What happened
On 13 August 2001, a Fokker 50, registration G-UKTH, was operating a scheduled passenger service from Amsterdam Schiphol to Teesside Airport. While flying in uncontrolled airspace approximately 35 nautical miles southeast of Teesside, the aircraft was receiving a Radar Advisory Service from Pennine Radar. Simultaneously, a US Air Force McDonnell-Douglas F15E was conducting a training exercise in the same general area.
As the Fokker 50 was descending, the air traffic controller alerted the crew to fast-moving traffic in their vicinity. During this exchange, the aircraft's Traffic Alert and Collision Avoidance System (TCAS) issued a Traffic Advisory (TA), followed quickly by a Resolution Advisory (RA) commanding a descent. The crew responded to the RA by increasing their rate of descent. However, the TCAS instructions then shifted, demanding a climb. The commander intervened to increase the pitch attitude, trading airspeed for a high rate of climb to avoid the perceived threat.
During the maneuver, the crew observed the conflicting traffic's data tag indicating it was 300 feet below them. Shortly after, the TCAS issued a "CLEAR OF CONFLICT" alert. The commander then visually identified a fast-moving aircraft, which was later confirmed to be the McDonnell-Douglas F15E, passing away to the left.
The investigation
The investigation examined radar data, flight recorder information, and onboard video from the fighter jet. Radar replays confirmed that the two aircraft experienced a significant reduction in separation. At one point, the vertical separation between the aircraft tracks was estimated to be approximately 6/00 feet.
Data from the Fokker 50 flight data recorder showed that the aircraft underwent a sudden pitch-up to an 8-degree attitude, resulting in a vertical acceleration of nearly 2g and a 40-knot reduction in airspeed. The investigation also reviewed the air traffic control communications, noting that the controller had identified the conflict via Short Term Conflict Alert (STCA) and had provided traffic information to the crew.
Findings
- The minimum lateral separation between the two aircraft was approximately 800 metres, with a vertical separation of roughly 1,500 feet at the closest point of approach.
- The McDonnell-Douglas F15E crew was unaware of the Fokker 50's presence, as the aircraft did not change its course or climb rate during the encounter.
- The Fokker 50 crew's rapid response to the TCAS Resolution Advisory was the primary factor in preventing a much closer encounter, which could have resulted in a vertical separation of less than 100 feet.
- The Pennine Radar controller's decision not to provide specific heading instructions was based on the assessment that such instructions might have inadvertently steered the Fokker 50 into the path of the fighter jet.