Overview
Two loss-of-separation incidents occurred at an airport on the same day, involving aircraft on runway 24 right (24R). In both cases, a controller cleared an aircraft to position on the runway and subsequently cleared a landing aircraft to the same runway, requiring the landing aircraft to overshoot. No injuries or damage were reported.
First Occurrence
At approximately 1309 eastern daylight saving time (EDT), an Air Ontario de Havilland DHC-8 (ONT331) was cleared to position on runway 24R. The controller, working combined north and south tower positions, did not include a take-off clearance in his transmission to ONT331. The flight crew attempted to contact the controller, but their transmission was not heard due to simultaneous communication with another aircraft. About one minute later, a Canadian Airlines Douglas DC-10 (CDN02) was cleared to land on the same runway. The controller then noticed ONT331 had not departed and issued a take-off clearance, but as CDN02 approached the threshold, the controller cancelled ONT331's take-off clearance and instructed CDN02 to overshoot. The acting supervisor observed the event but thought a take-off clearance had been issued and that ONT331 was slow to depart.
Second Occurrence
At approximately 1332 EDT, the controller cleared an Air Canada Airbus A320 (ACA127) to position and hold on runway 24R. About one minute later, while ACA127 was still holding, the controller cleared a Canadian Airlines Airbus A320 (CDN962) to land on the same runway. During approach, the crew of CDN962 reported an aircraft on the runway. The controller immediately instructed CDN962 to go around. The controller had not issued a take-off clearance to ACA127 and forgot it was on the runway. He did not use any visual cue to remind himself, as the flight data strip was difficult to slide. The shift supervisor was working the clearance delivery position at the time.
Contributing Factors
The controller had an accumulated sleep debt and was fatigued. He reported feeling fatigued before his shift but believed he could perform. The controller thought he had cleared ONT331 for take-off when he had not, and forgot about ACA127 after positioning it. These lapses appear consistent with sleep-loss effects. Additionally, the flight data strips were of non-uniform width, making some difficult to slide, and the controller sometimes used alternative reminder methods. During the second occurrence, the controller did not use any visual cue. The weather included visibility of 2 statute miles in rain showers and mist, and clouds at 700 feet. The threshold of runway 24R was 1.5 miles from the control tower. Factors affecting visibility included aircraft paint scheme (ACA127 was predominantly dark), traffic on taxiways, and weather. The ASDE may not have displayed a return for ACA127.
Investigation
The Transportation Safety Board released a report concluding that the two losses of separation occurred when the controller instructed aircraft to position on the active runway and subsequently cleared landing aircraft without assured separation. Contributing to the occurrences was the probable impairment of the controller's performance by sleep debt. Staffing at the time was in accordance with unit policies.