Introduction
On April 18, 2011, at 17:06 eastern daylight time, a wake turbulence operational error occurred at Andrews Air Force Base (ADW) in Camp Springs, Maryland. The incident involved two inbound aircraft: Executive 1 Foxtrot (EXEC1F), a Boeing 737, and Reach 3115 (RCH3115), a heavy Boeing C-17. ADW was operating with a south flow, landing on runway 19L. The parallel runway 19R had been closed for approximately one year for major renovation.
Sequence of Events
At 17:00, the Boeing 737 was 20 nautical miles northeast of the airport, heading 230 degrees at 6,000 feet with an indicated airspeed of 210 knots. The C-17 was 7 miles east of the airport, heading 360 degrees at 3,000 feet, indicating 240 knots. Both aircraft were about 20 flying miles from the runway 19L landing threshold and were receiving radar vectors from the Deale/ADWAR sector controller at Potomac Terminal Radar Approach Control (TRACON) for instrument landing system (ILS) approaches.
At 17:01, the controller directed the Boeing 737 to reduce speed to 190 knots and instructed the C-17 to turn left to heading 350 degrees and descend to maintain 2,000 feet. The aircraft were 12 miles apart. The controller then directed the Boeing 737 to descend to 5,000 feet and vectored the C-17 to a heading of 270 degrees, placing it on a base leg for runway 19L.
At 17:02:30, the Boeing 737 was cleared to descend to 4,000 feet, with the aircraft 5.34 miles apart. Twenty-five seconds later, the controller vectored the Boeing 737, now approximately 15 miles from the runway, to heading 210 degrees to join the localizer. The separation between the C-17 and the Boeing 737 was 4.45 miles on a converging course with a closure rate of 180 knots.
At 17:03, the controller directed the C-17 to fly heading 220 degrees to join the localizer and cleared it for the ILS runway 19L approach. The controller issued a wake turbulence advisory to the Boeing 737, stating, "Executive one foxtrot caution wake turbulence you’ll be four miles in trail of a C-seven, heavy C-17." The Boeing 737 acknowledged. At that moment, the aircraft were 3.62 miles apart. The minimum required wake turbulence separation for a Boeing 737 following a heavy C-17 was 5 miles.
At 17:03:30, the controller descended the Boeing 737 to 3,000 feet. The Boeing 737 requested a slower speed and was instructed not to exceed 170 knots. The C-17 was directed to maintain a speed no slower than 170 knots until five miles from the runway. At the time the C-17 was instructed to contact ADW tower, its groundspeed was 152 knots, while the Boeing 737's groundspeed was 170 knots.
At 17:04:25, the controller cleared the Boeing 737 for the approach and instructed the pilot to reduce to final approach speed. The Boeing 737 and C-17, both on the final approach course, were separated by 3.3 miles longitudinally and 1,600 feet vertically, with groundspeeds of 153 and 155 knots, respectively.
At 17:05:20, the controller directed the Boeing 737 to contact ADW tower. The Boeing 737 was seven miles from the runway and 3.3 miles behind the C-17. As both aircraft continued, the C-17 slowed to 110 knots groundspeed, while the Boeing 737 maintained 150 knots before slowing to 140 knots at 17:05:57, when separation reduced to 2.84 miles. At 17:06:44, the C-17's groundspeed was 106 knots and the Boeing 737's was 121 knots, with separation reduced to 2.81 miles longitudinally and 800 feet vertically.
Controller Actions
The Deale/ADWAR sector controller had 21 years of experience and had been at Potomac TRACON for 6 years. He stated that he confused the minimum wake turbulence separation requirements for a Boeing 737 following a heavy C-17, believing it to be 4 miles instead of the required 5 miles. He attributed this to thinking of the separation for a Boeing 757 following another 757 at Washington Reagan National Airport.
The Potomac Terminal Mount Vernon Operational Supervisor (MTVOS) was responsible for oversight. During interviews, the MTVOS stated he was monitoring a trainee in a busy sector and was aware that the Deale/ADWAR sector had only two aircraft, which he considered a light workload, so he did not focus on it. The controller was relieved from position shortly after the incident and went on break before the MTVOS reviewed the event.
Outcome
At 17:04:43, the C-17 checked in with ADW tower five miles from the runway, over the final approach fix, and was cleared to land. At 17:05:39, approximately six miles from the runway, the Boeing 737 called ADW tower and reported inbound on the ILS approach. The tower local controller advised of the heavy C-17 traffic ahead, and the Boeing 737 reported having the C-17 in sight. The tower cleared the Boeing 737 to land in sequence behind the C-17.
When the Boeing 737 was four miles from the runway, the tower advised that S-turns were approved to increase spacing, as the C-17 might not have cleared the runway before the Boeing 737 crossed the threshold. The Boeing 737 acknowledged and executed S-turns.
The MTVOS received a call from the ADW front line manager (FLM) advising that the Boeing 737 was on a 5-6 mile final executing S-turns. Shortly after, the FLM called to report the Boeing 737 was going around. The MTVOS reported the go-around to the operations manager (OM), who determined an operational error had occurred.
At 17:07:04, approximately two miles from the runway threshold, the Boeing 737 pilot reported to tower, "it doesn’t look like we’re going to make this." The tower directed a go-around with left closed traffic for runway 19L. The Boeing 737 entered a left downwind and landed uneventfully.
After the go-around, the FLM advised the local controller that S-turns were not authorized for an aircraft on an instrument approach, and since the Boeing 737 had not canceled its IFR flight plan, it should have been transferred back to Potomac TRACON for another approach.
Upon returning from break, the Deale/ADWAR sector controller was debriefed and stated he was unaware of any problems until then. According to facility management, the controller had been certified in the Mount Vernon area since 2007 and had been involved in four previous operational errors involving coordination. He was de-certified and later re-certified after training.