Incident Overview
On 21 July 2000 at 1715 UTC, an airprox (near collision) occurred in the Zurich Terminal Control Area (TMA), east of EKRIT. The involved aircraft were Swissair flight SWR693, an Airbus A321 (registration HB-IOK), and HB-DET, a Mooney M-20 light aircraft operated privately from Birrfeld.
Sequence of Events
SWR693 was inbound from Lisbon under radar guidance on an assigned heading of 010°, approximately 3.5 NM northeast of EKRIT. The aircraft had just reached assigned flight level 60 when the commander spotted a light aircraft silhouette about 500 meters ahead at nearly the same altitude and heading. The Swissair crew opted to perform a small descent of about 300 feet to overtake the unknown aircraft from below. This allowed them to keep visual contact and identify it as HB-DET, a Mooney. After the maneuver, SWR693 climbed back to FL60. The crew estimated the minimum lateral distance at 0.1 NM and the initial vertical separation at 0 feet, increasing to about -300 feet during the avoidance.
HB-DET had departed Birrfeld with the pilot and his six-year-old daughter, intending a flight towards the Black Forest area. The pilot had obtained a weather briefing, checked NOTAMs, and filed a flight plan without a specific route description. After takeoff, he climbed to 4000 feet on a heading of 290°. He then selected a pre-programmed waypoint "Schluchsee" on his GPS and followed the resulting bearing, intending to climb further to about 5000 feet. The pilot stated he was aware of altitude restrictions in TMA sectors, specifically that in TMA sector 4 a flight level of 5500 feet required ATC clearance. Later, he noticed an airliner at his 3 o'clock position flying away. After landing at Birrfeld, he was contacted by Zurich Tower and informed of the airspace violation and identification by the Swissair aircraft.
Findings
- Both aircraft were operating in Class C controlled airspace.
- SWR693 was IFR in continuous contact with Zurich Arrival West Sector (APW).
- HB-DET was VFR and had no communication with any ATS unit except Birrfeld.
- At 17:14:23 UTC, SWR693 reported the encounter to ATC, and 13 seconds later provided the registration HB-DET.
- At 17:14:27 UTC, a radar symbol without correlation (no label or altitude) appeared for 24 seconds, coinciding with the position of SWR693, almost certainly representing HB-DET.
- The controller briefly observed the primary radar echo but the Swissair crew had already executed the avoidance maneuver.
- The Mooney had no autopilot, and the pilot recalled leaving the transponder in standby mode.
Assessment
The identity of HB-DET as the conflicting aircraft is unquestioned, confirmed by the Swissair crew. The radar symbol without correlation for 24 seconds is highly likely HB-DET, as a second unknown aircraft at the same position is improbable. The pilot of HB-DET was aware of altitude restrictions and was familiar with the route. The unauthorized crossing of 5500 feet in TMA sector 4 was unintentional and unnoticed by the pilot. Whether the presence of his six-year-old daughter was a distraction remains unknown. The encounter posed a high collision risk; the Swissair crew's alertness and avoiding action prevented a potential impact.
Probable Cause
The incident resulted from the pilot of HB-DET flying through Zurich TMA 4 at an altitude requiring authorization without contacting the responsible ATC unit, thereby preventing the controller from providing separation from SWR693.