What happened
On March 11, 2011, at approximately 11:37 UTC, two aircraft engaged in flight training operations experienced a loss of separation near Famalicão, Portugal. The first aircraft, a Cessna 172R, registration CS-DGM, had departed from Porto Airport (LPPR) for a training mission in the Guimarães area. The second aircraft, a Cessna 172N, registration CS-DCU, departed from Maia Municipal Aerodrome (LPVL) to operate in the Santo Tirso area.
Both aircraft were operating within the Porto Terminal Control Area (TMA), frequently transitioning between controlled Class C airspace and uncontrolled Class G airspace. During the flight, the two aircraft approached each other closely, reaching a lateral separation of approximately 0.5 NM and a vertical separation of only 200 to 300 feet. No injuries or damage to the aircraft occurred during the encounter.
The investigation
The GPIAAF investigation examined radio communications between the aircraft and Porto Approach (APP) controllers, as well as operational procedures. The investigation focused on the discrepancy between the air traffic services provided and the expectations of the flight crews. Investigators noted that while the aircraft were operating in Class G airspace, the frequent instructions to maintain specific altitudes or headings—necessitated by IFR traffic crossing the area—created a perception among the pilots that they were under continuous radar surveillance and active separation services.
Furthermore, the investigation found that radar coverage at low altitudes was limited. For the CS-DGM, the radar signal was intermittent, meaning the controller could not positively identify the aircraft's position via radar at all times. The controller relied on flight progress strips to track the aircraft's position.
Findings
- The loss of separation resulted from a misunderstanding of responsibilities between air traffic controllers and the pilots.
- Controllers assumed that pilots were aware of the lateral boundaries of their assigned working areas and were maintaining their own separation.
- Pilots operated under the assumption that they were being actively controlled and monitored via radar, expecting the controller to provide corrective instructions to prevent collisions.
- The frequent transitions between Class C and Class G airspace, combined with instructions to descend or change course due to IFR traffic, fostered a false sense of being under active radar separation services.
- The controller was unable to provide effective radar separation due to technical limitations in radar coverage at low altitudes and the intermittent signal of the CS-DGM.