PHOENIX — Boeing engineers figured the joints holding the skin in place on their older 737 jetliners would begin to wear, but only as the planes neared retirement. They never expected it to happen in middle age.
Caught off guard when a piece of the fuselage on a Southwest Airlines jet peeled away last week, they are rushing to create inspection and repair instructions for hundreds of similar planes.
Federal aviation officials issued an emergency order Tuesday that requires inspections of certain types of 737s.
Southwest, which operates nearly all of the U.S.-registered 737s requiring urgent inspections, inspected its planes and found five with the same types of cracks suspected of causing the 5-foot-long hole to open on Flight 812 on Friday. While those planes are being repaired, the rest are again heading into the skies.
The failure raised concerns about the adequacy of safety inspections that failed to catch the problem even though nearly two dozen other instances of metal fatigue were spotted during an inspection of the Southwest plane a year earlier.
And it also focused attention on the specific 737 model. That model was redesigned after similar joint problems caused a huge section of the roof of an Aloha Airlines jet to break off in 1988. A flight attendant was sucked out and died.
"We want to understand why we saw the extent of tearing on the aircraft and this size of a rupture so that we can prevent it from happening again," National Transportation Safety Board Chairwoman Deborah Hersman told reporters Tuesday.
Boeing never expected failures in the riveted skin joints running along the top of the 737-300, 737-400 and 737-500 models until the planes were much older, said Paul Richter, Boeing's top engineer for older 737s.
Richter said Boeing also didn't anticipate the need to inspect for cracking on the redesigned lap joints — where two pieces of the fuselage skin overlap — until a 737 had reached 60,000 pressurization cycles, the number of takeoffs and landings.
The Southwest jet, which made an emergency landing in Yuma, Ariz., had about 39,000 cycles and was 15 years old. Pressurizing and depressurizing the cabin can weaken the aluminum skin and cause cracking.
Boeing's "service bulletin" and the FAA order will require inspections after 30,000 cycles. The company has written repair plans for three of the five Southwest planes with cracks and is examining the rest.
The aircraft maker said 579 airplanes in all will eventually need the stepped-up inspections, though it did not give a breakdown about the numbers in use in the United States and overseas.
Boeing's service bulletin requires inspections within 20 days for all of the planes in the fleet with more than 30,000 cycles. The FAA will require a repeat inspection every 500 flights, Richter said.
The FAA is requiring about 175 planes to be inspected at once, with nearly all of the 80 in the United States belonging to Southwest. Carriers in other nations will be covered by the order because of agreements those countries hold with the FAA.
The only way to find the subsurface cracks or the ones hidden between the overlapping pieces of the fuselage would be to use an electromagnetic technology known as eddy current.
Boeing started making the 300 model in 1993 and delivered the last of the "classic" series in 2000.