Southwest Airlines said Tuesday it has finished inspecting its 79 older Boeing 737-300 planes and five of them have small, subsurface cracks in the aluminum skin. The airlines operations were returning to normal after about 600 cancellations over the weekend and another 70 on Monday.
Meanwhile, the FAA says it wants to take a look at the aging U.S.-registered 737s - 78 belonging to Southwest and two at Alaska Airlines. Its emergency order is aimed at finding weaknesses in the metal exterior, but virtually all of the affected aircraft have already been inspected.
Southwest grounded 79 Boeing 737-300s for inspections after a jet leaving Phoenix lost pressure Friday, forcing pilots to make an emergency landing 125 miles away in at a military base in Yuma, Ariz. A piece of fuselage tore open 34,000 feet up, leaving a hole about 5-feet long in the roof of the cabin. None of 118 passengers or crew members were injured in the incident.
Southwest said it has already considered whether or not any other aircraft in its fleet may need to be inspected.
"We will be looking to see if this issue does relate to other airplanes in the fleet. But right now, we feel that through issuing this service bulletin that will take care of those airplanes that need attention but for the remaining fleet, I don't believe that there are structural deficiencies," said Robert Sumwalt, National Transportation Safety Board member.
One source of stress to an aircraft that's particular to Southwest is the number of times it takes off and lands. Southwest planes get quite the workout, each of them averaging about six flights a day. That's being looked at as a major contributing factor to the relatively young aircraft on Friday.
Cracks can develop from the constant cycle of pressurizing the cabin for flight, then releasing the pressure upon landing.
Since there had been no previous accidents or major incidents involving metal fatigue in the middle part of the fuselage, Boeing maintenance procedures called only for airlines to perform a visual inspection.
The FAA's emergency order will require initial inspections using electromagnetic devices on some Boeing 737 aircraft in the -300, -400 and -500 series that have accumulated more than 30,000 takeoffs and landings. It will require repetitive inspections at regular intervals.
The 15-year-old Southwest jet in Friday's incident had logged 39,000 pressurization cycles, a measurement of the number of takeoffs and landings. That's 7.2 cycles every day for every year it has been in service.
Planes that have 30,000 cycles or have been in service for 15 years are considered about halfway through their useful life.
Boeing Co. said Monday that it will issue guidance this week on how airlines should do checks on the affected airplanes now in service. An estimated 1,800 airplanes, including -300, -400, -500 model 737s, are affected by the aircraft maker's service bulletin.
Southwest officials said the Arizona flight was given a routine inspection on Tuesday and underwent its last so-called heavy check, a more costly and extensive overhaul, in March 2010.
The Associated Press contributed to this report.