What happened
On 2 June 2010, an Air France Airbus A318, registered F-GUGJ, and a private Pilatus PC-12, registered EC-ISH, nearly collided while cruising near the OLRAK waypoint in the Bordeaux FIR. The PC-12 was on a ferry flight from Switzerland to Spain, while the A318 was performing a scheduled passenger flight.
During the climb, the crew of the PC-12 noticed a discrepancy between their two altimeters. One unit indicated FL 270, while the other indicated FL 290. The pilot contacted Air Traffic Control (ATC) to seek verification of their true altitude. ATC consulted military coordination centers, which confirmed via secondary radar that the aircraft was at FL 270. Relying on this external confirmation, the PC-12 crew continued the flight, believing the left-side instrument was correct.
However, the A318 was actually cruising at FL 290. As the A318 approached the PC-12, the crew of the Airbus felt slight oscillations and visually spotted the other aircraft very close, slightly above and to the right. The A318 pilot performed an avoidance maneuver to the left. The two aircraft passed with an estimated horizontal separation of only 15 to 30 meters and a vertical separation of approximately 100 feet. Neither the onboard TCAS nor the ground-based Short Term Conflict Alert (STCA) triggered an alarm.
The investigation
The investigation focused on why the PC-12's altitude information was incorrect and why the error was not detected by ATC or safety systems. Investigators examined the PC-12's pneumatic systems and found a leak in the static pressure line supplying the pilot-side barometric and airspeed unit.
Technical analysis of the hardware revealed a deformed connector at the junction with the cabin differential pressure indicator. This deformation made a reliable seal impossible. While the leak was not detected during standard ground impermeability tests, the vibrations and pressure changes during flight caused the tube to move, triggering the leak. The investigation also looked into the reliability of the altitude data provided by ATC and the effectiveness of the aircraft's automated collision avoidance systems.