Following our previous discussions on the Concept of Operations (ConOps), Ground Risk Classification (iGRC and Final GRC), and Mitigation Strategies, we now move to the next step of the Specific Operations Risk Assessment (SORA): Determination of the Initial Air Risk Class (ARC).

This step is crucial for evaluating the risk of a mid-air collision between an unmanned aircraft (UA) and manned aircraft within the operational volume. It provides the foundation for determining strategic and tactical mitigations in subsequent steps to minimize air risk.

Understanding the Initial Air Risk Class (ARC)

The Initial Air Risk Class (ARC) is a qualitative classification of the rate at which a UAS would typically encounter a manned aircraft within its operational airspace. It serves as a baseline assessment of airspace collision risks before any mitigations are applied.

The ARC is influenced by several factors, including:

• Altitude of operations

• 
  

  
  

• Controlled vs. uncontrolled airspace

• 
  

  
  

• Proximity to airports, heliports, or urban areas

• 
  

  
  

• Use of segregated or typical airspace

The higher the ARC classification, the greater the unmitigated risk of mid-air collision. ARC-a airspace has the lowest risk, while ARC-d represents the highest risk.

Step 4 Task Description

1. Defining the Operational Volume

The first step in determining the initial ARC involves identifying the vertical limits of the operational volume:

• Define the upper boundary of the flight geography.

• 
  

  
  

• Determine the contingency volume (maximum height the UA may reach if it exits the planned flight geography before returning to it).

• 
  

  
  

• Establish contingency procedures in case of deviations beyond the planned altitude.

2. Airspace Collision Risk Mapping

The Competent Authority, Air Navigation Service Provider (ANSP), or UTM/U-Space Service Provider may publish airspace collision risk maps based on airspace characterization studies. These maps provide a direct assignment of the initial ARC and should be used where available.

If an official air collision risk map exists for the operational area, operators can refer directly to it to determine the ARC and proceed to Step 5 (Application of Strategic Mitigations).

3. Identifying the Initial ARC Using the SORA Decision Tree

If no air collision risk maps are available, the SORA decision tree is used to classify the ARC based on:

• The type of airspace (controlled vs. uncontrolled)

• 
  

  
  

• Whether operations occur near airports or heliports

• 
  

  
  

• Whether the operational volume is in an urban vs. rural area

• 
  

  
  

• Whether the operation takes place in typical vs. atypical airspace

Classification of Airspace and ARC Levels

The ARC categories define the baseline air risk level of a UAS operation before any mitigations are applied:

1. ARC-a (Lowest Air Risk Environment)

• Defined as airspace where the risk of collision between a UAS and manned aircraft is inherently low.

• 
  

  
  

• Typically found in reserved, restricted, or segregated airspace.

• 
  

  
  

• May also apply to very low-altitude operations in areas where manned aircraft activity is rare (e.g., industrial sites, close to obstacles).

2. ARC-b (Moderate Air Risk Environment)

• Airspace where manned aircraft operations occur but at low density.

• 
  

  
  

• Examples include rural uncontrolled airspace with limited general aviation activity.

• 
  

  
  

• Tactical mitigations (such as electronic conspicuity) may be required in later steps.

3. ARC-c (High Air Risk Environment)

• Airspace with moderate to high levels of manned aircraft traffic.

• 
  

  
  

• Includes uncontrolled airspace in suburban or semi-urban environments.

• 
  

  
  

• Typically requires both strategic and tactical mitigations to lower air risk.

4. ARC-d (Highest Air Risk Environment)

• Airspace with frequent manned aircraft operations.

• 
  

  
  

• Includes controlled airspace near major airports, heliports, or dense urban environments.

• 
  

  
  

• Requires Mandatory coordination with ANSP and robust detect-and-avoid capabilities.

• 
  

  
  

• May require alternative solutions such as certification under manned aviation regulations.

Challenges in Assigning the Initial ARC

• Multiple airspace environments: The operational volume may extend across different airspace classifications, requiring a risk assessment for each environment.

• 
  

  
  

• Conservative ARC assignment: The default ARC assignment is intentionally conservative. However, authorities may adjust the classification upwards if the assumptions in the decision tree are invalidated.

• 
  

  
  

• ANSP consultation: Operators should engage with the ANSP or Competent Authority to verify ARC assumptions and airspace characteristics.

Final Outcome of Step 4

By the end of Step 4, the operator should have:

• Identified the airspace collision risk within the operational volume.

• 
  

  
  

• Documented references and methodologies used to determine the initial ARC.

• 
  

  
  

• Prepared to apply strategic mitigations in Step 5 to reduce the ARC if necessary.

Conclusion

Step 4 of the SORA process provides a structured method for assessing the intrinsic air risk of UAS operations. By understanding the different ARC classifications, operators can take appropriate measures to mitigate mid-air collision risks in subsequent steps.

At AirHub Consultancy, we support enterprises in navigating airspace risk assessments, compliance with U-space regulations, and air traffic coordination. Our AirHub Drone Operations Platform offers tools for assessing operational volumes and strategic mitigations.

Stay tuned for our next blog, where we explore Step 5 of SORA: Application of Strategic Mitigations to Reduce Air Risk!