A wake turbulence operational error occurred at Andrews Air Force Base (ADW), Camp Springs, Maryland on April 18, 2011 at 17:06 eastern daylight time. The error involved two aircraft that were inbound to ADW; Executive 1 Foxtrot (EXEC1F), a Boeing 737 and Reach 3115 (RCH3115), a heavy Boeing C-17. ADW was on a south flow landing runway 19L. The parallel runway at ADW, runway 19R, had been closed for approximately one year undergoing major renovation.
At 17:00, the B737 was 20 miles northeast of the airport heading 230 degrees at 6000 feet indicating 210 knots. The C-17 was 7 miles east of the airport heading 360 degrees at 3000 feet indicating 240 knots. Both aircraft were approximately 20 flying miles from the ADW runway 19L landing threshold. Both aircraft were being provided radar vectors by the Deale/ADWAR sector controller at Potomac Terminal Radar Approach Control (TRACON) for instrument landing system (ILS) approaches to runway 19L at ADW.
At 17:01, the Deale/ADWAR sector controller directed the B737 to reduce speed to 190 knots and directed the C-17 to turn left heading 350 degrees and to descend and maintain 2000 feet. The two aircraft were 12 miles apart. Continuing to sequence both aircraft, the Deale/ADWAR sector controller directed the B737 to descend and maintain 5000 feet and radar vectored the C-17 to a heading of 270 degrees. The radar vector put the C-17 on a base leg for runway 19L.
At 17:02:30, the B737 was directed to descend and maintain 4000 feet. The two aircraft were 5.34 miles apart. Twenty-five seconds later, the Deale/ADWAR sector controller radar vectored the B737 that was approximately 15 miles from the runway, to fly heading 210 degrees to join the localizer. The C-17 and the B737 were separated by 4.45 miles on a converging course with a closure rate of 180 knots.
At 17:03 the Deale/ADWAR sector controller directed the C-17 to fly heading 220 degrees to join the localizer and cleared the C-17 for the ILS Runway 19L approach to Andrews Air Force Base. The Deale/ADWAR sector controller provided a wake turbulence advisory to the B737 stating “Executive one foxtrot caution wake turbulence you’ll be four miles in trail of a C-seven, heavy C-17”. The B737 acknowledged the wake turbulence advisory. The aircraft were 3.62 miles apart when the wake turbulence advisory was issued. The minimum required wake turbulence separation for a B737 following a heavy C-17 was 5 miles.
At 17:03:30 the Deale/ADWAR sector controller descended the B737 to 3000 feet. The B737 requested a slower speed and was directed to fly no faster than 170 knots and, prior to having the pilot of the C-17 contact ADW tower, directed the C-17 to keep his speed at no slower than 170 knots until five miles from the runway. When the time the C-17 was directed to contact ADW tower, its groundspeed indicated 152 knots while the B737 groundspeed indicated 170 knots.
At 17:04:25 the Deale/ADWAR sector controller cleared the B737 for approach and directed to the pilot to reduce to final approach speed. The B737 and C-17, both on the final approach course, were separated by 3.3 miles longitudinally, 1,600 feet vertically, and flying at 153 and 155 knots indicated groundspeed respectively.
At 17:05:20 the Deale/ADWAR sector controller directed the B737 to contact ADW tower. The B737 was seven miles from the runway and 3.3 miles behind the C-17. As both aircraft continued on approach, the C-17 slowed to 110 knots groundspeed, while the B737 continued at 150 knots groundspeed, before slowing to 140 knots groundspeed at 17:05:57 when the separation between the two aircraft reduced to 2.84 miles. At 17:06:44 the C17 groundspeed was 106 knots and the B737 groundspeed was 121 knots when the separation between the two aircraft reduced to 2.81 miles longitudinally and 800 feet vertically.
The Deale/ADWAR sector controller had been a controller for 21 years and had been a controller at PCT for 6 years. He declared that he confused the minimum wake turbulence separation requirements for a B737 following a heavy C-17 as 4 miles instead of the required 5 miles between aircraft. The controller stated that he was thinking of 757 following 757 separation at Washington Reagan National Airport and confused the two applications during this incident.
The PCT Mount Vernon Operational Supervisor (MTVOS) was responsible for oversight of the Deale/ADWAR sector controller and other controllers working the Mount Vernon area. During controller interviews, the MTVOS stated that he was monitoring a trainee working one of the busiest sectors in the Mount Vernon area, the TYSON/FLUKE combined sector. The MTVOS stated that he was aware that the Deale/ADWAR sector controller position only had two aircraft, the B737 from the northeast and a C17, and with such a light workload, was not the focus of his attention. The Deale/ADWAR sector controller was relieved from position and went on break shortly after the incident and before the MTVOS had reviewed the event.
At 17:04:43 the C-17 checked in with ADW tower 5 miles from the runway, over the final approach fix and was cleared to land.
At 17:05:39, approximately 6 miles from the runway, the B737 called ADW tower and advised they were inbound on the ILS runway 19L approach. The ADW tower local controller advised the B737 that they had heavy C-17 traffic in front of them and the B737 reported that they had the C-17 in sight. The ADW tower local controller cleared the B737 to land, in sequence, behind the C-17.
When the B737 was 4 miles from the runway the ADW tower local controller advised the B737 that S-turns were approved. These were offered to the B737 because the preceding C-17 was going to the end of the runway on landing rollout and might not have cleared the runway when the B737 crossed landing threshold. Regulations require that a runway be clear prior to a succeeding arrival aircraft cross the landing threshold of the same runway. The B737 acknowledged and turned to the left off the final approach course and then turned to the right toward the final approach course.
The MTVOS received a telephone call from the ADW front line manager (FLM) advising that the B737 was on a 5-6 mile final executing S-turns, where an aircraft on final make a series of turns to increase spacing from the preceding aircraft, on final to maintain separation from the preceding arrival. Shortly thereafter, the ADW FLM called to advise that the B737 was going around. Since all go-around’s were to be documented, the MTVOS reported the go-around to the operations manager (OM) on duty. The OM reviewed the incident and determined that an operational error had occurred between the B737 and the C-17
At 17:07:04, approximately two miles from the runway threshold, the pilot of the B737 reported to ADW tower “it doesn’t look like we’re going to make this”. The ADW tower local controller directed the B737 to “go-around make left closed traffic runway 19L.” The B737 entered a left downwind for runway 19L at PCT and landed uneventfully.
At about the same time that the B737 was reporting that they were not going to make the approach, the ADW FLM called the PCT MTVOS and advised that the C-17 and B737 were 3 miles apart and that ADW had the B737 doing S-turns on final. According to the ADW FLM, the call to PCT was to provide a heads up that the approach sequence between the C17 and the B737 was non-standard. The FLM did not expect a response from PCT but assumed that the PCT MTVOS would look into and address the situation as required. Approximately 1 minute later the ADW FLM called PCT and advised the MVTOS that the B737 was going around and that the aircraft was going to stay in the ADW tower pattern.
After the B737 executed the go-around, the FLM advised the local controller that s-turns were not authorized for an aircraft on an instrument approach and that since the B737 did not cancel his instrument flight rules flight plan, the aircraft should have been transferred back to PCT for another IFR approach.
After returning from his break, the Deale/ADWAR sector controller was de-briefed by the OM regarding the operational error. The controller professed that, until the de-brief, he was not aware of any problems with the C17 and B737 arrival. According to PCT facility management, the Deale/ADWAR sector controller had been certified at the Mount Vernon area since 2007 and in that time had been involved in 4 operational errors involving coordination errors. The controller was de-certified as a result of these errors and subsequently re-certified after training.

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