DALLAS (AP) — Investigators say a Southwest Airlines jet that crashed unusual “Dutch roll” in flight it was parked outdoors during a severe storm and then underwent routine maintenance, after which the pilots noticed strange movements in the rudder pedals.
After the May 25 incident, Southwest mechanics found “Substantial” damage in the tail of the aircraft, where the rudder is located, but the National Transportation Safety Board said Tuesday it has not determined when the damage occurred.
The plane, a Boeing 737 Maxit was suspended for more than a month, but resumed flights last week, according to data from Flightradar24.com.
The Dutch roll is an oscillating, rhythmic combination of yaw, or the tail sliding to the side and the wingtips bobbing up and down. The Southwest jet experienced the motion at 34,000 feet and again after descending to 32,000 feet while flying from Phoenix to Oakland, California.
The condition can be dangerous, and modern planes have a “yaw damper” to stop the oscillations that characterize Dutch roll.
After the plane touched down, Southwest mechanics found fractures in the metal bracket and ribs that secure a backup power control unit to the rudder system. Investigators examined the damaged parts last week in Ogden, Utah.
The NTSB said the plane was parked overnight at the New Orleans airport on May 16 during thunderstorms with high winds of up to 84 mph, heavy rain and a tornado warning.
On May 23, the plane underwent scheduled maintenance, and later the pilots noticed the rudder pedals moving when the yaw damper was engaged. The pilots on the May 25 flight felt the pedals moving during the Dutch rotation and even after landing, the NTSB said.
John Cox, a former airline pilot and now a safety consultant, said the NTSB’s preliminary report indicates the plane was likely damaged during the storm. He said near-hurricane-force winds may have caused the parked jet’s rudder to rock back and forth.
Cox said there was “no way in the world” the Dutch roll could have caused such severe damage, nor does he think it was related to maintenance work.
“I don’t see it as a Max problem. I don’t see it now as a 737 problem,” he said. “I see it as something unique.”
Southwest inspected its 231 Max jets last month and found no other instances of damage to the rudder propulsion units and no problems in the new planes it has received since then, according to the NTSB.
Dallas-based Southwest declined to comment.
It could be a year or more before the NTSB determines a probable cause for the incident.
