Southwest found more cracks in grounded 737-300s, a plane Boeing expected would fly much longer before developing fuselage fatigue. Image: CC gTarded/Flickr

Last Friday, Southwest Airlines faced a public relations nightmare when a five-foot crack broke open in the fuselage of a Boeing 737-300 in mid-flight. Southwest grounded its older 737-300s, and inspections revealed five additional planes with cracks in the aluminum skin. Boeing expressed surprise that cracks developed in the planes long before metal fatigue was expected to set in.

Southwest pushes Boeing 737-300s hard

When the crack opened in the fuselage of Southwest flight 812 from Phoenix to Sacramento Friday, the cabin depressurized and passengers said they could see blue sky through the hole in the roof of the plane. Southwest flies a fleet of 548 737-300s–more than any other airline. Southwest makes more short flights, putting the planes through several takeoff and landing cycles a day. Each takeoff and landing stresses the fuselage skin as the cabin pressure changes. The average age of Southwest’s fleet of 737s is 11 years old, and each plane makes about six flights a day. Airlines inspect planes with greater frequency as they get older, and the 737 damaged Friday is 15 years old. According to Southwest, the plane underwent a routine inspection on March 29 and had an extensive overhaul in March 2010.

Fuselage cracks a familiar problem for Southwest

Fuselages have torn on planes operated by Southwest before. In 2009, an 18-inch gash opened in a Southwest Boeing 737-300 flying between Nashville and Baltimore at 35,000 feet, forcing an emergency landing in West Virginia. That year Southwest paid a $7.5 million Federal Aviation Administration fine for skipping required inspections for fuselage cracks on nearly 60,000 flights. After the incident Friday, Southwest grounded 79 older Boeing 737-300 planes, causing about 600 flight cancellations over the weekend and another 70 Monday. After Boeing and Southwest inspected the grounded planes using a low-frequency current to analyze the aluminum skin for weakness, cracks were found in five planes. Southwest expected to have nearly all of the grounded 737-300s back in service Wednesday. The planes with cracked skin will stay grounded until Boeing and Southwest comes up with a plan to repair them.

Boeing overestimates fuselage integrity

Southwest accounts for a majority of the “high-cycle” 737s flying in the U.S. A high-cycle plane, such as the plane damaged on the Phoenix flight, has completed up to 39,000 takeoff and landing cycles. A senior Boeing engineer told the New York Times that the company expected the aluminum skin and the supporting joints on the 737-300 to endure 60,000 flight cycles before airlines should to be concerned about fuselage cracks. On Tuesday the FAA issued an emergency directive that all Boeing 737-300s with  more than 35,000 flight cycles must be inspected within the next five days. All 737-300s with 30,000 to 35,000 flight cycles must be inspected within 20 days. Boeing said all 737-300s, 400s and 500s built from 1993 to 2000 will eventually be reaching those flight cycle numbers, a total of 570 planes.


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