A small drone’s accidental threat to people on the ground was the only hazard considered by a U.S. task force that recommended registration procedures for such aircraft.
The Transportation Dept. task force on Nov. 21 said all unmanned aircraft systems (UAS) weighing between about 0.55 lb and 55 lb should be registered with the FAA. That range covers maximum takeoff weight, including the drone, payload and any other associated weight. The aviation agency set the upper limit last February in outlining rules for small drones flying in civil airspace. Task force members wrestled with a lower limit Nov. 3 to 5 in what one task force leader called “long and very painful debates.”
Midair collisions with helicopters and other aircraft were not evaluated “because of the lack of data on UAS-aircraft collisions” and on ingestion of UAS parts in another aircraft’s engine or impacts with its propeller and rotor. The task force did focus on the risk of a small drone failing in flight and killing or seriously injuring someone on the ground as it falls. Members did rough calculations, drawing on a 2010 U.K. Defence Ministry study on the lethality of debris from accidental explosions as well as a 2012 study by U.S. not-for-profit federal research and development organization MITRE.
Task force members accepted the previous studies’ findings that an object with a kinetic energy level of 80 joules (or about 59 ft-lb) has a 30% probability of lethality when striking a person in the head. They determined that this equated to an object weighing 250 grams (or slightly more than 0.55 lb) traveling at a terminal velocity of 25 meters/sec (or about 57 mph). That led to their recommendation for registration of small drones as light as 0.55 lb.
Assuming that a small drone would fly over populated areas and would fail once every 100 flight hours, the members calculated the probability that a drone as light as that would fall and kill someone once every 20 million flight hours. That compares to an acceptable risk level one in 1 billion for commercial aviation and an actual risk level of five in 100,000 for general aviation, said the task force’s report. The threat a 0.55-lb drone poses “seems to present a reasonably acceptable risk level,” it said.
The task force reached general agreement on the lower weight, but some task force members “believed it was too conservative” and might “negatively impact the credibility” of a registration program and “lessen compliance levels” by requiring registration of some drones “generally considered to be in the ‘toy’ category,” the report said. It added that some members questioned why small drones “would ever be required to exceed the current general aviation risk level.”
Other members argued that “there should be no registration exemption” of any size.