Incident Overview
On a visual approach to Runway 26L, United Flight 409, a Boeing 737, encountered an uncommanded roll of approximately 23 degrees at an altitude of 882 feet above ground level (AGL). The flight crew successfully corrected the roll and executed a go-around, followed by an uneventful second approach and landing.
Flight Details
The Boeing 737 was following a Boeing 757 that was on a visual approach to the parallel Runway 26R. The two runways are separated by 900 feet from centerline to centerline, with the threshold of Runway 26L offset about 1,300 feet east of Runway 26R. At the time of the incident, the two aircraft were 1.32 nautical miles apart in an offset trail position, both on 3-degree glide slopes.
Weather and Wake Turbulence
According to flight recorder data, at the onset of the incident the wind was from the northwest at 5 to 10 knots. The surface wind was recorded from 360 degrees at 10 knots. The wake vortices from the preceding Boeing 757 had settled about 100 feet and moved laterally toward the glide path of Runway 26L. Weather data indicated possible interaction between the wake turbulence from the 757 and an outflow boundary moving through the area.
Crew Response and Landing
The crew immediately corrected the uncommanded roll and initiated a go-around. The second approach to Runway 26L was conducted without further incident, and the aircraft landed safely.