Boeing plans to issue a Service Bulletin to describe inspection techniques for Boeing 737s similar to the Southwest Airlines 737-300 whose fuselage skin ruptured while on a scheduled flight from Phoenix to Sacramento on April 1. Flight 812 diverted to Yuma, Ariz., for an emergency landing at 5:07 p.m. after a hole developed in the top of fuselage.

Mechanics from Southwest Airlines, under the supervision of NTSB investigators, yesterday removed a section of the ruptured fuselage skin and prepared to send it to NTSB headquarters in Washington, D.C., for in-depth analysis.

Meanwhile, NTSB investigators conducted additional inspections of other portions of the lap joint along the fuselage of the accident airplane and found evidence of additional cracks.

Over the past few days, Southwest Airlines has conducted inspections on several of its 737 airplanes, three of which showed cracks in the lap joints. Southwest said it expects to finish inspecting all 79 of the airplanes it grounded as a result of the incident by tomorrow night.

While Boeing develops the specifics of the Service Bulletin, the NTSB recommends a “focus” on the left and right lap joints on all similar 737 airplanes that have flown as many cycles as the accident airplane. Once Boeing releases the Service Bulletin, the FAA will determine whether to make it mandatory for all similar 737s.

Southwest’s inspections forced the cancellation of 300 flights yesterday and the airline expected to cancel another 70 today. The NTSB has scheduled a press briefing on the matter for 1:30 p.m. PST at Yuma County Airport.