FTW97LA070

An Air Wisconsin Airline flight, operating as United Express Flight 684, had been cleared to descend from FL190 to 13,000 feet on the Thompson One Arrival into Denver. As the airplane was passing 14,000 feet, it encountered unreported and unanticipated clear air turbulence. Two flight attendants, making their pre-landing check of the cabin, were seriously injured. One flight attendant received fractures to her pelvis, collarbone, and two ribs. The other flight attendant sustained a fractured ankle.

On December 20, 1996, approximately 1750 mountain standard time, a British Aerospace BAe 146-200A, N606AW, operated by Air Wisconsin Airlines Corporation, d/b/a United Express Flight 684, sustained no damage when it encountered severe turbulence 30 miles northeast of Denver, Colorado. There were no injuries to the two cockpit crew members and 12 passengers, but two flight attendants sustained serious injuries. Visual meteorological conditions prevailed, and an IFR flight plan had been filed for the scheduled domestic passenger flight conducted under Title 14 CFR Part 121. The flight originated at Eagle, Colorado, on December 20, 1996, approximately 1730.

The flight had been cleared to descend from FL190 to 13,000 feet on the Thompson One Arrival into Denver. According to the captain's statement, they encountered "one sharp clear air turbulence jolt. Duration was approximately 20 to 30 seconds with moderate turbulence the remainder." The airplane landed without further incident and as it was being taxied to the gate, a passenger contacted the cockpit crew via interphone to advise that both flight attendants had been injured.

According to the first officer's statement, the airplane was passing 14,000 feet at 240 knots when speed increased to 270 knots, followed by a severe downdraft. The airplane was "displaced only in pitch, not roll." The turbulence encounter lasted approximately 10 to 15 seconds, followed thereafter by only occasional light turbulence. A passenger then notified the crew via intercom that both flight attendants had been injured. The airplane landed at Denver at 1805 and was met by paramedics who transported the injured flight attendants to a Denver General Hospital.

The flight attendant most seriously injured sustained a fractured pelvis, fractured collarbone, and two fractured ribs. In her written statement, she said it had been a smooth flight. She was walking forward to complete the passenger landing check when the airplane hit turbulence and she was thrown off her feet, striking the ceiling.

The other flight attendant suffered a fractured ankle. In her written statement, she said the previous flight to Eagle had been routine with no significant weather and very little turbulence. As the airplane began its descent into Denver, the FASTEN SEATBELT sign was illuminated as she and the other flight attendant made their pre-landing checks. She stood to give the passengers instructions over the public address system while the other flight attendant walked through the cabin. She did not recall the turbulence encounter.

According to the Denver Air Route Traffic Control Center (ARTCC) and TRACON (terminal radar approach control), no turbulence warnings were issued to United Express flight 684 because they had not received any turbulence reports from other inbound flights.

The digital flight data recorder was removed from the airplane and shipped to NTSB's laboratory in Washington, DC, for readout and analysis. According to the laboratory's report, "the aircraft was descending through 14,000 feet when significant excursions in the aircraft's vertical and lateral acceleration occurred. During this time the indicated airspeed increased from 272 to 284 knots. Four seconds after the disruptions began the autopilot disconnected... Immediately thereafter the aircraft's pitch increased about 8 degrees and the vertical acceleration spiked sharply going from a minimum 0f -.75 g's to a maximum of 2.37 g's in about one second. These events were accompanied by a right roll of approximately 6 degrees, yet during this time the aircraft yawed to the left. The airbrake was applied...and the aircraft began to decelerate."