Cracks and corrosion in bonded skin panels may prove to be the aerospace equivalent of concrete blight in the construction industry. Case in point: a US Airways Boeing 737-300 with 35,710 flight cycles was found with a 10-inch crack on a fuselage lap joint, prompting the FAA to issue an airworthiness directive calling upon operators of 737-200 through -500 models to inspect them both externally and internally and to effect repairs.

Sources say the FAA came within a hairsbreadth of issuing an emergency AD. Instead, the agency declared an effective date of July 14 to the AD (No. 2003-14-06) posted July 9 on the Federal Register.

On the airplane where the problem was discovered, multi-site cracking on a lap splice over the windows ran from just aft of the flight deck to just forward of the wing spar. Some of the spider web of cracks linked up to form a single 10-inch long crack. The premature cracking was attributed to a manufacturing defect that led to delaminated skin doublers.

According to a US Airways official, the cracking was discovered during a C check just before the July 4 holiday weekend. "The FAA was notified immediately," the official said.

Recall the China Airlines 747 that crashed May 25, 2002 in the Taiwan Straits appeared to have been the victim of fatigue cracking from an improper repair. The explosive 1988 decompression of an Aloha Airlines 737 with 89,680 flight cycles, more than twice as many as the US Airways jet, clearly casts a fearful shadow, stimulating the FAA to issue this AD. It calls upon operators to conduct detailed external inspections of 63 feet of the lap joint. The FAA appears to have concluded that the Boeing solution needs to be checked carefully and may have undershot significantly.

There may be a larger implication to this development. Under the supplemental structural inspection program (SSIP) put in place after the Aloha disaster, 737s undergo their first major inspection at 50,000 cycles. Given the discovery of delamination at 35,000 flight cycles, the first SSIP has de facto been accelerated with publication of this AD to take place some 30 percent sooner than previously. Structures expert Martin Aubury offered observations on this case.

The Case in Context: (By Martin Aubury, former head of aircraft structures, Australia's Civil Aviation Safety Authority): "Prior to 737 production line number 291 [which included the Aloha accident aircraft] ... two adjacent skins overlapped and three rows of rivets were backed up with cold bonding. Because the countersunk rivet head was deep in a 0.036-inch-thick skin, it created a sharp 'feather edge' at the rivet hole. This area became a critical stress raiser whenever the bond failed and led to fatigue cracking in the top skin, at the top row of rivets.

"The fix was to drill out the top row of flush rivets and replace them with button-head rivets. In fact, it was a mandatory action after the Aloha accident.

"Post line number 291, cold bonding ceased and the inner, hot bonded 'waffle' sheet was extended to become a doubler along the lap joint. Thus there was a three-thickness overlap. Provided the bond between the two outer thicknesses was sound, the rivets were effectively countersunk into a 0.072-inch-thick skin. This arrangement eliminated the main cause of fatigue cracking at the top row of rivets on the outer skin.

"For aircraft both pre- and post-line number 291, nothing was done about the (initially less critical) inner sheet at the bottom row of rivets. There are no countersunk holes in the inner skin, so there is less stress concentration and longer fatigue life as a result. But sooner or later, fatigue would happen and cracks would be internal and much more difficult to detect. This seems to be the one issue now coming to a head.

"It took some persuading, but the FAA eventually did call for enhanced inspections of the inner skin at the bottom row. Whatever inspections were instituted did prevent a repeat of Aloha but obviously were not adequate to prevent damage-and potential blowout-found on the US Airways jet."