What happened
On 16 December 2011, a Saab 2000, operating as DWT 9401, was conducting a ferry flight from Geneva to Lugano. While joining the PINIK holding pattern, the crew received clearance from Lugano aerodrome control to descend to 6,000 feet, using a QNH of 995.
Shortly after, a Cessna Citation C510 (registration I-STCD), operating as MXY 451 from Biggin Hill to Lugano, contacted the same controller and was cleared to descend to flight level 70. At approximately 14:44 UTC, the Traffic Alert and Collision Avoidance System (TCAS) in the Saab 2000 issued a "monitor vertical speed" advisory. Simultaneously, the crew of the Cessna Citation C510 observed an opposing aircraft via their Traffic Advisory System (TAS). The two aircraft crossed paths in opposite directions with a lateral separation of only 0.1 NM and a vertical separation of 500 feet. Both aircraft subsequently landed safely at Lugano-Agno airport.
The investigation
The Swiss Accident Investigation Board (SAIB) established that the incident occurred in Italian airspace, but the investigation was delegated to Switzerland. The inquiry focused on the air traffic control clearances and the technical accuracy of the radar display used by the Lugano tower.
Investigators found that the altitude readings on the radar display (DFTI) were not reflecting the actual altitudes at Lugano. Instead, the system was displaying altitude information based on the atmospheric pressure and transition level used at Zurich-Kloten airport. Because the pressure difference between Zurich and Lugano was approximately 11 hPa, the radar display showed an error of roughly 300 feet, which obscured the controller's ability to maintain proper separation.
Findings
- The primary cause was the air traffic controller issuing a descent clearance to an aircraft near the transition level, which, due to the prevailing atmospheric pressure, led to a dangerous convergence with another aircraft.
- The controller was working alone at the time of the incident.
- The radar display provided inaccurate altitude information because it was calibrated to Zurich's pressure settings rather than Lugano's.
- The incident resulted in an ICAO Category B airprox, representing a considerable risk of collision.