What happened
On March 4, 2018, at approximately 15:00 local time, a near-collision occurred near Sarnen, Switzerland, involving two light aircraft at Kägiswil airfield. The first aircraft, a Piper J3C-65/L-4 with registration HB-ODH, was performing a visual approach to runway 03. To minimize noise over the nearby town of Sarnen, the pilot was executing a right-hand side slip on a steep descent path.
Simultaneously, a Robin DR 300/180 R, registration HB-EDB, was arriving from the opposite traffic pattern following a glider tow operation. As the HB-ODH pilot turned into the final approach, they suddenly observed the faster-moving HB-EDB appearing closely in front and slightly below on the right side. The pilot of the HB-ODH immediately initiated a go-around to avoid a collision. The two aircraft came within approximately 50 m of each other at the same altitude.
The investigation
The investigation examined the flight paths, radio communications, and environmental conditions at the time of the incident. Investigators reviewed the radio logs, which showed that while the pilot of the HB-EDB had broadcast their intentions, the HB-ODH pilot had not made any recent blind calls. Furthermore, previous transmissions from the HB-ODH were difficult to understand due to high engine noise interference.
Environmental factors were also scrutinized, including the position of the sun and the visibility of the terrain. The investigation also looked into the cockpit configurations of both aircraft, noting that the HB-EDB was equipped with a Flarm collision warning system, whereas the HB-ODH was not. Additionally, the status of the airfield's aeronautical documentation was reviewed.
Findings
Several contributing factors led to the dangerous encounter:
- The aircraft were utilizing different traffic patterns for the same runway, which increased the risk of a collision and made the second aircraft harder to detect.
- Sun glare and low contrast from the snow-covered mountains hindered the pilot of the HB-EDB from spotting the other aircraft.
- The pilot of the HB-ODH had restricted upward visibility due to the aircraft's high-wing configuration and the execution of the side slip.
- The pilot of the HB-EDB may have flown a continuous 180-degree turn directly into final approach without a proper base leg, which limited their downward visibility due to the low-wing design of the aircraft.
- Ineffective radio communication, characterized by a lack of recent blind calls from the HB-ODH and poor audio quality, prevented the pilots from coordinating their positions.
- The lack of a collision warning system in the HB-ODH meant there was no electronic alert to warn the pilot of the approaching aircraft.