As the taxiing aircraft passed, the ground controller instructed the lead tug driver to continue on Taxiway C and hold short of Runway 26. At 11:04, the two tugs proceeded onto Runway 17R, where a Jazz Aviation, De Havilland DHC-8-402, aircraft was beginning its take off roll. The flight crew noticed the tugs on the runway but decided to continue the take-off, considering it the safest option given the aircraft’s proximity to decision speed. The aircraft became airborne approximately 3,700 feet laterally away aircraft’s proximity to decision speed. The aircraft became airborne approximately 3,700 feet laterally away from the tugs and passed overhead at an altitude of 350 feet as the vehicles exited the runway.

The TSB investigation finds that the incident was not caused by mechanical or technical malfunctions, but rather because of human factors, including procedural drift, conflicting mental models, and a steep authority gradient between the air traffic controller and the tug driver. The report also highlights the role of the advanced surface movement guidance and control system (A-SMGCS), which was offline for a planned software update at the time of the incident.

One of the key findings of the investigation was the lack of recurrent training for tug operators. The lead tug driver held an Airside Vehicle Operator’s Permit (AVOP), which grants specific driving and aircraft-towing privileges on the airfield. However, the AVOP program at CYYC does not feature a retraining component, meaning that once the permit is issued, holders are not required to revisit the material or demonstrate their proficiency during its five-year validity period. The report notes that the driver diverted from the laid down procedure, most probably from “procedural drift”.

This procedural drift was compounded by the lead tug driver’s mental model of the situation, which conflicted with the ground controller’s expectations. The driver believed that turning around on a taxiway was not allowed, unless explicitly instructed by the controller. This impression, combined with his proximity to the runway holding position marking, and his previous experience of making detours onto runways during construction work, led him to conclude that entering the active runway was the only viable option. The report states, “The ground controller’s mental model was that the lead tug driver would continue the route the controller had instructed. This was contradictory to the lead tug driver’s mental model in which the only way to continue the route was to enter [the active] Runway.”

Another critical factor was the steep authority gradient between air traffic controllers and ground vehicle operators. The report explains that this gradient is inherently steep due to the differing nature of their roles. Controllers are tasked with monitoring and ensuring the safe movement of all vehicles and aircraft within their area of control, while ground vehicle operators are responsible for their own vehicles and following instructions. However, a too steep gradient can lead to situations where ground vehicle operators feel uncomfortable seeking clarification when they find a controller’s instructions unclear. The report warns, “If the authority gradient between an air traffic controller and a ground vehicle operator is not proactively managed, a ground vehicle operator may not feel comfortable asking for clarification if he or she considers an instruction to be unclear or erroneous.”

The investigation also examined the impact of the A-SMGCS outage on the incident. The system, which provides real-time surveillance of aircraft and vehicle traffic on airport manoeuvring areas, was offline for a planned software update at the time. To mitigate the loss of this critical tool, the control tower implemented a simplified movement posture which increased spacing for arriving and departing aircraft, and designated specific runways for landings and take offs. However, the report notes that relying primarily on additional time and aircraft spacing during A-SMGCS outages could increase the risk of hazardous situations, like runway incursions.