What happened
On August 1, 2011, an ATR 42-500, registration F-GPYO, was climbing through FL 106 toward FL 110 while operating a scheduled passenger flight from Annecy to Paris Orly. Simultaneously, a Decathlon aircraft, registration LX-FUN, was cruising at FL 105 on a flight between La Mole and Luxembourg.
While operating within the Chambéry TMA, the air traffic controller provided traffic information to both crews, noting their relative positions. The crew of the F-GPYO reported they did not have visual contact with the LX-FUN but noted a TCAS information alert. During the encounter, the F-GPYO crew performed a series of maneuvers, including a 30-degree right turn, which the controller interpreted as a visual avoidance maneuver. However, the crew subsequently experienced a TCAS Resolution Advisory (RA) "Climb" alert. Following the RA, the F-GPYO crew performed further maneuvers before eventually leveling off at FL 110. The minimum separation achieved between the two aircraft was 950 feet vertically and 1 nautical mile horizontally.
The investigation
The investigation focused on the coordination between the air traffic controller and the F-GPYO crew, as well as the use of TCAS technology. The controller testified that they intended for the F-GPYO to pass behind and above the LX-FUN to maintain separation. The controller also noted that while a Short Term Conflict Alert (STCA) was triggered on the ground station, they did not take action because they believed the F-GPRY crew was performing a visual avoidance maneuver.
The crew of the F-GPYO stated that the traffic information provided by the controller was insufficient to determine if their flight paths were conflicting. They reported that they used the TCAS display as a primary tool for separation, specifically reacting to the appearance of the traffic symbol on their navigation display and the subsequent RA alert.
Findings
- The primary cause of the incident was the decision by the crew of the F-GPYO to use TCAS traffic information as a tool for separation.
- Insufficient communication between the air traffic controller and the F-GPYO crew contributed to the event.
- There was a discrepancy in separation strategies: the controller intended to maintain a following/overtaking pattern, while the crew implemented a maneuver to increase distance from the TCAS-displayed target.
- The crew did not seek further clarification from the controller regarding the potential for a flight path conflict.