DEN93IA044

On a visual approach to runway 26L, united flight 409, a Boeing 737, experienced an uncommanded roll of about 23 deg at an altitude of 882' AGL. The crew was able to correct the roll & made a go-around to an uneventful second approach & landing. The Boeing 737 was following a Boeing 757, which was on a visual approach to runway 26R. The two runways were 900' apart from centerline to centerline & the threshold of 26L was offset about 1300' east of 26R. According to data from the flight recorder, the wind at the onset of the incident was from the northwest at 5 to 10 knots. The surface wind was from 360 degrees at 10 knots. The two aircraft were 1.32 nautical miles apart in an offset trail position. Both aircraft were on 3 deg glide slopes. The wake vortices had settled about 100' & moved laterally toward the glide path of 26L. Weather data showed evidence of possible interaction between wake turbulence from the 757 & an outflow boundry, which was moving through the area.

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.