What aircraft was involved and where did it happen?

The accident on 1999-02-26 involved a Boeing B737-300/Gulfstream Aerospace Gulfstream IV, at in der Nähe von Lambourne (UK), DE.

What was the probable cause of the accident?

The loss of separation was caused by an air traffic controller overlooking a potential conflict while managing other traffic, compounded by high closing speeds that reduced the available reaction time for both TCAS and STCA systems.

TCAS and ATC Conflict Averted Near Lambourne

A near-collision between a Boeing 737-300 and a Gulfstream IV was avoided in UK airspace through the combined use of TCAS resolution advisories and air traffic control interventions.

What happened

On February 26, 1999, a serious loss of separation occurred in the Lambourne sector of UK airspace between a German-registered Boeing 737-300 flying from Munich to London Heathrow and a Gulfstream IV traveling from Istanbul to Farnborough.

The B737 was operating in a holding pattern at flight level 120 after requesting to remain above the cloud layer. Simultaneously, the Gulfstream IV was descending through 15,000 feet. As the two aircraft approached each other on intersecting tracks, the Short Term Conflict Alert (STCA) was triggered for air traffic controllers.

Both aircraft received Traffic Advisory (TA) and Resolution Advisory (RA) alerts from their onboard TCAS units. The B737 pilot received a 'CLIMB' instruction and subsequently climbed to flight level 125. The Gulfstream IV crew, already executing a turn instructed by air traffic control, experienced significant lateral forces, reaching a bank angle of 63 degrees and 1.78G during the maneuver. The separation was eventually restored after the B737 followed TCAS instructions and the Gulfstream IV corrected its course.

The investigation

The investigation examined radar data from the London Area and Terminal Control Centre (LATCC), flight recorder data from the Gulfstream IV, and air traffic control communications. While the B737's flight data recorder had been overwritten, reconstructed radar tracks indicated that the B737 would have flown directly behind the Gulfstream IV had no corrective action been taken. The minimum separation reached was approximately 1.3 nautical miles horizontally and 400 feet vertically.

Investigators also reviewed the actions of the two controllers involved. The controller at the Lambourne sector had noted a potential conflict on flight level 120 but became preoccupied with planning subsequent arrivals. The controller at Heathrow Intermediate North issued a descent instruction to the B737 that ultimately conflicted with the TCAS RA received by the aircraft.

Findings

The primary cause of the loss of separation was the controller's failure to monitor the potential conflict on flight level 120 after becoming distracted by other traffic planning tasks.
The high closing speed between the two aircraft, caused by their opposing headings, resulted in a very short warning window for both the TCAS and STCA systems.
Conflicting instructions were issued during the event: the air traffic controller instructed the B737 to descend, while the TCAS issued a climb command. The pilot correctly prioritized the TCAS RA.
Adverse weather conditions, including a thick cloud layer, placed both aircraft in instrument meteorological conditions (IMC), which may have contributed to the crew's focus on flight parameters during the maneuver.