SAIL levels explained: Understanding UK SORA risk assessment

From low-risk missions to complex beyond visual line of sight (BVLOS) operations, SAIL levels shape the regulatory requirements operators must satisfy before receiving a SORA Operational Authorisation.

The importance of SAIL levels is becoming increasingly evident as organisations look to deploy scalable drone programmes.

heliguy™ has secured a UK SORA SAIL II Operational Authorisation to conduct remote BLVOS DJI Dock 3 missions on behalf of Network Rail. Our regulatory team can work with your organisation to help you build UK SORA applications.

Key takeaways

• SAIL stands for Specific Assurance and Integrity Level. It defines the depth of safety evidence an operator must provide, not an approval in itself.

• SAIL levels range from I (lowest assurance burden) to VI (highest), derived by combining a Ground Risk Class (GRC) score with an Air Risk Class (ARC) rating.

• GRC assesses potential harm to people on the ground; ARC assesses the likelihood of collision with manned aircraft. Both can be reduced through targeted mitigations.

• The Concept of Operations (ConOps) is the foundational document of any SORA application - it must be complete, accurate, and iterated until the SAIL level is both correct and achievable.

• Each SAIL level maps to Operational Safety Objectives (OSOs), each requiring Low, Medium, or High robustness of evidence.

• Higher SAIL levels (IV-VI) require formal Safety Management Systems, independent technical validation, and in some cases evidence approaching manned aviation standards.

Expert insight

"A common misconception is that SAIL is an approval in itself. In reality, the SAIL level defines the degree of assurance required to demonstrate that an operation can be conducted safely. Understanding this distinction is key to developing a successful UK SORA application."

— Tom Anderson, heliguy™ Regulatory Specialist

What is a SAIL level?

Specific Assurance and Integrity Level (SAIL) is a risk-based classification used within the UK SORA (Specific Operations Risk Assessment) methodology.

The SAIL level determines the depth and robustness of safety evidence, operational procedures, technical controls, and organisational structures that an operator must put in place before the UK CAA will grant authorisation to fly.

Lower-risk operations are subject to fewer requirements, while more complex missions require greater levels of technical, operational, and organisational assurance.

SAIL levels range from 1 to 6. The higher the SAIL level, the more robust the required safety mitigations and supporting evidence.

Why are SAIL levels important?

SAIL levels are effectively the mechanism that translates operational risk into regulatory requirements.

Once a SAIL level has been determined, it influences:

• Operational Safety Objectives (OSOs);

• Pilot and crew competency requirements;

• Technical mitigation requirements;

• Organisational responsibilities;

• Safety management expectations;

• Evidence and documentation requirements.

This ensures that safety measures remain proportionate to the risk profile of the operation.

For example, a simple rural survey mission may require relatively limited assurance, whereas a BVLOS inspection programme operating near critical infrastructure may require significantly greater operational controls and evidence.

How are SAIL levels calculated?

SAIL levels are derived by combining two independent risk assessments - Ground Risk Class (GRC) and Air Risk Class (ARC).

This determines the depth of evidence required across a set of Operational Safety Objectives (OSOs).

The three stages are sequential but iterative: an operator who finds their initial SAIL level unacceptably high can refine their operational concept, strengthen their mitigations, and re-assess.

A critical distinction: The SAIL level is not an approval. It is the assurance burden that an operator must satisfy to earn one. A SAIL II authorisation can still be refused if the OSO evidence is inadequate. A SAIL IV application can succeed if every objective is properly evidenced.

What is Ground Risk Class (GRC)?

Ground Risk Class quantifies the potential harm to people on the ground in the event of an uncontrolled UAS impact. It is a consequence measure, not a probability measure.

GRC is determined by factors such as:

• The maximum characteristic dimension and the maximum possible speed of the drone.

• The intrinsic GRC (iGRC) footprint based on flight volume, contingency volume, and initial ground risk buffer.

• The maximum population density within the iGRC footprint.

GRC is expressed as an integer from 1 (lowest consequence) to 10 (highest).

Intrinsic Ground Risk Class Table

The following table reproduces the iGRC determination matrix from Table 3 of UK Regulation (EU) 2019/947.

If population density values are not available, not accurate, or an applicant would rather use qualitative descriptors for the iGRC table, the following approximations may be used as guidance - as outlined in CAP 3239C: Determining Population Density.

The UK CAA may also allow applicants to provide appropriate population density maps.

Table 4 in UK Regulation (EU) 2019/947 presents the suggested optimal grid size for authoritative maps, based on different maximum operating heights.

GRC mitigations

Once the intrinsic GRC is established, operators can apply mitigations to reduce it.

UK Regulation (EU) 2019/947 (Annex B, Table 5) defines four mitigation classes: M1A, M1B, M1C, and M2.

The GRC reduction available from each depends on the robustness level (Low, Medium, and High) at which it is evidenced. Not all mitigations are available at all robustness levels.

"Robustness means the property of mitigation measures resulting from combining the safety gain provided by the mitigation measures and the level of assurance and integrity that the safety gain has been achieved."

- UK Regulation (EU) 2019/947, Artcle 2(5)

What is Air Risk Class (ARC)?

