Swiss investigators believe a remotely-piloted aerial vehicle might have been deliberately positioned above an air navigation waypoint for photography purposes before an airprox incident involving an Airbus A330-300.
Despite the good visibility, says Swiss investigation authority SUST, the crew could not detect the vehicle's presence in time for evasive action.
The Swiss International Air Lines aircraft had been conducting an ILS approach to Zurich following a service from Dar es Salaam on 6 May.
SUST says the vehicle had been positioned about 5,000ft above the MILNI final approach point – some 10.3nm from the airport. This coincides with the height an aircraft would expect to reach upon crossing the waypoint during an ILS descent to runway 34.
"It is conceivable that the position and height of the [vehicle] was deliberately chosen, with the intention of making close-ups of commercial aircraft," the inquiry states.
The vehicle was a multi-rotor craft about 1m in diameter and the A330 passed 10m underneath, it adds. None of the 185 passengers and 12 crew members was injured and the aircraft was undamaged.
SUST says a "similar situation" occurred in the approach path to Basel's runway 33 on 14 July last year.
The inquiry states that a collision with an aircraft is "only a question of time", given the prevalence of remotely-piloted drones.
It has detailed the A330 encounter as the Single European Sky collaborative research organisation SESAR embarks on a series of projects to investigate various safety and security aspects of drone technology.
Tasks in the two-year programme include exploring geo-fencing capabilities and the resistance of drones to unlawful interference, as well as the tracking and surveillance of operations to protect restricted areas such as airports.