UK SORA Air Risk
Understanding Air Risk Class (ARC) in UK SORA for Drone Operations
The Air Risk Class (ARC) is a key component of the UK Specific Operations Risk Assessment (SORA), classifying the risk of mid-air collisions between unmanned aircraft systems (UAS) and manned aircraft within the operational airspace. The ARC is not a separate permit, recognition, or classification granted by the UK Civil Aviation Authority (CAA). The operator determines and substantiates the initial ARC based on the airspace and expected encounter rate. After implementing accepted strategic mitigations, a residual ARC may be established, which then forms the basis for the required Tactical Mitigation Performance Requirements (TMPR).
What Does Air Risk Class Mean?
The ARC describes the air risk: the likelihood that the UAS will encounter manned aviation during the operation. UK SORA distinguishes between ARC-a to ARC-d. A higher class indicates a higher expected encounter rate and necessitates more stringent risk management in the air.
The ARC is alongside the Ground Risk Class (GRC). The GRC addresses the risk to people on the ground; the ARC addresses the risk of a collision with manned aviation. Both assessments have different subjects and should not be confused with each other.
- ARC-a: Minimal risk
- ARC-b: Low risk
- ARC-c: Moderate risk
- ARC-d: High risk
From Initial ARC to Residual ARC
The initial ARC results from the characteristics of the operational airspace and the expected presence of manned aviation. The operator must substantiate this with appropriate data and sources, including the relevant airspace, traffic patterns, flight altitudes, usage times, and other operational conditions.
Strategic mitigations can reduce the chance of an encounter. Examples include adjusting the operational area, limiting the time window, or other demonstrable restrictions before the flight. Only when the chosen mitigations are sufficiently substantiated according to the SORA methodology can a lower residual ARC be used.
- Adjust operational area
- Limit time window
- Implement other demonstrable restrictions
What Are the Implications of the Residual ARC?
The residual ARC determines which Tactical Mitigation Performance Requirements (TMPR) apply. The TMPR describes the required performance level for tactical mitigation during the operation, including detection, decision-making, command-and-control, and executing an avoidance action when necessary.
The concrete technical and operational implementation must align with the Concept of Operations (ConOps) and be demonstrably supported with procedures, competencies, configuration data, exercises, and evidence. A mere mention of an ARC class is therefore insufficient.
- Detection
- Decision-making
- Command-and-control
- Avoidance action
How Does ARC Feature in an Application?
In a bespoke SORA, the operator documents the steps taken, initial ARC, strategic mitigations, residual ARC, and associated TMPR in a traceable manner. The competent authority assesses this substantiation as part of the complete dossier for the operational authorisation; there is no separate ARC application or ARC granting.
In an applicable PDRA, the risk assessment and conditions are predefined. The operator must then demonstrate that the operation fully fits within those conditions. A PDRA is not a free method where the operator can independently choose another ARC.
- Document steps taken
- Initial ARC
- Strategic mitigations
- Residual ARC
Practical Checkpoints for the Operator
Ensure that the ConOps accurately describes the operational area, vertical limits, usage times, and relevant airspace users. Also, document which data for the encounter rate have been used, how current those data are, and what assumptions or uncertainties exist.
Ensure that every strategic and tactical mitigation is reflected in procedures, crew training, technical configuration, and evidence. When the operational area, airspace usage, or operational concept changes, the ARC substantiation must also be reassessed.
- Accurate ConOps description
- Document encounter rate data
- Reflect mitigations in procedures
- Reassess ARC with changes
Frequently asked questions
Is ARC a permit issued by the UK CAA?
No. ARC is a risk classification within UK SORA. The UK CAA assesses the substantiation as part of the dossier for an operational authorisation but does not issue a separate ARC certificate. ([caa.co.uk](https://www.caa.co.uk/drones/specific-category/uk-sora-based-operational-authorisations/getting-started-with-uk-sora/?utm_source=openai))
What is the difference between initial ARC and residual ARC?
The initial ARC results from the operational airspace before applying strategic mitigations. The residual ARC is the class that remains after accepted and sufficiently substantiated strategic mitigations have been applied. ([caa.co.uk](https://www.caa.co.uk/drones/specific-category/uk-sora-based-operational-authorisations/getting-started-with-uk-sora/?utm_source=openai))
What is the difference between ARC and GRC?
ARC addresses the risk of an encounter with manned aviation. GRC addresses the risk to people on the ground. Both are separate components of the SORA assessment. ([caa.co.uk](https://www.caa.co.uk/drones/specific-category/uk-sora-based-operational-authorisations/getting-started-with-uk-sora/?utm_source=openai))
Does every Specific-category operation need to determine its own ARC?
Not always. In a bespoke SORA, the ARC is substantiated according to the SORA steps. In an applicable PDRA, the relevant risk outcomes and conditions are predefined, and the operator must demonstrate that the operation fully fits within those. ([caa.co.uk](https://www.caa.co.uk/drones/specific-category/uk-sora-based-operational-authorisations/getting-started-with-uk-sora/?utm_source=openai))