Mass threshold for ’harmless’ drones

Today it is possible to buy small and cheap drones in toy stores, super markets, and on numerous online shops. Often, these drones are very light-weight and even though they are flown in back yards, sport fields, parking lots, and such places, they typically pose no lethal threat to people in the vicinity of the drone. Nonetheless, in many countries such drones are regulated by aviation rules that does not distinguish between these drones and the larger hobby or professional drones. Consequently such small drones are flow illegally. This has prompted the Danish Transportation Authority to suggest a category labeled ’Harmless’, which should be based on a mass threshold. To aid such a classification this work proposes a mass threshold of 250 grams, below which, we argue, it is reasonable to classify drones as ’harmless’ in the sense that the expected fatality rate is equivalent to that of manned aviation.

This threshold is found by combining probabilities for crashing a drone, for impacting a person, and for the impacted person to sustain a fatal injury. Drone technology is still in its infancy and statistical basis for these probabilities is therefore scarce. As a consequence the probabilistic approach in this work is based a numerous assumptions, such as the reliability of drones, the average mass to speed ratio, and the severity of drone inflicted injuries.