Calculating Ground and Air Risk with the New UK SORA Methodology for OSC Holders

Ian Titchener
Mar 17
4 min read

As the UK Civil Aviation Authority (CAA) gears up to implement the Specific Operations Risk Assessment (SORA) methodology in April 2025, Operational Safety Case (OSC) holders in the Specific category will encounter a transformative approach to assessing and mitigating risks for Unmanned Aircraft System (UAS) operations. Developed by the Joint Authorities for Rulemaking on Unmanned Systems (JARUS) and tailored for UK adoption, SORA introduces a structured, quantitative framework for calculating ground and air risks, replacing the qualitative methods of CAP 722A. This article delves into how SORA redefines these risk assessments, equipping OSC holders with a robust toolset for safer, scalable Beyond Visual Line of Sight (BVLOS) operations in compliance with UK Regulation (EU) 2019/947.

The Shift to SORA: A Quantitative Leap

Departing from CAP 722A’s qualitative reliance on operator judgement, SORA offers a systematic, step-by-step process grounded in quantifiable metrics. Set to become an Acceptable Means of Compliance (AMC) for Article 11 of UK Regulation (EU) 2019/947 in 2025, SORA categorises risks into Ground Risk Class (GRC) and Air Risk Class (ARC), culminating in a Specific Assurance and Integrity Level (SAIL) that dictates operational safety objectives (OSOs). For OSC holders, this shift necessitates updating existing safety cases to meet SORA’s rigorous standards, supported by a transitional period to facilitate adoption.

Ground Risk Calculation: From Intrinsic to Final GRC

SORA’s ground risk assessment evaluates the potential harm to people or property if a UAS loses control, starting with the Intrinsic Ground Risk Class (iGRC) based on three core factors:

UAS Characteristics: The size, weight (Maximum Take-Off Mass, MTOM), and kinetic energy (e.g., cruise speed or terminal velocity) determine impact potential. A larger UAS with higher energy might start at a higher iGRC than a smaller one.
Operational Scenario: This considers VLOS or BVLOS operations and the environment—controlled, sparsely populated, populated, or over gatherings. BVLOS in a restricted area might classify as “sparsely populated,” while urban settings elevate the risk.
Population Density: Using datasets like the UK Census 2021 (noted in recent X discussions), SORA estimates people at risk. Operations in controlled zones typically see lower density, adjusting the iGRC accordingly.

The iGRC ranges from 1 (lowest) to 10 (highest). A mid-sized UAS operating BVLOS in a sparsely populated area might begin at an iGRC of 4 or 5, depending on its energy profile.

Mitigations then reduce the iGRC to the Final GRC:

Strategic Mitigations: Restrictions like daylight-only flights or controlled access zones lower risk, potentially reducing iGRC by 1-2 points.
Tactical Mitigations: Systems such as parachutes or flight termination systems (FTS) further decrease risk, with high-robustness solutions cutting iGRC by 1-3 points.
Emergency Response Plan (ERP): A well-defined ERP, coordinating with local authorities, can trim additional points.

A Final GRC of 7 or below is required to stay within SORA’s scope; exceeding this shifts the operation to the Certified category. An iGRC of 5 might drop to 3 with geo-caging, restricted access, and a medium-robustness ERP, ensuring tolerable safety levels.

Air Risk Calculation: From Initial to Residual ARC

Air risk under SORA gauges the likelihood of a mid-air collision (MAC) with manned aircraft, beginning with the Initial Air Risk Class (ARC):

Airspace Type: ARC ranges from ARC-a (lowest risk, e.g., segregated or atypical airspace) to ARC-d (highest, e.g., busy controlled airspace). Segregated zones typically start at ARC-a, while proximity to active airspace might raise it to ARC-b.
Encounter Probability: Traffic density, drawn from airspace data (e.g., AIP, NOTAMs), shapes the ARC. Segregated areas see minimal manned traffic, keeping ARC low.
Operational Volume: The flight geography and contingency areas define exposure, with remote or low-traffic zones reducing risk.

The Residual ARC adjusts the initial ARC with mitigations:

Strategic Mitigations: Pre-flight coordination and NOTAM issuance (aligned with CAP 3040 principles) minimise encounters, potentially lowering ARC-b to ARC-a.
Tactical Mitigations: Electronic Conspicuity (EC) via ADS-B (978 MHz transmitter, dual-frequency receiver per RTCA DO-282B) boosts visibility, complemented by high-intensity anti-collision lighting for daylight flights.
Detect and Avoid (DAA): Optional DAA systems can further mitigate risk near airspace boundaries, though they’re less critical in segregated zones.

An initial ARC-b might drop to ARC-a with EC, NOTAMs, and airspace coordination, reflecting a low residual risk in controlled settings.

From GRC and ARC to SAIL: Setting Safety Objectives

The Final GRC and Residual ARC combine to yield the Specific Assurance and Integrity Level (SAIL), from I (lowest) to VI (highest). A Final GRC of 3 and Residual ARC-a might result in SAIL II, requiring standard OSOs like pilot training and UAS maintenance. Higher SAILs (IV-VI) demand advanced mitigations, such as certified FTS, backed by robust evidence.

Implications for OSC Holders

For OSC holders, adopting SORA entails:

Revising OSCs: Qualitative safety cases must transition to SORA’s quantitative framework, recalculating GRC and ARC. The CAA’s transitional period (details pending Q4 2024 consultation) supports this shift.
Data Challenges: Reliance on 2021 Census data for ground risk, flagged on X, may pose accuracy issues in 2025, particularly near dynamic population zones.
Operational Benefits: SORA’s standardised approach enables scalable BVLOS operations, streamlining approvals for complex missions in controlled environments.

The UK SORA methodology heralds a new era for OSC holders, delivering a precise, data-driven framework for ground and air risk assessment. By quantifying risks and applying targeted mitigations, SORA ensures safer, compliant UAS operations. As OSC holders refine their safety cases by April 2025, close collaboration with the CAA will unlock SORA’s potential, advancing UAS integration into UK airspace.