Guest blog by Anthony Venetz from Across Safety Development

Love them or hate them, drones are now a part of the aviation environment. They represent a huge spectrum of equipment types, use cases and operators, and somehow they all need to be managed so as to ensure the safety of manned aviation and people on the ground. The BowTie methodology is playing a key role in that process. Being a company specializing in the use of BowTie across all sectors of the aviation industry, we at Across Safety have been very engaged in that process.

Pies in the sky?

You might scoff that a world full of drones delivering pizzas is ‘pie in the sky’ and I’d agree that does seem far-fetched. But civil drones are already delivering really important cargoes in a wide range of scenarios around the world (OK, I know pizzas are important but let’s keep this in perspective please).

In several Swiss city’s drones already make routine deliveries of pathology samples between hospitals and labs, saving time and reducing traffic congestion. In Rwanda and Ghana, over 20,000 deliveries of urgent medical supplies have been made to people otherwise isolated by poor infrastructure. In Vanuatu, UNICEF has set up vaccine deliveries to remote villages, with 1-month old Joy Nowai being the world’s first child to be given a vaccine delivered commercially by drone.

Aside from deliveries, drones are used for environmental protection missions (sniffing the emissions of ships, monitoring pollution in cites, etc.), they are used in search and rescue missions and firefighting, they are used in construction, rail and powerline surveys, they are used for monitoring crop health, they are used for film and TV footage, the list really does go on and on. So when am I going to get to BowTies? Soon, I promise.

Rulemaking on unmanned systems

Regulators obviously need to keep pace with drone activities and that brings us to the Joint Authorities for Rulemaking on Unmanned Systems (JARUS). This organization is made up of regulators from 61 countries, as well as the European Aviation Safety Agency (EASA) and Eurocontrol. Their mission is to recommend a single set of technical, safety and operational requirements for all aspects linked to the safe operation of drones.

Specific Operations Risk Assessment (SORA) tool – Bowtie

National Aviation Authorities (NAAs) receive an ever-growing number of applications for commercial drone activities and they obviously need a consistent and practical approach to the assessment of those applications. To aid that process JARUS developed the Specific Operations Risk Assessment (SORA) tool. It is essentially a BowTie that works from back to front. What I mean by that is well usually a BowTie would work from left to right and help you determine your consequence risk exposure through a consideration of the threats and barriers.

In the case of SORA, we work from right to left in a multi-step approach. I’m going to abbreviate and simplify things a little here but essentially, we start by looking at the consequences related to the drone’s physical characteristics (it’s size and mass) and the use area (the proximity to population centers or manned airspace). This generates a risk score for the 2 consequences identified in SORA: the ‘air risk class’ and the ‘ground risk class’. Based on these, we now work back towards the top event at the center of the BowTie by looking at the barriers available between the consequences and the top event, and how effective those barriers are. This leads to a ‘significance’ score for the top event.

In the SORA BowTie, 4 threats have been identified (related to human, technical and environmental issues) and a set of 24 barriers are defined as the ways these threats should be managed. The final step in the SORA process is to assign ‘robustness’ (or effectiveness) requirements for each of these barriers based on the top event score determined earlier. The various robustness levels require varying degrees of assurance and reliability backed up by relevant evidence. It’s a really innovative way to utilize BowTie and it’s been extremely interesting working with operators and regulators to put together safety cases in this exciting industry.

Depending on the specific circumstances, it ends up looking something like this (click to enlarge):


More information

If you’d like to know more about how this all works check out our Farnborough International Airshow presentation ‘Drones: Safety, Risk and Regulations’ at , or contact Across Safety by email at

So, no pizzas for now but keep your eyes skyward for future developments in this fast-moving space.


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