Examining the Age-Old Question: What Are the Risks of Flying Drones Over People?

The Alliance for System Safety of UAS through Research Excellence (ASSURE), the Federal Aviation Administration’s (FAA) designated Center of Excellence, has released the results of a study that was undertaken to understand the risks of flying small unmanned aircraft systems (sUAS) over people.

ASSURE says the study sought to answer the following questions: “How big of a UAS can we fly directly over a person, and if it does fall and hit someone, what is the risk of a serious injury?”

Though the FAA says it can’t yet definitively answer the questions, this new study has made a start toward better understanding the risks of allowing drones to fly over people.

Notably, according to DJI, the report has concluded that sUAS are, however, far more safe to operate around people than earlier models had assumed. Specifically, it showed that drones have unique aerodynamic and structural properties that mitigate the force involved in a collision, the company says.

Mississippi State University (MSU), which leads ASSURE, designed advanced, real-world simulations by using supercomputing resources at MSU’s High Performance Computing Collaboratory.

“We developed and validated human head-UAS computer models to determine the thresholds and severity levels for traumatic brain injury for different injury scenarios,” says Raj Prabhu, the lead investigator for the MSU team and an assistant professor in the Department of Agricultural and Biological Engineering.

The MSU researchers used quantifiable injury metrics – including Abbreviated Injury Standard, Gadd Severity Index and Head Impact Criterion – to define safe and unsafe zones.

“It’s essential to understand what happens when a UAS hits a person in order to develop safer designs and materials that lower injury risk and severity should a collision occur,” Prabhu says.

According to the FAA, the research team also reviewed more than 300 publications from the automotive industry and consumer battery market, as well as toy standards and the Association for Unmanned Vehicle Systems International database. The team also conducted crash tests, dynamic modeling and analyses related to kinetic energy, energy transfer and crash dynamics.

When the studies were complete, personnel from NASA and the U.S. Department of Defense, FAA chief scientists, and other subject matter experts conducted a strenuous peer review of the findings, the FAA says.

In the end, according to the agency, the studies identified three dominant injury types applicable to small drones:

  • Blunt force trauma – the most significant contributor to fatalities;
  • Lacerations – to avoid, blade guards are required for flight over people; and
  • Penetration injuries – difficult to apply consistently as a standard.

The research also showed that multi-rotor drones fall more slowly than do pieces of metal of the same mass, thanks to a higher drag on the drone. Unlike most drones, wood and metal debris do not deform and will transfer most of their energy to whatever they hit.

Computation image by Dr. Raj Prabhu: The color contours represent von Mises shear stress as the UAS impacts with the forehead of the human head model.

As explained by DJI, ASSURE concluded that a DJI Phantom 3 drone falling on a person’s head has a 0.03% chance or less of causing a head injury, compared with a 99% risk of head injury from blocks of steel or wood with the same weight. While other materials transfer their force to a person’s head in a collision, a DJI Phantom 3 drone absorbs much of the energy – resulting in much less energy being transferred. (DJI notes that though it has supplied ASSURE with materials for testing, as well as technical expertise, it played no other role in the report’s preparation or conclusions.)

An earlier scientific report from DJI concluded that drones weighing up to 2.2 kilograms can be safely flown with the lowest risk to people – far higher than the FAA’s 250-gram threshold used for registration purposes. DJI says its report showed the 250-gram standard was based on “poorly chosen data and deeply flawed assumptions,” including an almost 50-year-old model of casualties caused by shrapnel during a nuclear war.

“ASSURE’s report is the first thorough scientific study of the risk drones pose to people on the ground, and we are pleased that it validates our own findings that earlier measurement standards grossly overstate the risks of injury from a drone,” says Brendan Schulman, DJI’s vice president of policy and legal affairs. “ASSURE’s work provides a deeper scientific understanding of the kinetic and aerodynamic factors which make drones far safer than some had thought. We look forward to more detailed research that will ensure drone safety requirements and regulations are based on measurable risk – not on fear, misunderstandings or outdated standards.”

Walter Stockwell, DJI’s director of technical standards, adds, “This report represents groundbreaking work to understand how drone impacts would occur in the real world. This will be an important guide as the industry works to make standards for drones that fly over and near people.”

The FAA says the ASSURE team recommended continued research to refine the metrics developed. The team members suggested developing a simplified test method to characterize potential injury and validating a proposed standard.

“The ASSURE team has produced peer-reviewed research results for air-to-ground impact, and that will allow the FAA to regulate sUAS operations over people based on facts,” states MSU’s Marty Rogers, the director of ASSURE. “We could not be prouder of this team or the quality of research results we’re producing.”

The University of Alabama-Huntsville, the University of Kansas and Embry-Riddle Aeronautical University also contributed to the FAA-commissioned report, which can be found here.

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