The Importance of SORA in Specific UAS Operations: Mastering SAIL Differences for Optimal Compliance, Efficiency, Reliability and Robustness

In the dynamic and growing world of Unmanned Aircraft Systems (UAS), safety and regulatory compliance are fundamental aspects. The Specific Operations Risk Assessment (SORA) procedure has become an essential tool for conducting risk assessments for operations included in the specific category, according to the European UAS regulation established by the European Union Aviation Safety Agency (EASA). This meticulous procedure was released by the Joint Authorities for Rulemaking on Unmanned Systems (JARUS) organization and later adopted by EASA to provide a common methodology for guaranteeing safety in a UAS operation.

This methodology analyzes various critical aspects of a UAS operation, such as the type of aircraft, the type of operation, and the associated risks both on the ground and in the air. According to the combined level of risks, SORA defines up to six SAILs (Specific Assurance and Integrity Level) that determine the robustness of the specific Operational Safety Objective (OSO). Therefore, the proper implementation of SORA not only defines the level of risk of an unmanned mission but also provides a structured framework for risk assessment and mitigation.

In this article, we want to briefly review some of the main topics where we can find differences among the SAIL:

• Ground Risk Class (GRC): It is a quantitative measure, from 1 to 7, of the risk that a person on the ground can be struck by the UAS in case of loss of control. To determine the GRC, the applicant must consider the maximum dimension of the UAS, the intended operational scenario, and the overflown area, ensuring compliance with both qualitative and quantitative criteria.
• Air Risk Class (ARC): It is a qualitative measure, determined by the qualification from ARC-a to ARC-d, of the risk of a mid-air collision based on the operational airspace defined in the Concept of Operations (ConOps). Risks are reduced through strategic mitigations, like specific operating times, and tactical mitigations, such as detect and avoid systems. UAS operators must collaborate with airspace service providers for necessary authorizations and to enhance UAS detectability.
• Robustness of the Flight Control System (FCS): This point is related to design-oriented OSOs, OSO #5 being particularly relevant. OSO #5 involves conducting a Functional Hazard Assessment (FHA), which derives the level of reliability required for the UAS subsystems, including the FCS. The higher the SAIL, the lower the failure probability and the more redundancy strategies are required for critical UAS systems.
• Need for Document Evidence: Depending on the SAIL, the operator must provide specific documentation. For low-risk levels, a declaration is generally sufficient. For medium-risk levels, evidence must be provided to show that the UAS has been designed according to established procedures. In the case of high-risk operations, this evidence is typically validated by the competent aerospace authority.
• Authorization Process: It relates to the regulatory authority or agency responsible for reviewing and approving the operation. For operations up to SAIL III, approval may be granted by the National Aviation Authorities. However, for higher-risk missions, EASA must oversee the design of the aircraft.
• Operator Training Requirements: To ensure safe and efficient UAS operations, operators must provide competency-based training, with higher levels requiring validation by the competent authority. 

Given this context, it is logical to deduce that the higher the SAIL, the greater the technological complexity required by the system. Therefore, a low or medium-risk operation cannot be compared to the complexity and requirements of those of a high-risk or even of the certified category (e.g., eVTOL, UAM and air taxis, which involve transport of people). For the latter, systems such as FCS must be designed in accordance with standards (e.g., DO-178C, DO-254) and subject to certification processes equivalent to those of manned aircraft, which would be disproportionate for a medium-risk operation (up to SAIL III or IV). This is why UAV Navigation-Grupo Oesía focuses on modular and efficient architectures adapted to the different SAILs in order to achieve the required level of safety without penalizing the weight and size of critical components of the guidance, navigation and control system.

Therefore, it is essential to keep up to date with the evolution of the regulations and to have a good characterization of our system. Proof of this is our proactive participation in standardization working groups led by regulatory organizations such as the European Organization for Civil Aviation Equipment (EUROCAE) or the reliability studies such as the one performed for the VECTOR-600 autopilot, which has demonstrated a probability of loss of mission per flight hour of 1.809E-05 under its operating conditions, ensuring reliability and robustness.

At UAV Navigation-Grupo Oesía, we are firmly committed to supporting our clients in the authorization processes. With a deep understanding of all the necessary details, our team is dedicated to ensuring the success of our clients' operations. From the initial stages of an authorization process to the final execution of the mission, we offer comprehensive advice that covers all aspects of the SORA procedure. Our experience and dedication allow us to guide our clients through each step of the process, ensuring that every operation meets the highest standards of safety and efficiency.