A paper well worth reading
Andrew V. Shelley Aviation Safety Management Systems Ltd, Andrew Shelley Economic Consulting Ltd, [email protected]
On 16 December 2015, the FAA issued Interim Final Rule 80 FR 78593, requiring the registration of all unmanned aircraft weighing more than 250g. The 250g weight requirement was set on the basis of analysis by the UAS Task Force of ground fatalities per hour of operation, assuming a free fall from 500ft.
The results in Figure 6 suggest a considerably different outcome than that derived by the UAS Task Force.
The difference between the UAS Task Force and the current analysis arises from two sources: first, the safety goal adopted by the UAS Task Force is based on a ground fatality rate approximately 100 times higher than the actual ground fatality rate; second, the UAS Task Force assumes a very low rate of people exposed to the falling aircraft.
The UAS Task Force assumed a population density of 0.0039 people/m2 and that just 20% of that population is exposed to the falling aircraft. An American Football field is 360ft (109.73m) long and 160ft (48.77m) wide (NFL, 2015), so it has an area of 5,351.5m2 . The assumptions employed by the UAS Task Force are equivalent to just four people evenly spaced on that field. The probability of hitting a person in the event of a failure is obviously very low, and in part drives the results of both the UAS Task Force and the Micro UAS ARC in recommending no restrictions on unmanned aircraft weighing 250g or less.
During a football game, each team has 11 players on the field, and there may be 7 officials on the field; considering these 29 people alone, the density of unsheltered people is over 7 times higher than that assumed by the UAS Task Force. The density obviously becomes much higher again when the total number of players on each team, the coaches, and spectators are considered. Figure 7 shows the effect of relaxing the safety goal to the value assumed by the UAS Task Force: all modelled unmanned aircraft are able to fly at 500ft AGL over areas of low population density, and all aircraft are able to operate at greater heights over people at higher population densities. The UAS Task Force results are reasonable in the context of considering whether an unmanned aircraft should be able to fly over a populated area with people conducting normal day-to-day tasks, many of them inside buildings or vehicles. However, those results substantially understate the risks of flying above crowds, or even above loose groups of people. A 1.5kg unmanned aircraft flying above a crowded sidewalk, for example, would pose an unacceptably high risk, whereas the same aircraft flying across houses and backyards with few people outside would likely pose an acceptable level of risk.