BOEING 737 near Camp Springs, MD — 2011-04-18

Probable cause

The misapplication of wake turbulence and instrument flight rules (IFR) separation criteria by air traffic controllers.

NTSB narrative

A Boeing 737 (B737) and a heavy Boeing C-17 were being provided radar vectors for instrument landing system approaches to the runway by a controller at the Potomac Terminal Radar Approach Control (TRACON) facility. The controller vectored the C-17 onto the base leg of the traffic pattern ahead of the B737 to join the localizer for the approach. At this point, the airplanes were separated by 4.45 miles on a converging course with a closure rate of 180 knots. The C-17 was subsequently vectored to join the localizer and cleared for the approach. The controller then issued a wake turbulence advisory to the B737 and informed the pilot he would be 4 miles in trail of the C-17. The pilot of the B737 acknowledged the advisory, but, at this point, the airplanes were 3.62 miles apart. The minimum required wake turbulence separation was 5 miles. Both airplanes were directed to contact the air traffic control tower local controller. As both airplanes reduced their speeds for landing during final approach, their separation was reduced to 2.81 miles. The minimum instrument flight rules separation was 3 miles. The local controller authorized the B737 to make "S" turns on final approach to increase separation; however, when the B737 was 2 miles from the landing threshold, the B737 pilot told the local controller, "it doesn't look like we're going to make this," since the C-17 was still conducting the landing roll on the runway. The B737 pilot was instructed to perform a go-around, entered a left downwind for the runway, and landed uneventfully. The TRACON controller responsible for the wake turbulence operational error reported after the incident that he confused the minimum wake turbulence separation requirements for a B737 following a C-17 with the requirement for a B757 following a B757. The B757 requirement was 4 miles. The TRACON controller had been involved in 4 other operational errors in the 4 years preceding this incident.

NTSB coding

• Near midair/TCAS alert/loss of separation · Approach-IFR final approach
• Decision Making / Judgment