Flight safety authorities in the United States are investigating two separate Boeing 737 flight incidents operated by Southwest Airlines.
The US National Transportation Safety Board (NTSB) said on Friday, local time, it is investigating why a Boeing 737 MAX 8 "rolled" during a flight last month.
The NTSB said the plane experienced what the crew said was a "Dutch roll" at 34,000 feet while en route from Phoenix, Arizona to Oakland, California on May 25. Such lateral asymmetric movements are named after a Dutch ice skating technique and can pose serious safety risks.
The board said pilots regained control of the plane, landed it safely and no-one among the 175 passengers and six crew were injured during the incident.
In a subsequent inspection, Southwest found damage to structural components, the NTSB said.
The Federal Aviation Administration (FAA), which said damage was seen in a stand-by power-control unit, said it is also investigating and working "closely with the NTSB and Boeing to investigate this event".
Boeing declined to comment on the Dutch Roll incident, referring questions to Southwest, which said it is participating in the investigation.
Plane dropped 4,400 feet a minute
Separately, the FAA confirmed it is investigating another Southwest 737 MAX 8 passenger flight in April that came about 400 feet (122 metres) of the ocean off the coast of Hawaii after bad weather conditions prompted pilots to bypass a landing attempt at Lihue airport in Kauai.
During the go-around, the first officer "inadvertently pushed forward on the control column while following thrust lever movement commanded by the autothrottle," according to a June 7 memo seen by Reuters, and the plane began to descend rapidly hitting a maximum descent rate of about 4,400 feet a minute.
The memo said "safety data confirmed the crew received a "DON'T SINK" oral warning followed by a "PULL UP" oral warning, but the first officer later said the crew did not hear the warnings.
The pilots in a post-debrief said seeing the severity of the flight "through the animations was a significant, emotional event," the Southwest memo said, adding the crew participated in comprehensive corrective actions and the airline is reviewing data and trends related to its procedures, training, standards, and performance.
Southwest said in a statement "the event was addressed appropriately as we always strive for continuous improvement".
Bloomberg News first reported the Hawaii flight.
Airplane troubles
The two incidents are the latest in a series of troubles for the aviation industry.
Boeing aircraft have been involved in a number of incidents.
In January, a door panel flew off a Boeing 737 MAX 9 operated by Alaska Airlines, which was later found to be missing four bolts.
An engine cowling fell off a Boeing 737 MAX 9 operated by Southwest and struck its wing flap during take-off on a flight in Denver in February.
More recently a Boeing 777-300ER hit by severe turbulence in May, dropped 54 metres, resulting in a 73-year-old man's death and dozens of serious injuries.
Closer to home, a LATAM flight in March from Auckland to Sydney suddenly dropped causing injury to 50 passengers in the Boeing 787-9.
Airbus has also had troubles this year.
A Japan Airlines Airbus A350 airliner burst into flames after a collision with a Coast Guard aircraft at Tokyo's Haneda airport in January.
Five of the six crew on board the Coast Guard plane died and at least 17 people on the passenger plane were injured.
Reuters/ABC