Air Risk Class assesses the likelihood of an encounter with manned aircraft during the operation.

There are four levels of ARC:

1. ARC-a (minimal risk)

2. ARC-b (Low risk)

3. ARC-c (Medium risk)

4. ARC-d (High risk)

The initial ARC is set by the airspace classification and typical traffic density of the operational area before any mitigations are applied.

This flow diagramme (Figure 5, UK Regulation (EU) 2019/947) provides an air risk characterisation flow chart describing the UK SORA air risk model characterisation process.

Key elements within the flowchart are explained below.

ARC Mitigations

ARC can be reduced through either strategic mitigations — applied at the planning stage and embedded in the ConOps — or tactical mitigations that operate in real time during flight.

For BVLOS operations where no visual observer is present, tactical ARC mitigations become critical. The absence of a human observer must be compensated by technical or procedural means — typically electronic conspicuity combined with ATC coordination or a DAA solution — and this must be demonstrated to the CAA's satisfaction through evidence of system reliability and failure mode management.

Deriving the SAIL Level

With a final (post-mitigation) GRC and a final (post-mitigation) ARC established, the SAIL level is read directly from the combination matrix below.

This is the definitive matrix as published in the UK CAA SORA guidance and cross-referenced with JARUS SORA v2.5, as adapted by 2019/947.

SAIL Levels And Operational Safety Objectives (OSOs)

Once the SAIL level is determined, it drives a set of Operational Safety Objectives.

OSOs define what safety performance must be demonstrated, covering everything from pilot competency and equipment reliability to emergency procedures and organisational oversight.

They are the mechanism by which SAIL translates into a concrete evidence checklist.

Each OSO is assigned a required robustness level at each SAIL level: Low, Medium, and High.

The table below, taken from UK Regulation (EU) 2019/947, shows the corresponding OSOs per SAIL. In the table:

• NR means not required.

• L means low robustness.

• M means medium robustness.

• H means high robustness.

SAIL levels in practice

SAIL levels are not theoretical constructs — they define the real-world regulatory pathway for every Specific Category drone operation in the UK.

A few illustrative examples show how the GRC/ARC combination plays out across typical operation types:

Rural VLOS survey, small UAS (<1 m), sparse population, Class G low-level airspace:

GRC 1–2, ARC-a → SAIL I. Minimal evidence burden; basic operational procedures and pilot competency documentation sufficient.

Semi-rural BVLOS infrastructure inspection, medium UAS (1–3 m), sparse population, Class G with limited GA traffic:

GRC 2–3 (mitigated), ARC-b → SAIL II. Documented procedures, C2 link reliability evidence, electronic conspicuity, and route-specific risk assessment required.

Suburban VLOS inspection, medium UAS, populated area, proximity to ATZ:

GRC 4–5, ARC-c → SAIL IV. Formal SMS, structured training, independent equipment validation, and ATC coordination required.

Large UAS BVLOS corridor, controlled airspace, near major aerodrome:

GRC 5+, ARC-d → SAIL V–VI. Evidence approaching manned aviation standards; independent safety review likely required.

heliguy™ obtains SAIL II for remote BVLOS Dock operations

heliguy™ recently secured a UK SORA SORA II to operate DJI Dock 3s BVLOS, on behalf of Network Rail.

The flights - at sites in Romford and Glouster - will be conducted hundreds of miles away by heliguy™ pilots in our Remote Operations Command Centre.

The authorisation required GRC and ARC assessments tailored to a live railway corridor environment, a ConOps that addressed remote launch, recovery, and monitoring without on-site crew, and OSO evidence packages built from the ground up for a drone-in-a-box deployment at scale.

It shows how obtaining SORA is possible and how a well-designed operational concept - with careful geographic containment and targeted ARC mitigations - can keep remote BVLOS deployments within a manageable assurance level.

How heliguy™ can help

Successfully navigating UK SORA requires more than familiarity with the methodology.

It requires the ability to design an operational concept that genuinely minimises risk, the technical understanding to select and evidence appropriate mitigations, and the regulatory experience to know what the CAA expects at each SAIL level - and how to deliver it.

heliguy™ has invested in its regulatory team and wider infrastructure to accelerate UK SORA success and has a track record of achieving regulatory milestones.

We support organisations across the full authorisation lifecycle, including:

• Operational concept development

• Ground Risk and Air Risk assessments

• OSO evidence packages

• BVLOS strategy and drone-in-a-box deployment planning

• CAA application management

• Ongoing regulatory compliance

Summary

SAIL levels translate operational risk into a proportionate regulatory requirement - a calibration of how much safety evidence an operator must produce to earn an Operational Authorisation from the UK CAA.

The critical insight is that SAIL is not fate. A well-designed operational concept - one that scopes its geography carefully, applies the right mitigations, and iterates the ConOps until the risk picture is optimised - can legitimately achieve a lower SAIL level than an operation that accepts default assumptions.

Every SAIL level is earnable through good operational design.

References and sources

• UK Regulation (EU) 2019/947
• CAP3239C: Determining Population Density
• JARUS SORA 2.5