Delivery firm changes drone design after propeller detaches and hits man's head

The operator has redesigned the propeller hub, and has committed to changing from the current configuration to a new design consisting of a single propeller mounted on each arm. File picture: Dan Linehan

A delivery company has had to re-design its drones after a propeller came off and hit a man in the head, narrowly missing his eyes.

A report by the Department of Transport’s Air Accident Investigation Unit (AAIU) concluded the wrong type of propellers were being used and a propeller bolt hadn’t been on tight enough.

As a result of the investigation, the unnamed operator changed the way its propellers are configured on its fleet of drones. On July 14, 2022, a remote pilot working for the operator was tasked with a delivering a package to an address in Balbriggan, Co Dublin.

It took off from the top of a multi-storey carpark in Balbriggan and was flying at 71 metres above ground when there was a sudden shake less than 60 seconds into the flight. This caused an onboard vibration which in turn led to a propeller blade falling off and the drone losing control.

It fell at a rate of nearly 30km/h before its onboard parachute was deployed and it landed on a public footpath.

A crowd of around 30 people had gathered at the accident site and there was, the investigators noted, “some interaction” between members of the public and one of the drone company managers. It was here that a person said they had been struck by what was described as “a propeller blade fragment”.

After the drone was recovered, all flight operations were temporarily ceased until a cause for the accident could be established.

Later, it emerged that another member of the public was mowing his lawn when he was struck by one of the drone’s propellers. He told investigators he did not notice anything unusual prior to being struck due to the noise from his lawn mower.

He said he sustained a small cut to his head but did not require stitches or hospitalization.

Cause of accident

The probable cause of the accident was described as “fatigue failure of a bolt securing a propeller blade to the rear-right upper motor resulting in propeller blade separation from the hub”.

Contributory causes included the fact that there was “insufficient torque (tightness) of the propeller blade retaining bolt” and a “vibration-induced failure of the rear-right lower motor”.

Investigators found the wrong type of propeller was being used in the drone’s two-propeller configuration on each of its arms.

The propeller manufacturer stated the polymer propeller used on the drone “should not be used” in a coaxial configuration, which is where two propellers operate in the same axis of rotation". Such configurations generate more thrust to allow drones to carry heavier loads.

The operator subsequently redesigned the propeller hub, and has committed to changing from the current configuration to a new design consisting of a single propeller mounted on each arm.