What happened
On the morning of September 5, 2012, a private training flight was being conducted in a Reims Aviation F152, registration HB-CNI, departing from Triengen. The pilot intended to use the reduced visibility conditions for a navigation exercise. During the flight, the aircraft encountered clouds near Wolhusen, causing the pilot to lose geographic orientation while struggling to stabilize the aircraft. After several minutes, the pilot regained visual references using a smartphone to identify landmarks, including the southern end of Lake Sempach. To return to Triengen, the pilot turned toward Sempach, unknowingly entering the Emmen control zone (CTR).
Simultaneously, an Eurocopter EC 120B helicopter, registration HB-ZFM, was performing a commercial sightseeing flight. The helicopter pilot had received permission from Emmen tower to transit the CTR at a low altitude. While flying, the air traffic controller noticed a primary radar echo and issued a traffic advisory to the helicopter pilot regarding an unidentified VFR aircraft in the area. Shortly after this warning, the helicopter pilot spotted the aircraft at a 12-to-1 o'clock position. The helicopter pilot performed an intuitive rightward maneuver, resulting in the two aircraft passing within approximately 50 meters of each other at nearly the same altitude.
The investigation
The SUST investigation examined the flight paths, meteorological conditions, and the actions of both pilots and the air traffic controller. The investigation established that the pilot of the HB-CNI was flying without contacting Emmen tower and had the transponder turned off, under the mistaken belief that it was only required above 7,000 feet. The investigation also reviewed the weather forecasts, which indicated critical visibility conditions for VFR operations due to a low cloud ceiling and mist.
Findings
- The pilot of the HB-CNI assessed the weather too optimistically and chose a flight tactic that led into increasingly restricted visibility.
- The decision to turn back was made too late, leading to the aircraft entering clouds and the pilot losing orientation.
- The aircraft entered the control zone without the pilot's knowledge of the airspace boundaries or communication with air traffic control.
- The pilot of the HB-CNI did not use the transponder and failed to utilize a portable GPS device that was available in the cockpit.
- The air traffic controller's timely traffic advisory to the HB-ZFM pilot was a critical factor that mitigated the severity of the encounter.