What happened
On July 3, 2000, a serious air proximity incident occurred in the Shannon Oceanic Transition Area (SOTA). A Swissair MD-11, operating as SWR 127 from Boston to Zurich, experienced a sudden loss of radar identification while cruising at FL 320. Simultaneously, an American Airlines B76 and operating as AAL 176 was climbing from FL 290 toward FL 370.
For approximately eight minutes, the aircraft's radar track vanished from the controller's display. During this period of invisibility, the radar controller issued a climb clearance to the American Airlines aircraft. The pilot of the Swissair flight, observing the other aircraft through visual contact and receiving multiple TCAS warnings, performed an emergency 10-degree right turn. This maneuver prevented a collision, as the American Airlines aircraft was observed passing within 0.5 nautical miles of the Swissair wing.
The investigation
The AAIU investigation focused on why the aircraft disappeared from radar and why the controllers failed to notice the loss of contact. Technical analysis revealed that the radar signal from the MD-11 was suppressed by a "ghost" processing function within the MSSR ERM 870 extractor. This software feature, intended to manage false tracks, inadvertently suppressed real aircraft replies when signals from multiple aircraft overlapped (garbling).
On the human factors side, the investigation found a significant breakdown in communication. After the initial contact, there was no further radio communication between Shannon Radar and the Swissair crew for eight minutes. Furthermore, the Radar Controller and the Planning Controller failed to communicate regarding the missing flight progress annotations or the lack of onward clearance for the Swissair flight.
Findings
- The primary cause was a software anomaly in the radar extractor that caused the loss of the MD-11 radar signal.
- The loss of signal led the controller to issue a climb clearance to AAL 176 without realizing the conflict risk.
- A breakdown in communication occurred between the Radar and Planning Controllers, as neither questioned the lack of follow-up for the Swissair aircraft.
- The TCAS on the American Airlines B767 was inoperative at the time, meaning the crew of AAL 176 was unaware of the near miss until contacted by the Swissair pilot.
- The decisive actions and high situational awareness of the Swissair pilot prevented the collision.
Safety action
Following the incident, the "ghost" processing function was disabled by engineering. The investigation led to safety recommendations for the IAA to conduct technical investigations into the Secondary Surveillance Radar System and to review training procedures for Radar and Planning Controllers.