Incident Overview
On 4 October 2018, at approximately 15:45 local time, the pilot of a TB10 Tobago, registered HB-KAU, was returning from a sightseeing flight to Grenchen Airport (LSZG). While approaching the Whiskey reporting point over Lake Biel, the pilot contacted Grenchen traffic and reported that a larger drone, black and red in color, had flown toward him at an altitude of 3400 ft AMSL. The pilot estimated the distance of the drone at 20 to 30 meters and its weight at 2 to 3 kg. The pilot continued the approach without further unusual events.
Aircraft and Pilot Information
The aircraft, a single-engine light aircraft TB10 Tobago, was owned and operated by Flugsportgruppe Grenchen. The pilot, a Swiss national born in 1947, held a Private Pilot Licence for Aeroplanes (PPL(A)) issued by the Federal Office of Civil Aviation (BAZL) under EASA regulations. The pilot had accumulated approximately 520 total flight hours, with about 200 hours on the accident type, and about 8 hours in the last 90 days, including 5 hours on type.
Incident Details
The Whiskey reporting point is part of a published arrival and departure route for Grenchen Airport and is depicted on the area chart. Drones can be programmed to operate autonomously and beyond visual line of sight, flying to defined positions and altitudes.
Analysis
The Swiss Transportation Safety Investigation Board (STSB) analyzed that despite the estimated dimensions and weight of the drone, early visual detection from the aircraft was difficult to impossible. It was considered pure chance that HB-KAU did not collide with the drone at that altitude. A collision with a drone of such mass at an approximate speed of 100 knots (about 180 km/h) could have devastating consequences. The pilot acted cautiously by immediately informing the aerodrome traffic controller, thereby indirectly alerting other pilots on the frequency.
The STSB found it notable that the drone was positioned over the Whiskey waypoint at a typical entry altitude, suggesting that the drone's position and altitude may have been intentionally chosen to capture close-up images of passing aircraft.
Probable Cause
No probable cause was stated in the investigation report.
Recommendations
The STSB noted that beyond ongoing efforts by the BAZL in international bodies to establish a U-Space/UTM system, bridging measures should be implemented promptly. From a flight safety perspective, it is particularly desirable to enable or improve the detectability of unmanned aircraft for directly affected parties as well as third parties such as air traffic control (detect and avoid).