SORA Ground Risk Class (GRC) Explained for UK Operators

Ground Risk Class — GRC — is one half of the risk picture in a SORA. It answers a simple question: if this aircraft lost control and came down, how dangerous would that be to people on the ground? Understanding how GRC is set, and how you can lower it, is essential to any Specific Category application.

Intrinsic GRC: the starting point

The intrinsic GRC is determined before any mitigation. It depends on two main inputs:

• The aircraft's characteristic dimension — broadly, its size, which correlates with the kinetic energy it could deliver on impact. Categories typically step up around 1 m, 3 m and 8 m wingspan or rotor span.
• The operational scenario — the type of area overflown, ranging from a controlled ground area (access restricted), through sparsely populated areas, to populated areas and, at the extreme, assemblies of people.

A small drone over a controlled area produces a low intrinsic GRC. A large drone over a populated area produces a high one. The methodology presents this as a table, and you read off the intrinsic value from the intersection of aircraft size and scenario.

Reducing GRC with mitigations

The intrinsic GRC is rarely the final figure. Two families of mitigation can bring it down:

M1 — reducing the number of people at risk

If you can demonstrably reduce how many people are exposed, the GRC falls. Examples include restricting access to the overflown area, conducting the flight at times of low footfall, using ground observers to clear the area, or shrinking the operational volume so fewer uninvolved people are beneath it. The credit you earn depends on the robustness of the measure.

M2 — reducing the effect of ground impact

If you cannot remove people, you can reduce the harm an impact would cause. A parachute recovery system, a frangible airframe, or other energy-absorbing design features reduce the severity of a ground strike. Again, the credit scales with how robustly the measure is implemented and proven.

Robustness matters

Every mitigation is assessed at low, medium or high robustness. Robustness combines integrity (does the measure genuinely reduce the risk?) and assurance (can you prove it does?). A parachute with rigorous test data and a maintenance regime earns more credit than one fitted without evidence. Claiming a mitigation without being able to substantiate it will not survive CAA scrutiny.

From intrinsic to final GRC

The final GRC is the intrinsic value adjusted down by the credits earned from M1 and M2. There is a floor: you cannot reduce the GRC below certain limits no matter how many mitigations you stack, which prevents very high-risk operations from being argued down to trivial levels.

Why GRC matters so much

The final GRC, combined with the residual Air Risk Class, determines the SAIL — and the SAIL sets the entire burden of proof for your operation. A lower GRC can be the difference between a manageable application and one that demands disproportionate engineering and assurance work. This is why operators invest effort in honest, well-evidenced ground-risk mitigation rather than simply accepting the intrinsic value.

A practical approach

When building the GRC part of your SORA: describe the overflown area precisely, choose the smallest credible operational volume, and identify mitigations you can actually demonstrate. Be conservative where evidence is thin. The GRC tables and reduction values are revised between methodology editions, so always work from the version the CAA currently references and treat older worked examples as illustrative.

Done well, the GRC analysis is not a hurdle but a design tool — it shows you exactly where reducing exposure or impact energy will pay off in a lower SAIL and a more achievable safety case.