Southwest Airlines canceled 70 more flights Monday as it inspected older planes for cracks in the fuselage. The airline grounded dozens of planes over the weekend after a Boeing 737-300 sprang a 5-foot hole in the roof. It happened shortly after takeoff from Phoenix.
A Southwest Airlines spokesperson said Monday that inspections in between flights vary from day to day and place to place, but at a minimum, when the 737 landed that morning at Midway, there would have been an exterior walk-around by crew members.
Of course, the eventual problem was on the roof, fatigue cracks in the metal 25 feet up-- not visible in the walk-around at Midway and not subject to any pre-flight inspection requirement.
Federal aviation records show that the Southwest plane, tail number N632SW, was frequently at Midway, the carrier's busiest airport.
A flight tracking website reveals that, last Friday, the plane from Baltimore to Chicago landed at Midway just after 11 a.m. Records show the Boeing 737 was at Midway on the ground for about one hour and then took off on the rest of its schedule, flying south across Illinois to St. Louis, and finally that afternoon to Phoenix.
"All required aircraft inspections were up to date and no discrepancies were found," said Robert Sumwalt, National Transportation Safety Board.
According to Southwest maintenance records, when the jetliner landed at Midway the morning of April 1, there had been a service difficulty report filed just five days earlier, one of 38 such reports related to cracks, damage or corrosion in the fuselage; although, that number is not unusual for a 15-year-old aircraft.
Three of the service difficulty reports concerned damage from the fuselage section that ruptured after takeoff from Phoenix on Friday.
Fuselage metal fatigue was the subject of an article in a trade publication last month, so if there were concerns, now there are questions about why that specific area of every 737 was not subject to federally-mandated inspections.
"The manufacturer, the FAA and the industry have not believed that this particular lap joint on this model airplane was one that warranted attention on aircraft that only had this many takeoffs and landings," said Sumwalt. "So this was an area that could fail until we see it now."
Inspections by Southwest since the accident have turned up three additional 737s with similar cracking on the fuselage.
Tuesday, federal authorities will order emergency inspections of 175 Boeing 737s, planes that make frequent takeoffs and landings. Stress can produce the kind of weakness that seems to have caused the Southwest roof to peel back last week.