An investigation by the Swiss Accident Investigation Board revealed that an Airbus A340 on approach to Zurich Airport was forced to take avoiding action when a glider was on a collision course.
On 11 August 2012 an Airbus A340-313, registration HB-JMN, was on a scheduled flight from San Francisco to Zurich. After an uneventful flight, the crew of flight LX39 received clearance to descend to 4000 ft QNH from the Zurich Final air traffic control officer at 13:32:04 UTC. At this time the aircraft was in Class C airspace in terminal control area (TMA) LSZH 2 at an altitude of 6000 ft QNH, with a rate of descent of 2500 ft/min and an indicated airspeed of 245 KIAS (knots indicated airspeed).
At the same time, an ASW 20 glider, registration HB-1519, which had taken off from the Bohlhof glider airfield on a training flight at 12:59 UTC, was located on the southern boundary of TMA LSZH 2 at an altitude of just over 4700 ft QNH.
While turning onto the localiser axis, the third pilot of the A340-313, who was in the central third occupant seat in the cockpit, surprisingly caught sight of the glider, which was at the same altitude on a collision course. He warned the two pilots conducting the flight and a pronounced avoidance manoeuvre was initiated. The recordings show a maximum bank angle of 36 degrees to the left and an increase in attitude to approximately 5 degrees, which generated a normal acceleration of 1.6 g. At this time the aircraft was still in Class C airspace in TMA LSZH 2 at an altitude of 4700 ft QNH, with a rate of descent of 350 ft/min and a speed of 248 KIAS.
According to the recordings of the two flight paths, the two aircraft passed at approximately the same height at a lateral distance of approximately 260 m.
The air traffic control officer (ATCO) was unable to detect the glider at any point as it was not equipped with a transponder and therefore could not be detected by radar. The ground based and aircraft-based safety nets were not able to respond for the same reason.
After the avoidance manoeuvre, the A340 landed on runway 14 in Zurich at 13:38 UTC. The glider continued its flight and landed at Bohlhof glider airfield at 13:59 UTC.
The near collision is attributable to the fact that a glider, without a respective clearance, was in airspace class C in which a commercial aircraft was directed below the minimum radar vector altitude.
The following factors were identified as the direct cause of this near collision:
– Lack of risk awareness on the part of the glider pilot.
– The ATCO issued a descend clearance to an altitude which was, in the airspace in which the clearance was given, below the minimum radar vector altitude for instrument flights, without monitoring a possible violation.
The following factor was identified as a systemic cause:
– The absence of a compatible safety system for gliders, commercial aircraft and air traffic control which could have warned of the dangerous convergence.
The following was identified as a contributing systemic factor:
– The air navigation services company did not realise that the minimum radar vector altitude was occasionally violated when clearance to descend was issued.
The following factors were identified neither as causal nor as contributory but as systemic factors to risk:
– The airspace structure around Zurich airport is complex, make it demanding for crews to use and for air traffic control officers to manage.
– The airspaces around Zurich airport are, regarding their vertical dimension, designed in a way that also relatively small mistakes can already lead to dangerous situations.
- Final Report No. 2208 (PDF)