Incident Overview
A Beech A36 aircraft, registration N2057A, was vectored for an instrument landing system (ILS) approach to Runway 17L while a heavy Boeing 747 was vectored for an ILS approach to Runway 18R. The arrival controller stated he had staggered the A36 behind the 747 with at least three miles of separation. After the A36 was established inbound on the localizer, it encountered wake turbulence from the 747. The pilot recovered and landed the aircraft safely.
ATC and Flight Paths
According to the controller, the 747 was approaching the ILS 18R localizer from the west when its crew was instructed to make a hard right turn to join the localizer. During this turn, the 747 descended from 5,700 feet to 4,100 feet, then continued descending to 3,000 feet while inbound on the approach. Radar data revealed that the controller had delayed the vector for the 747 to the 18R localizer, and the 747 overshot its localizer, converging to within 600 feet of the 17L localizer.
Separation and Wake Turbulence
As the A36 turned to intercept the 17L localizer, it had an interval of approximately 90 seconds behind the 747. Investigation found evidence that wing tip vortices from the 747 settled at a rate of 400 to 600 feet per minute and drifted toward the A36's flight path. Standard procedures required five miles of separation when a small aircraft is flying directly behind and within 1,000 feet below a heavy jet.
Conclusion
The incident highlights a deviation from required separation minima, but no causal factors are stated in the source.