More Southwest flights canceled

Southwest started grounding airplanes after one of its Boeing 737-300 planes made a safe emergency landing Friday at a military base in Arizona. Investigators and Boeing engineers began inspecting similar 737s for problems. Inspectors have found small, subsurface cracks in three more Southwest Airlines planes that are similar to those suspected of causing a jetliner to lose pressure and make an emergency landing in Arizona, according to federal investigators.

According to The Associated Press, 19 other Boeing 737-300 planes inspected using a test developed by the manufacturer showed no problems and will be returned to service. Checks on nearly 60 other jets are expected to be completed by Tuesday, the airline said.

NTSB board member Robert Sumwalt said Boeing was developing a "service bulletin" for all 737-300 models with comparable flight cycle time as the ruptured jet, which was 15 years old.

There are 931 such models in service worldwide, 288 of which are in the U.S. fleet.

Boeing's bulletin would strongly suggest extensive checks of two lines of "lap joints" that run the length of the fuselage. The NTSB has not mandated the checks.

Friday's flight carrying 118 people rapidly lost cabin pressure after the plane's fuselage ruptured -- causing a 5-foot-long tear.

The tear along the riveted joint near the roof of the plane above the midsection shows evidence of extensive cracking that hadn't been discovered in routine maintenance -- and probably wouldn't have been unless mechanics had specifically looked for it -- officials said.

"What we saw with Flight 812 was a new and unknown issue," said Mike Van de Ven, Southwest's executive vice president and chief operating officer.

Sumwalt said the rip was a foot wide, and that it started along a joint where two sections of the 737's skin are riveted together. An examination showed extensive pre-existing damage along the entire tear.