What happened
On April 26, 2002, a Piper Dakota (HB-PHW) was conducting a positioning flight from Zurich to Samedan. Due to an approaching weather front, the pilot opted to depart via VFR Route 3, requesting a departure from runway 32 to avoid delays. During the takeoff sequence, the air traffic controller instructed the pilot to set the transponder to "squawk standby."
While the Boeing MD-11 (HB-IWI) was on its final approach to runway 28, the crew reported seeing an aircraft on their right at a similar altitude, approximately 400 to 500 meters away. The controller subsequently identified the aircraft as the Piper and ordered the HB-PHW to turn immediately left to a heading of 360 degrees to avoid a collision.
The investigation
An investigation by the Swiss Transportation Safety Investigation Board (SUST) established that the HB-IWI was operating under Instrument Flight Rules (IFR) while the HB-PHW was flying under Visual Flight Rules (VFR). The investigation found that the HB-PHW had drifted approximately 2 nautical miles south of its assigned flight path.
Investigators noted that the pilot of the HB-PHW was unfamiliar with the specific VFR route, as it was his first time flying it. Furthermore, the controller's instruction to set the transponder to standby meant the aircraft was not visible on the secondary radar. The investigation also found that the primary radar was unable to detect the light aircraft at low altitudes (approximately 1,000 feet) near the airport.
Findings
Several contributing factors led to the airprox incident:
- Loss of radar surveillance: The instruction to set the transponder to squawk standby prevented the controller from tracking the aircraft via secondary radar.
- Navigational error: The pilot of the HB-PHW likely misidentified local landmarks, leading to a significant deviation from the assigned VFR route.
- Environmental factors: Strong westerly winds at low altitudes likely contributed to the aircraft drifting off course.
- Radar limitations: The primary radar system failed to provide a reliable track for the light aircraft at low altitudes near the airfield.
- Pilot workload: The pilot's primary focus was on escaping the approaching weather front, which may have impacted navigational precision.