1999 Loss of Separation Near Lambourne VOR Involving a Boeing 737-300 and Gulfstream IV (N77SW)

Incident Overview On 26 February 1999 at 0830 UTC, a loss of separation occurred in Class A controlled airspace under instrument meteorological conditions near Lambourne VOR. The aircraft involved were a Boeing 737-300, registration D-ABEK, and a Gulfstream IV, registration N77SW. No injuries or damage were reported. ## Aircraft and Operations The Boeing 737-300 was operating a public transport flight inbound to London Heathrow from Munich. It carried 6 crew and 95 passengers. The Gulfstream IV was operating a private flight inbound to Farnborough from Istanbul. It carried 3 crew and 3 passengers. Both aircraft were equipped with TCAS, although ACAS was not yet mandatory in UK airspace at the time. ## Sequence of Events The Gulfstream IV was descending through cloud when the conflict developed. The Boeing 737 was holding at FL120 near Lambourne. At 0830 UTC, the Boeing 737 reported TCAS traffic at one o'clock while turning outbound in the hold. The controller issued an avoiding-action descent to FL110, but the aircraft then executed a TCAS climb to FL125. The Gulfstream IV reported a traffic warning and was instructed to turn left to a heading of 180 degrees. It subsequently reported clear of traffic at FL116. ## Air Traffic Control and Warning Systems The two aircraft were under the control of different controllers. The conflict generated an ACC Short Term Conflict Alert (STCA) and both aircraft received TCAS Resolution Advisories. The STCA displayed a high-severity alert. The time from the first alerts to the estimated point of minimum horizontal separation was approximately 25 seconds. ## Outcome The Gulfstream IV passed through the Boeing 737's 12 o'clock position at a range of 1.3 nautical miles with 400 feet of vertical separation. The prompt avoidance manoeuvres by both aircraft prevented a closer encounter.