Drone safety refers to the operation of Unmanned Aerial Vehicles (UAVs) in a controlled and safe manner. Safety encompasses the developer (pilot), environment (other people, animals, vehicles, building etc.) and the drone itself (hardware, software). This article also includes a checklist template intended to be a starting point for establishing a culture of safety when working with autonomous UAV platforms.

Drone related injuries are unfortunately common and range from minor cuts or bruises to life altering injuries like damaged eyesight, cut tendons, or burnt down homes. While some injuries are due to pilot error, others unfortunately result from software errors or poor flying conditions.

For unfortunate examples of drone related injuries, see these change.org and reddit posts.

Platform safety

Propellers

Drone propellers spin at high speeds – fast enough to slice through human tissue, bone, tree branches, and other obstacles. Propellers are also prone to damage caused by collisions with hard surfaces like the walls of buildings or tree trunks. Some professional drones come with built in propeller (prop) guards which are designed to protect both the platform and the pilot from physical harm. The video below shows how prop guards can protect internal walls.

Thankfully, designing and 3D printing custom prop guards for drones is relatively straightforward as outlined in this tutorial. Prop guards increase the overall weight of the platform and will thus affect the flight time and aerodynamics, but this tradeoff is often worthwhile.

Tethering

A tether is a physical cord that connects a drone to the ground. Such systems often provide physical power and communication, thus enabling longer than normal flight times within pre-specified areas. Typical applications include surveillance, large event monitoring or security missions (eg. border control). Tether systems are however heavy, very expensive and rarely used by ordinary users.

Software

Most flight control software such as PX4 Autopilot and ArduPilot have built-in safety features like pre-flight checks and pre-arm safety checks to verify that sensors, calibrations, and configurations are optimal for safe takeoff. Additional safety checks can also be programmed into the flight controllers or companion boards depending on specific requirements.

PX4 supports configuration of failsafes which specify conditions for flying and associated actions that are taken if these conditions are not met. Actions include landing, returning to a specific position, or hovering in hold mode. Safety conditions that can be specified include low battery, object detection (collision avoidance), RC or data signal loss, and geofencing. Similarly, the ArduPilot open source software also provides failsafe mechanisms to monitor the radio, battery, vibration, parachute, EKF and dead reckoning parameters.

LiPo batteries

Most drones use LiPo batteries which can catch fire if charged incorrectly, accidentally shorted or damaged severely. People have reported entire rooms burning down due to LiPo battery fires. The video below demonstrates LiPo fires in a controlled experiment.

It is important to charge the batteries outside, in a fire bag, with a fire extinguisher nearby. Only use approved balanced chargers (never try to manually balance charge the battery) and select the correct battery capacity and type on the charger before starting. Lastly, never leave the battery unattended while charging.

Lithium battery fires are particularly dangerous because water will not extinguish the flames. A specialised class D fire extinguisher is required.

Batteries that have been in bad accidents should be safely disposed of, as a safety precaution, since internal damage can also result in fires.

Battery charge

The thrust of multicopter motors is what generates lift. If the motors do not get enough power (due to low battery voltage), the drone could flip or fly erratically or even crash in a matter of seconds. Communication links with the ground station or RC controller could also be lost due to low battery voltages, resulting in fly away accidents.

To prevent these scenarios, always fully charge the drone and RC controller LiPo batteries, and the ground station laptop battery. In addition to using fully charged batteries, it’s important to have a good intuition of the typical flight times your battery affords you, and to set parameters in the RC controller and ground control software to give verbal warnings for low battery voltage.

Pilot safety

PPE

Personal Protective Equipment (PPE) is important when operating drones to prevent eye and tissue injury. A recent paper – from the 2021 IEEE International Conference on Intelligence and Safety for Robotics (ISR) – titled “Evaluation of the effectiveness of protective glasses for small UAV propellers: a report on preliminary experiments” showed that protective glasses certified by impact standards reduce the risk of injury from small UAV propellers. The video below, from the authors, illustrates this well.

Based on this research, wearing EN166B or ANSI Z87.1 certified googles is advisable when operating drones. When operating drones outdoors, it is advisable to also wear a visibility vest which signals to others to approach with caution.

Propellers can injure body tissue through lacerations and amputations. This often happens on the hands, arms or face as shown in the video below.

To prevent this from happening, never arm the vehicle if you plan to approach it, remove the propellers when working directly on the platform (eg. loading new firmware, testing the motors) and consider wearing protective gloves. Cut resistant impact gloves and hard hats are designed to withstand cut and impact forces and could help prevent serious injuries, especially when using drones that takeoff or land in a hand.

Emergency stop, hold, or return

Before using any drone, it is important to understand and test the emergency systems. In the PX4 ecosystem, the emergency stop and various flight modes can be set up using the QGC software.

Switching to hold or return mode allows the pilot to temporarily hover the drone (in order to hopefully regain control), or to return to a pre-specified position.

The emergency stop can be activated in QGC or via a mapped radio controller switch. This button cuts off power to the motors which crashes the drone in an emergency. Practise using this feature until it becomes second nature in a panic situation.

Environment safety

The importance of not flying in public spaces where there are other people, cars, buildings, animals, or large infrastructure cannot be overstated. Find a secluded open space with no wind, electric pylons, telephone lines, trees, animals, people, buildings or other obstacles nearby. Do not fly at night unless prior permission is obtained from the relevant authorities.

Unfortunately, several incidents where drones fell from the sky injuring pedestrians have been reported around the world. To reduce potential accidents, always fly in controlled environments maintaining good line of sight and adhere to the local regulations.

Working in simulation is a good way to mitigate some of these risks and test out algorithms before deployment in the real world.

Checklists

In robotics, having a printed list of steps to follow before, during and after running the robot platform is important. Apart from improving safety, such guidelines also increase efficiency and help with live debugging during demos. Below are three checklist templates for before, during, and after flying drones.

Pre-flight checklist

• Wearing visibility vest, protective eye-wear and gloves
• No obstacles (pylons, people, cars, buildings etc.) in environment
• Clear weather, no rain or wind (good line of sight)
• No interference (radio towers, electric pylons etc.)
• Flat level surface for takeoff
• Drone frame (ESC, flight controller, motors, sensors, and props) not damaged
• All drone sensors clean
• LiPo battery fully charged
• LiPo battery secured using Velcro straps
• Ground station laptop battery fully charged
• All propellers secured and in correct orientation
• Connect LiPo battery to power up drone
• Stand at least 5m away from the takeoff point
• Turn on RC controller (check battery level is above 98%, radio connection)
• Turn on ground station software (check telemetry connection)
• Software pre-flight checks passed (including GPS lock)
• Software failsafes are enabled
• Maximum flight altitude is set
• Arm the drone (check propellers spin correctly)
• Test emergency stop button
• Test hold mode works (hover for a minute)
• Test return home or return-to-launch works (if enabled)
• Select a flight mode, arm, takeoff and fly

In-flight checklist

• Maintain a clear line of sight
• Be alert for any warning signals

Post-flight checklist

• No obstacles (people, cars, buildings) near the landing site
• Stand at least 6m from the landing site
• Land on level ground
• Disarm the drone (all propellers stopped)
• Disconnect and remove the LiPo battery
• Place LiPo battery in a fireproof bag
• Inspect drone frame (ESC, flight controller, motors, sensors, and props) for damage
• Wipe any dust or dirt off the sensors
• Charge LiPo battery outside at base/home
• Store LiPo battery in fireproof bag