What happened
On May 1, 1999, an Air Canada Airbus A320 (ACA270) and a Canadian Airlines Boeag 737 (CDN960) experienced a loss of radar separation while operating near Calgary International Airport. The Airbus A320 was arriving from Edmonton, cleared to descend to 6,000 feet. Simultaneously, the Boeing 737 had departed runway 34 and was climbing toward flight level 250.
During the sequence, the departure controller cleared the Boeing 737 to a heading of 090 degrees. To facilitate the arrival of the Airbus A320, the departure controller applied a special function indicator code to the arriving aircraft's radar data block, which allowed the aircraft to descend below 10,000 feet before reaching the designated arrival dump point. As the aircraft progressed, the two planes passed within 2 nautical miles of each other at approximately 9,700 feet altitude, failing to maintain the required 3 nautical mile lateral or 1,000-foot vertical separation.
The investigation
The investigation focused on the actions of the controllers at the Calgary Terminal Control Unit. It was established that the departure controller cleared the departing aircraft to an altitude exceeding 9,000 feet without first ensuring sufficient separation from incoming traffic. Furthermore, the controller utilized a "look and go" monitoring method rather than the established "nine/ten split" procedure, which is designed to provide more robust vertical separation during complex maneuvers.
Investigators also examined the management of the departure controller, noting that he had been involved in a separate separation incident the previous day. While the shift manager had allowed him to return to duty, there was no formal assessment of his performance the following morning, and the team supervisor was unaware of the recent irregularity. Additionally, the controller's recent work schedule had been highly irregular, involving frequent changes and significant overtime.
Findings
- The departure controller authorized the departing aircraft to climb above 9,000 feet before confirming adequate separation from the arrival.
- A radar data block code was applied to the Airbus A320, removing the altitude restriction that would have kept it above 10,000 feet.
- A lack of effective communication and coordination between the departure and arrival controllers was a primary factor.
- The arrival controller identified a potential conflict but failed to take decisive action or communicate the concern in a way that prompted a response from the departure controller.
- The departure controller opted for a high-monitoring "look and go" method instead of using the safer "nine/ten split" separation procedure.