On Sunday, April 23, 1993, at 2020 mountain daylight time, a Boeing 737-522, N905UA, operated by United Airlines as flight 409 experienced an uncommanded roll at approximately 1,000 feet above ground level (AGL) while conducting a visual approach to runway 26 left at Stapleton International Airport, Denver, Colorado. There were no injuries to the 2 pilots, 3 flight attendants, or 128 passengers, and the aircraft was not damaged. The aircraft was being operated under the provisions of 14 CFR Part 121 when the incident occurred. Visual meteorological conditions prevailed and an IFR flight plan was on file. The flight originated from San Antonio, Texas, at 1830.

At the onset of the roll, according to the crew, maximum power was applied and a go around initiated. The roll was corrected by the crew and an uneventful landing was made on the next approach.

Radar data provided information that the incident aircraft was following a Boeing 757 which was conducting a visual approach to runway 26R. The two runways are 900 feet apart centerline to centerline and the incident aircraft was 1.34 miles (32 seconds)in trail behind the B757. According to flight recorder data from both aircraft, wind at the time of onset of the occurrence was from the northwest at 5 to 10 knots. The wind values changed to the northeast on the incident aircraft, during the occurrence, and reached a peak value of 30 knots. According to flight recorder data, the incident aircraft achieved a maximum deflection of 23.027 degrees left wing down during the uncommanded roll. This occurred at a radio altitude of 882 feet above ground level. A flight recorder factual report and radar data is attached.

In addition, research was conducted by NOAA in Boulder, Colorado, in the area of the interaction of wake turbulence with an outflow boundary. The research provided information that wake turbulence can be lifted up as an interaction with the boundary, and that an outflow boundary was moving through the area at the time of the incident.

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