DALLAS (AP) — A Southwest Airlines jet that experienced an unusual “Dutch roll” during flight had been parked outside during a severe storm and was undergoing routine maintenance after the pilot noticed unusual movement of the rudder pedals, investigators said.
After the May 25 accident, Southwest mechanics found “substantial” damage to the plane’s tail, where the rudder is located, but the National Transportation Safety Board said Tuesday it was unable to determine when the damage occurred.
The plane, a Boeing 737 MAX, had been grounded for more than a month but resumed flying last week, according to data from Flightradar24.com.
A Dutch roll is a rhythmic combination of yaw and wingtip up-and-down motion that a Southwest Airlines jet experienced at 34,000 feet during a flight from Phoenix to Oakland, California, and again after descending to 32,000 feet.
This condition can be dangerous, and modern airplanes are equipped with “yaw dampers” to stop the vibrations that characterize Dutch roll.
After the plane landed, Southwest mechanics found cracks in metal brackets and ribs that secure a backup power control to the rudder system. Investigators inspected the damaged parts in Ogden, Utah, last week.
The plane was parked overnight at the New Orleans airport on May 16 during a thunderstorm with wind gusts of up to 84 mph, heavy rain and a tornado watch, according to the NTSB.
On May 23, the plane underwent routine maintenance, after which the pilot noticed the rudder pedals moving when the yaw dampers were engaged. On a May 25 flight, the pilot felt the pedals moving during the Dutch roll and after landing, according to the NTSB.
John Cox, a former airline pilot and safety consultant, said the NTSB’s preliminary report indicated the plane was likely damaged during the storm. He said hurricane-force winds could have caused the parked jet’s rudder to swing violently back and forth.
Cox said it was “absolutely inconceivable” that the Dutch roll could have caused such severe damage and he didn’t believe it was related to maintenance work.
“I don’t see this as a Max issue. I don’t see this as a 737 issue right now,” he said. “I see this as a one-off issue.”
The NTSB said Southwest inspected 231 MAX jets last month and found no damage to the rudder powertrain area, and has not found any issues with any new planes it has received since then.
Dallas-based Southwest Airlines declined to comment.
It could take more than a year for the NTSB to determine the cause of the accident.
