What happened
On 26 February 1999, a Boeing 737-300, registration D-ABEK, operating a public transport flight from Munich to London Heathrow, and a Gulfstream IV, registration N77SW, operating a private flight from Istanbul to Farnborough, experienced a loss of separation near the Lambourne VOR. Both aircraft were operating in Instrument Meteorological Conditions (IMC) within Class A controlled airspace.
The Boeing 737-300 was performing a holding pattern at FL120 after the crew requested to maintain that level to remain clear of cloud cover. Simultaneously, the Gulfstream IV was descending through FL150 toward FL120. At approximately 0830 hrs, the Boeing 737-300 crew reported a TCAS traffic advisory. The aircraft subsequently executed a TCAS Resolution Advisory (RA) climb to FL125. At the same time, the Gulfstream IV crew reported a traffic warning and received an immediate instruction from air traffic control to perform an avoiding action turn. Separation was eventually regained following the TCAS manoeuvre by the Boeing 737-300 and a heading change by the Gulfstream IV.
The investigation
The investigation involved a review of radar data from the London Area and Terminal Control Centre, as well as Flight Data Recorder (FDR) analysis from the Gulfstream IV. While the Boeing 737-300 FDR data was unavailable due to being overwritten, the Gulfstream IV data confirmed the aircraft reached a maximum bank angle of 63 degrees during its avoidance manoeuvre.
Investigators examined the actions of both air traffic controllers. The Heathrow Intermediate North Director was unaware of the conflict until the Short Term Conflict Alert (STCA) activated, at which point they provided a descent clearance. The Lambourne Sector Controller had intended to use FL120 for the Gulfstream IV but had made a mental note to wait until the Boeing 737-300 vacated the level. However, the controller subsequently issued a descent clearance for the Gulfstream IV to that level, overlooking the potential conflict.
Findings
- The geometry of the conflict, involving high closing speeds due to the aircraft flying on opposing headings, resulted in a late-stage detection by both the STCA and TCAS systems.
- The controller issued a descent clearance to the level occupied by the other aircraft after overlooking a previously noted potential conflict.
- The Boeing 737-300 crew correctly prioritised the TCAS Resolution Advisory over air traffic control instructions.
- Both the STCA and TCAS systems functioned as intended, providing the necessary alerts to prevent a collision.