Background
On 12 August 2000 at 1512 UTC, an airprox (near midair collision) occurred between Swissair flight SWR235 (Airbus A320, registration HB-IJL) and an unidentified glider in controlled airspace class C over the EKRIT holding fix. SWR235 was inbound from Tunis and had just entered the holding pattern, reaching assigned flight level 60.
Events
The first officer of SWR235 sighted a glider emerging from a left turn, moving away in a north-northwest direction. The flight crew estimated the vertical separation at approximately 300 feet below (glider lower) and lateral distance at 0.4 nautical miles. The glider did not transmit a transponder signal and had no radio communication with any air traffic service unit. At the time of the event, the controller's radar display showed no primary radar return from the glider. Shortly after, a primary blip appeared at the position of the encounter.
Findings
Both aircraft were operating in Zurich TMA 4, class C airspace. SWR235 was under instrument flight rules and in continuous radio contact with Zurich Arrival West Sector (APW). The glider was not communicating with ATC and carried no operational transponder. Investigators queried pilots from three nearby glider airfields (Fricktal-Schupfart, Birrfeld, Schaffhausen); all six potential pilots were ruled out based on credible statements about their routes and altitudes. Foreign border airfields were not included to limit investigative scope.
Cause
The investigation determined that the incident resulted from an unidentified glider transiting the Zurich TMA 4 without its pilot contacting the responsible air traffic control unit. Consequently, the controller was unable to issue clearances or instructions to separate the glider from the IFR flight SWR235.