Federal investigators are trying to learn what caused a footwide hole to blow open as a Southwest Airlines jet flew high above West Virginia, reviving concerns about lax maintenance practices that embarrassed the carrier last year.

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DALLAS — Federal investigators are trying to learn what caused a footwide hole to blow open as a Southwest Airlines jet flew high above West Virginia, reviving concerns about lax maintenance practices that embarrassed the carrier last year.

“We’re working as quickly as possible to figure out what happened to this plane,” The Federal Aviation Administration (FAA) Administrator Randy Babbitt said in a prepared statement.

Southwest said it visually inspected its fleet of 181 Boeing 737-300s Monday night after the incident and found no problems.

The flight from Nashville, Tenn., to Baltimore/Washington International Airport carried 126 passengers. One person seated near the hole, which witnesses said was about the size of a football, told reporters that it caused the loudest noise he had ever heard. The plane, flying at 34,000 feet, lost cabin pressure, and passengers had to use oxygen masks from overhead bins.

The FAA dispatched an accident investigator and a Boeing 737 structural specialist, along with a local inspector, to Charleston to participate in the National Transportation Safety Board’s accident investigation.

The 737-300 series makes up a third of Southwest’s fleet of 544 aircraft, which have an average age of 10 years.

Bill Voss, president of the nonprofit Flight Safety Foundation, said the situation could involve ground damage or undetected fatigue cracks in the fuselage, a known problem for aging aircraft.

An FAA spokesman said the Southwest jetliner was delivered in 1994. As a 300-series plan, it is one technological generation behind Boeing’s latest 737 planes, the 600 through 900 series. Because of past problems with cracks, the planes are supposed to be inspected rigorously.

“It does seems like fatigue to me, based on what I’ve learned,” said Fred Mirgle, chairman of the aviation-maintenance-science area of Embry-Riddle Aeronautical University. “I don’t think Southwest is an unsafe airline, but they’ve had some issues.”

The FAA fined Southwest $10.2 million last year after regulators determined that the airline had knowingly flown 46 jets that were overdue for fuselage inspections required by a 2004 regulation. Six jets were found to have cracks in their fuselages.

As part of a settlement in March, Southwest paid a reduced fine, $7.5 million, but added maintenance staff and rewrote some of its maintenance manuals.

Maintenance records for the plane show it underwent a heavy review in early January, during which its frame was stripped apart and put back together. That review revealed some cracks but nothing out of the ordinary for a plane of its age, FAA officials said.

In 1988, a section of a Boeing 737 operated by Aloha Airlines blew open at 24,000 feet. A flight attendant was sucked through the hole and killed.