What happened
On an approach to Halifax International Airport, two aircraft entered a dangerous situation at the intersection of runways 24 and 33. The first aircraft, an Air Nova de Havilland DHC-8, was arriving from Sydney, Nova Scotia, and was initially instructed to hold short of the runway 24 intersection as part of a simultaneous intersecting runway operation (SIRO). The second aircraft, an Inter-Canadien ATR 42, was arriving from Charlottetown, Prince Edward Island, on a visual approach to runway 24.
Seeking to optimize the arrival sequence, the airport controller cancelled the hold short restriction for the de Havilland DHC-8, assuming it could land before the arriving Inter-Canadien flight. However, the controller later realized the de Havilland DHC-8 could not comply with a new hold short instruction. While the controller instructed the crew of the ATR 42 to perform a missed approach, the crew requested to land further down the runway instead. The controller subsequently cleared the ATR 42 to land on runway 2 and, as the aircraft crossed the intersection, the de Havilland DHC-8 was simultaneously entering that same intersection.
The investigation
The investigation examined the controller's management of the arrival sequence and the deviation from established SIRO procedures. Investigators looked into the radar data, which showed the de Havilland DHC-8 was traveling approximately 60 knots faster than the ATR 42. The investigation also reviewed the controller's visual perspective from the tower, noting that a shallow viewing angle and a lack of time to re-evaluate the situation contributed to an erroneous perception that the intersection was clear.
Findings
- The airport controller abandoned the standard SIRO procedure without verifying that enough separation existed to switch to sequential landing operations.
- The controller cleared both aircraft to land without ensuring that one would be stopped before the intersection or that the intersection had been cleared by the first aircraft.
- The pilot of the Inter-Canadien ATR 42 did not immediately execute the missed approach instruction, which brought the two aircraft into close proximity with minimal vertical separation.
- The use of an ad-hoc, non-standard procedure increased the required level of vigilance and safety checks beyond what is typically necessary under standard protocols.