What happened
Two distinct runway incursions occurred involving multiple aircraft on runway 2D4R. In the first incident, an Air Ontario de Havilland DHC-8 (ONT331) was cleared to position on the runway. Shortly after, a Canadian Airlines Douglas DC-10 (CDN02) was cleared to land on the same runway. The controller failed to issue a take-off clearance to the de Havilland DHC-8, and although the flight crew attempted to request clearance, the transmission was not heard by the controller. Upon realizing the aircraft was not departing, the controller instructed the Douglas DC-10 to overshoot the runway, which the crew performed safely.
A second loss of separation occurred approximately half an hour later. An Air Canada Airbus A320 (ACA127) was cleared to position on runway 24R, but the controller subsequently cleared a Canadian Airlines Airbus A320 (CDN962) to land on the same runway. The crew of the Airbus A320 (CDN962) alerted the tower that an aircraft was on the runway, prompting an immediate go-around instruction. Both incidents resulted in zero fatalities and no injuries.
The investigation
Investigators examined the controller's workload and the physical environment of the tower. At the time of the first event, the controller was managing both north and south tower positions simultaneously. During the second event, the shift supervisor was occupied with clearance delivery, leaving the controller without immediate supervisory oversight for runway positioning.
Technical issues with flight data strips were also identified. The controller had been using an unconventional method of lifting the strip holder to create a visual reminder because some strips were too wide to slide easily. In the second incident, the controller was unable to use this visual cue because a wide strip prevented the usual movement. Furthermore, visibility was reduced due to rain and mist, and the dark paint scheme of the Airbus A320 (ACA127) made it difficult to spot against the background. The investigation also noted that the airport's surface movement radar (ASDE) likely failed to provide a return for the aircraft on the runway.
Findings
- The controller was likely impaired by accumulated sleep debt and fatigue.
- The controller mistakenly believed a take-off clearance had been issued to ONT331 during the first event.
- The controller forgot the position of ACA127 during the second event.
- There was no standardized visual reminder system for controllers to track aircraft positioned on active runways.
- A combination of low visibility, aircraft color, and ground traffic prevented the controller from visually acquiring the aircraft on the threshold.