by Elizabeth Lakoff Paquette, Manager, Safety Business Partner NAV CANADA, Rachelle Léger, Manager, Safety Programs and Product Implementation NAV CANADA, and Nicolas Jean, Manager, Area Control Centre Operations NAV CANADA
On July 16, 2019, at 14:26:02Z, Carrier 3396 contacted the Terminal Departure (DE) controller at NAV CANADA and was instructed to climb to flight level 210 and to turn right direct a fix, on course. Just over three minutes later, Carrier 3296, who had departed the same airport, contacted the same DE controller and was instructed to climb to 9 000 ft (as there was conflicting arrival traffic descending to 10 000 ft) and to turn right direct the same fix, on course.
At 14:31:21Z, as Carrier 3396 reached the vertical boundary of the DE controller’s airspace, the DE controller instructed Carrier 3396 to contact the adjacent controller. However, Carrier 3296 mistakenly acknowledged the change in frequency. This was the first instance of call sign confusion to occur in this scenario.
Although the carrier was disidentified in this scenario, this is an actual example of a safety event that occurred because of call sign confusion.
An aircraft call sign is a group of alphanumeric characters—a unique identifier—that is used to identify an aircraft in air-ground communications. Call signs come in various forms: some are the aircraft’s civil registration (such as C-GAPG, spoken: “Charlie Golf Alpha Papa Golf”); some are three-letter telephony call signs (such as ACA, for Air Canada); some are two-letter call signs (AB for Air Bravo); and some are military (like Voodoo or Batman).
The allocation of call signs to aircraft, which seems logical for managing, tracking or scheduling of multiple aircraft in a fleet, can have such unintended consequences on the ground or in the air, as similar call signs can result in call sign confusion.
Call sign confusion was formally identified as one of NAV CANADA’s top operational safety risks as far back as 2018 and has remained a risk since that time. This risk, which is not limited to Canada, is a contributing factor in some safety-related events.
What is call sign confusion? Call sign confusion involves two or more aircraft, having similar call signs, operating on the same frequency or under the responsibility of the same controller or specialist, and one of the following situations occurs:
- A pilot takes and executes a clearance intended for another aircraft without air traffic services (ATS) noticing it.
- Air traffic control (ATC) clears or instructs the wrong aircraft, believing it is the correct one, and neither pilot identifies the error.
In either case, the similarity of the call signs creates a hazardous situation that could result in an event jeopardizing the safety of aircraft.
When Carrier 3396 queried the DE controller to confirm the frequency change was for them, the DE controller responded, “Carrier 3296 just remain with me then, thank you, and maintain nine thousand [feet], traffic.” Although Carrier 3396 rightly questioned the DE controller, the DE controller did not realize that it was Carrier 3396 that called them and that they were talking to, not Carrier 3296. Unfortunately, Carrier 3396 did not correct the DE controller.
Common causes of call sign confusion can take different forms:
- operator or flight schools allocate consecutive call signs (C-GMFC and C-FMFC)
- operator schedule flights with similar call signs to be in the airspace at the same time (ABC123 and XYZ123)
- call signs coincidentally contain the same alphanumeric characters in a different order (same company - ABC123 and ABC132, or two different companies - AB1234 and BA2314)
- call signs containing repeated digits (ABC555)
Figure 1: Example of de-identified flight strips, where three aircraft with similar sounding idents were in the same controller's airspace.
In the last year, NAV CANADA identified 34 safety-related events of call sign confusion by the specialist/controller, the pilot or both. In each instance, similar call signs were the main contributing factor.
Other contributing factors were:
- failure of operator to identify the risk of similar call signs (numbering process)
- expectations and/or anticipations (confirmation bias)
- sub-optimal situation awareness
- poor radio transmission techniques
- deviation from proper phraseology
- delays in schedules
At 14:31:40Z, Carrier 3296 made initial contact with the adjacent controller, who was busy speaking to someone else on another line at the time. A few seconds later, the adjacent controller proceeded to instruct Carrier 3396 to proceed direct to another fix and to climb to flight level 250. Carrier 3296 read back the instructions. Again, Carrier 3296 accepted instructions for Carrier 3396. Despite being cleared to a fix that was not on their flight plan, Carrier 3296 did not identify that the instructions were not intended for them. The controller, seeing only Carrier 3396 in their airspace and expecting Carrier 3396 to respond, did not capture the error either.
At almost the same time, the DE controller instructed Carrier 3296 to “stay with me, maintain nine thousand ft.” Carrier 3296 did not respond. This perplexed the DE controller, and they then queried whether Carrier 3396 was still on their frequency. Carrier 3396 responded that they believed that Carrier 3296 had accidentally taken their assigned frequency. This was the moment the DE controller seemed to realize what had occurred and immediately called for Carrier 3296 who, again, did not respond.
Call sign confusion impacts the performance and cognitive load of both pilots and air traffic service professionals. Plus, in an environment where pilot, controller and/or specialist workload and interruption or distraction occur, it can be easy for any individual to miss that an error has occurred.
At 14:32:12Z, the DE controller made a call to the adjacent controller on the hotline, as surveillance data indicated that Carrier 3296 had started climbing out of 9 000 ft; this was the moment where the aircraft would leave a safe altitude and potentially conflict with the arriving aircraft. The adjacent controller answered the hotline call, and the DE controller immediately instructed them to transfer Carrier 3296 back to their frequency and to “stop them at nine; descend them to nine now.” Then, the DE controller immediately instructed the Arrival controller to turn the conflicting traffic near the path of Carrier 3296.
What is most concerning is that the exposure to similar sounding call signs is rising. There has been an alarming increase in the number of similar call sign instances reported and captured at NAV CANADA. With increased exposure comes increased risk of call sign confusion for both ATS personnel and the pilot.
At 14:32:27Z, the adjacent controller requested the DE controller to clarify to which aircraft they were referring. The DE controller advised that Carrier 3296 was on their frequency, climbing and in conflict with an arriving aircraft. The adjacent controller stated that they were not talking to Carrier 3296. The adjacent controller had no idea that they were actually speaking to Carrier 3296 and not to Carrier 3396, as their display would have shown Carrier 3396 in their airspace, not Carrier 3296.
At 14:33:04Z, surveillance data indicated that the lateral spacing between Carrier 3296 and the conflicting arriving aircraft had reduced to 3.1 miles laterally, increasing; vertical spacing between the aircraft was 800 ft and increasing in airspace, where 1 000 ft vertical and 5 miles lateral spacing were required.
As Canada’s air navigation service provider, NAV CANADA has implemented several risk controls, such as procedures for when a specialist/controller identifies similar sounding idents as well as established new aviation occurrence reporting procedures for capturing instances of call sign confusion. NAV CANADA is also working with carriers to help identify cases of similar sounding idents and has produced reports for two of Canada’s major carriers. These reports contain a national snapshot of the risk, including the number of aviation occurrence reports of call sign confusion.
NAV CANADA remains unwavering in our efforts to reduce call sign confusion in commercial operations. Carriers, operators and flight schools can all help by increasing the awareness of those that plan, schedule or assign call signs to try to reduce and eliminate the occurrence of similar sounding idents.
We believe that through collaboration and with commitment from all stakeholders to reduce the exposure to similar sounding idents, we can successfully see a reduction in potential instances of call sign confusion.