparent lack of replacement parts. In comments on the proposed directive, the airline said it checked with Boeing and found that the OEM did not have any replacement frames or frame kits in stock.

Southwest requested that the FAA withhold the final rule until Boeing has built up “adequate stock” to support possible frame replacements.

Southwest also noted new Boeing service bulletin instructions issued earlier this month that, if accomplished, could push the initial inspection threshold out to 45,000 cycles.