Federal officials said "severe clear air turbulence" is suspected to have injured nearly a dozen people on a Feb. 17 Denver-to-Billings flight, after which five people were taken to the hospital.
The National Transportation Safety Board began investigating the incident a few days after it happened and released its preliminary report on United Airlines Flight 1676 and the jarring turbulence it ran into shortly before beginning its descent into Billings.
The flight "encountered severe clear air turbulence while in cruise flight at flight level 340," which is an altitude of about 34,000 feet, the report said.
According to the National Oceanic and Atmospheric Administration, clear air turbulence (CAT) "is the bumpiness experienced by aircraft at high altitudes (above 18,000 feet) in either cloud-free conditions or in stratiform clouds. CAT occurs when undulations (known as gravity waves) in the upper atmosphere become steep and unstable, then break down into chaotic motion."
Such turbulence can often be difficult or impossible to detect ahead of time and, according to the NTSB report, the crew on Flight 1676 reported clear weather when the turbulence happened.
The NTSB said that, of the 114 passengers and crew aboard the flight, two suffered serious injuries while nine suffered minor ones. In a news statement after the flight, which landed safely in Billings after the turbulence, United said that three crew members and two passengers were taken to Billings hospitals.
All but one, a flight attendant, were released within a day. That attendant, whose name has not been released, remained hospitalized for several days.
While it wasn't immediately clear how long she remained in the hospital, the attendant's mother wrote a letter to the editor, published in the Gazette on March 10, thanking health care officials, emergency responders, United Airlines and the community of Billings for taking care of her daughter. The letter said her daughter spent time at St. Vincent Healthcare and New Hope Rehabilitation Center.
"I am happy to report that my daughter is now recovering in her own home," Ruth Behnke, of Washington, wrote in the letter.
The airplane, a Boeing 737-700, suffered minor damage, according to the report, and remained on the ground in Billings until it was flown two days later to Houston, where it arrived safely at a United maintenance facility.
A United spokeswoman said on Tuesday that the plane is now back in service and that United continues to work with the passengers from the flight.
"Our general manager and his entire staff greeted the aircraft and assisted passengers upon arrival in Billings," spokeswoman Christen David said in an email to the Gazette. "Shortly after the incident, our Customer Care team reached out to all of the passengers for whom we had contact information. Additionally, we continue to work with the passengers who need additional assistance."
While the NTSB has issued its preliminary report, its final report with a final decree on the cause of the turbulence and other information could take months to complete as investigators comb through interviews and other details from the flight.
"This is preliminary information, subject to change, and may contain errors. Any errors in this report will be corrected when the final report has been completed," the report said.