DALLAS — Investigators say a Southwest Airlines jet that exhibited an unusual “Dutch roll” motion during flight was parked outside during a severe storm after which underwent routine maintenance, after which pilots noticed strange movements within the rudder pedals.
After the May 25 incident, Southwest mechanics found “significant” damage to the tail of the plane, where the rudder is situated, however the National Road Safety Committee said on Tuesday that it was not yet known when the damage occurred.
The aircraft, a Boeing 737-800was grounded for greater than a month but resumed operations last week, in response to data from Flightradar24.com.
The Dutch roll is a swaying, rhythmic combination of yaw, or sideways sliding of the tail, and pitching of the wingtips. The Southwest jet experienced this motion at 34,000 feet and again after descending to 32,000 feet on the flight from Phoenix to Oakland, California.
This condition will be dangerous, and modern aircraft have a yaw damper to stop the vibrations typical of the Dutch roll motion.
After the plane landed, Southwest mechanics discovered fractures within the metal bracket and ribs that hold an emergency power unit to the rudder system. Investigators examined the damaged parts last week in Ogden, Utah.
The NTSB said the plane was parked overnight on the New Orleans airport on May 16 during thunderstorms with wind gusts as much as 85 mph, heavy rains and a tornado warning.
John Cox, a former pilot turned safety consultant, said the NTSB's preliminary report indicates the plane was more than likely damaged in the course of the storm. He said the near-hurricane-force winds can have caused the parked jet's rudder to swing forwards and backwards.
Cox said it was “absolutely impossible” that the Dutch Roll could have caused such severe damage and that he didn’t consider it was related to maintenance work.
“I don't see this as a Max problem. I don't see this as a 737 problem at the moment,” he said. “I see this as a one-off event.”
Southwest inspected its 231 Max jets last month and located no further damage to the rudder actuators, in response to the NTSB. No problems have been encountered in the brand new planes the corporate has received since then.
Dallas-based Southwest declined to comment.
It could take a yr or more for the NTSB to find out a probable cause for the incident.
image credit : www.mercurynews.com