What You Should Know About Flying Over People

The Physics of Falling Drones and the New FAA Rule

Falling Drones

It does not matter how or what caused the failure of a drone in flight, it matters who or what can be hit, hurt, damaged, or killed when it comes down!  An article published in a peer reviewed journal titled “A Model of Human Harm from a Falling Unmanned Aircraft: Implications for UAS Regulation”, states that “UAS have the potential to cause a range of harm and injury to human beings”. (Shelley 2016, 6)  The article presents a model to estimate the economic cost of a subset of injuries from multi-rotor UAVs falling from a certain height.  The model is based on free fall incidents only.  Basically, you are hovering your UAV over a crowd and the motors suddenly quit, what’s the potential outcome.

The article states that “Standard physics can be used to estimate the energy with which a falling multi-rotor unmanned aircraft could hit a person. An object free falling through the atmosphere experiences a downward force due to gravity, and an upward drag force due to air resistance.” (Shelley 2016, 8)  From there it calculates the impact energy of UAVs of various weight categories.  Two of the categories are especially of interest because they represent a couple of fairly common UAVs used commercially.   The 1.5kg UAV model is similar in weight to the DJI Phantom 4, the 8kg UAV is similar in weight to the DJI Matrice 300.  The 250g drone is considered by the FAA to be a microdrone and is similar to the DJI Mavic Mini.  Here are the impact energies modeled based on the height and weight of the UAVs modeled.

The article then went on to present the probability of a fatality if the UAV impacted a human.

The findings show the danger of flying over people and having a fatal incident if the UAV should fail and enter free fall.  Between 10-30 ft AGL both the 8kg (Matrice 300) and the 1.5kg (Phantom 4) UAVs can be fatal if they impact somebody on the ground!  Conversely, the 250g (Mavic Mini) only reaches a 1% probability of a fatality between 80-100 ft AGL.  From these results you can understand the FAA’s vested interest in strictly regulating the flight of UAVs over people in the name of safety.

Another article, “Failure Analysis for an Unmanned Aerial Vehicle Using Safe Path Planning” models failure modes of different UAVs, both a fixed wing (HTOL)  and a quad-copter (VTOL).   One of the things they calculated were crash trajectory and crash probability density (CPD) radius.  Note that VTOL UAVs don’t just fall straight down.  Also, the higher you are the greater the CPD.  This needs to be taken into consideration when avoiding flying over people.

Findings from their modeling for the VTOL UAV traveling at both 5 meters/second and 10 meters per second yielded the following results.

These findings show that the area of possible impact grows significantly with altitude and velocity.  Just because you are not directly over people when the UAV fails does not mean you will not impact them.  Combine that with the probability of a free fall incident that could cause a fatality if the UAV impacts someone, regulations and mitigating measures need to be in place when operating near and/or over people.

Flying Drones Over People – 2021 New FAA Rules

Can you fly drones over people here in the U.S.?  The quick answer is yes!  However, the fine print requires that you are in compliance with the requirements and are broadcasting your Remote ID during the operation.

On April 21st, 2021 new operating rules for civil sUAS (Part 107) went into effect that require remote identification (Remote ID) and allowing flights over people, over moving vehicles and at night when certain conditions are met.

Remote ID requires that drones transmit identification information to include their ID, location, velocity, control station location, time mark and emergency status.  Drones that do not have Remote ID can only operate within visual line of sight (VLOS) and within an FAA Recognized Identification Area (FRIA).  Drone manufacturers must make Remote ID standard by September 16, 2022.  All drone pilots required to register their UAS must operate their aircraft in accordance with the final rule on remote ID beginning September 16, 2023.

The Operations Over People rule applies if you are operating your drone under Part 107.  The ability to fly over people or moving vehicles depend on the risk your sUAS poses to people on the ground.  This rule also enables flight at night when certain conditions are met.  In order to operate under this new rule, you must pass the updated  initial Remote Pilot knowledge test or take the appropriate updated training course and comply with the Remote ID rule.

The Operations Over People rule has four distinct categories whose eligibility and requirements are detailed in FAA Part 107 Subpart D.  The categories are centered around the risk level of flying over people.  

Category 1 has the lowest number of conditions that must be met and is self-declared by the operator prior to operations.

  • Weighs 0.55 pounds or less on takeoff and throughout the duration of each operation under Category 1, including everything that is on board or otherwise attached to the aircraft; and
  • Does not contain any exposed rotating parts that would lacerate human skin upon impact with a human being.
  • No remote pilot in command may operate a small unmanned aircraft in sustained flight over open-air assemblies of human beings unless the operation meets Remote ID requirements.  (Standard or broadcast module)

This essentially applies to microdrones with rotor guards weighing less than 250g.

Categories 2 and 3 drones and operations need to meet performance based means of compliance and be listed on an FAA-accepted declaration of compliance.

Category 2 drone requirements are:

  • Aircraft weight between .55 and 55 pounds, including all equipment attached to the aircraft (lights, memory card, propeller guard), at the time of takeoff and throughout the duration of the flight.
  • Aircraft may not cause injury to a person equal or greater to the equivalent of 11 foot-pounds of kinetic energy upon impact from a rigid object.
  • Aircraft must not have any exposed rotating parts that could lacerate skin.
  • Aircraft may not fly over any “open-air-assemblies” unless the operation is compliant with Remote ID.

Category 3 drone requirements are:

  • Aircraft weight between .55 and 55 pounds, including all equipment attached to the aircraft (lights, memory card, propeller guard), at the time of takeoff and throughout the duration of the flight.
  • Aircraft may not cause injury to a person equal or greater to the equivalent of 25 foot-pounds of kinetic energy upon impact from a rigid object.
  • Aircraft must not have any exposed rotating parts that could lacerate skin.
  • Aircraft may not fly over any “open-air-assemblies”, and fit one of the following scenarios;
  • Operations are conducted in closed or restricted-access sites with all parties on notice of UAV operations, or;
  • The remote pilot does not maintain sustained flight over any people unless those individuals are directly participating in the operation and are protected by a structure or are inside a stationary vehicle. 

Category 4 requirements are:

The aircraft must possess a valid airworthiness certificate issued by the FAA under Part 21, and must be “operated in accordance with the operating limitations specified in the approved Flight Manual or as otherwise specified by the Administrator” and meet additional maintenance requirements.

These categories go from the easiest and least restrictive compliance to the hardest and most restrictive compliance requirements.  It also can be said they go from the least expensive requirements to the most expensive requirement to comply with.

In the section on falling drones we saw the impact of takeoff weight, altitude, and velocity has on the area of potential impact and the possibility of inflicting a fatal injury to someone on the ground.  

In the section on the new FAA Operations Over People rule we see their approach to reduce the risk to people on the ground from drone flight operations overhead in the case of an inflight failure.

As professional drone pilots we have a duty of care to those around our operations and complying with the regulations designed to mitigate potential bad outcomes.