The nose-pointed-down landing described by investigators violated the airline's normal procedures, Southwest said.
Flight 345 from Nashville, Tenn., skidded off the runway Monday and came to rest on its nose after the front landing gear crumpled. About 150 people were aboard the Boeing 737, and 16 passengers suffered minor injuries.
The National Transportation Safety Board released preliminary information about the accident late Thursday. The finding was based on video and other evidence, it said.
NTSB spokesman Eric Weiss would not say whether pilot error contributed to the hard landing or if the nose gear should have been able to withstand the impact.
"That's something we're going to look at during the investigation," Weiss said.
Whitney Eichinger, spokeswoman for Dallas-based Southwest, said the "landing scenario that the NTSB described is not in accordance with our operating procedures." She provided no further details.
About 4 seconds prior to touchdown, the plane's pitch was about 2 degrees nose-up, the NTSB revealed. When the plane touched the runway, it was pitched down about 3 degrees, it said.
Pilots are trained to land on the main landing gear, which are much sturdier than the nose wheel, and then gently ease the nose of the plane down until the nose wheel is on the runway.
Denny Kelly, a retired airline pilot and now a consultant and private accident investigator in Dallas, said that if the Southwest plane was pointing nose-down 3 degrees when it touched the runway, "That's a problem."
"If you are pitched down at all, you will land on the nose wheel. At touchdown, the nose should be pointed up at 3 to 5 degrees," Kelly said.
The NTSB had said Wednesday that the landing gear "collapsed rearward and upward into the fuselage, damaging the electronics bay that houses avionics."
It posted a photo showing the jet's electronics bay penetrated by the landing gear with only the right axle still attached.
Investigators recovered the flight data and cockpit voice recorders on Tuesday. They'll be analyzed by the NTSB.
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