What happened
On an instrument flight rules (IFR) flight from Teterboro, New and to Buttonville, Ontario, a PA-44-180 registered C-GLJM experienced a loss of altitude readout on its transponder. During the flight at 6000 feet, the pilot attempted to troubleshoot the Mode C issue by recycling the transponder. This action caused the aircraft's data to be moved to a "coast list" on the radar display.
While managing the coast list, the air traffic controller inadvertently deleted the flight plan for C-GLJM. When the aircraft's transponder was eventually re-acquired by the radar, it appeared as an unidentified, uncorrelated target without altitude or flight information. During a subsequent controller changeover, this aircraft was not mentioned because it did not appear as known radar traffic.
Shortly after, an Air Canada Jazz de Havilland DHC-8, operating as JZA7868, departed Toronto City Centre Airport for Ottawa. As the DHC-8 climbed eastbound, it passed in front of and below C-GLJM. At the closest point of approach, the two aircraft were separated by only 800 feet vertically and 1.3 nautical miles horizontally, violating the required separation minimums of 1000 feet vertically and 3 nautical miles laterally. No traffic alerts or avoidance vectors were issued to either crew.
The investigation
Investigators examined the radar data processing system and the controller's actions. They established that the controller's manual deletion of the entry from the coast list simultaneously deleted the aircraft's entire flight plan from the database. The investigation also looked into the handover procedures between controllers and the functionality of the transponder.
Findings
- The flight plan deletion was the primary reason the aircraft was overlooked, as the radar target lacked identifying flight data.
- The radar system design allowed a single action to delete both the coast list entry and the associated flight plan without a confirmation prompt.
- The controller's changeover briefing relied on matching radar targets to flight progress strips; because the target for C-GLJM was not recognized as active traffic, it was not cross-referenced with its strip.
- The presence of many uncontrolled VFR aircraft in the Toronto area may have desensitized controllers to unidentified targets.
- The practice of handing off departing aircraft to subsequent sectors prior to reaching airspace boundaries may have led to the assumption that unidentified targets were not a threat to climbing traffic.