#4 - The Air Risk Class (ARC)

The process used in the SORA to determine the Air Risk Class (ARC) is covered by steps #4, #5, and #6. The aim is to define the mid-air collision risk and apply mitigations to it. However, some risks remain uncovered by the SORA, for example, the probability of a UAS- UAS encounter, the risk due to wake turbulences, adverse weather or lost link. The airspace considered for this evaluation is the one defined in the operator’s ConOps and is specific to the operation planned.

To begin the assessment, the operator needs to find the initial ARC for the operational volume by taking over the category already established by the competent authority or, if this first option is not available, by using the chart provided in the JARUS Guidelines. This chart is shaped as a decision tree in which each type of environment corresponds to a level of risk and gives a result ranging from the lowest risk ARC-a, to the highest risk ARC-d. In total, there are 13 different collision risk categories and each of them is assigned a level from ARC-a to ARC-d depending on the characteristics of the airspace. In some cases, the risk in the operational volume happens to be different than the category it is placed in: this is usually due to the use of strategic mitigations. By using such optional strategies, the operator can claim a lower category for its assessment than the one initially assigned, but there should always be a discussion with the ANSP to assure that the level claimed is the correct one and that the strategic mitigations are sufficient to guarantee safety at all times. Two different types of strategic mitigations are available: mitigation by operational restriction and mitigation by common structures and rules. These means of mitigation have in common that they all need to be applied before take-off, but they differ in having their own specific limitations and ways to be applied, described in detail in the SORA Annex C. Typical examples of strategic mitigation by operational restriction would be to avoid performing operations during peak hours when the risk of collision is the highest or to geographically limit the operational volume to a lower-risk space like a remote sheltered corner of an airport area. These actions can all be taken by the operator who has control over them and can decide to put them in place if deemed necessary. On the other hand, mitigations by common structures and rules can be applied only by the competent authority and are not flexible. For them to be able to safely control the flow of traffic, they require all participants to follow the same set of rules that allows central management to provide separation. This type of mitigation is either available to the operator or not.

The air risk category retained after applying all strategic mitigations corresponds to the residual ARC and will serve to determine requirements in the last part of the Air Risk Process, step #6 Tactical Mitigation Performance Requirement (TMPR) and Robustness Levels. For a Residual ARC-a, there are no requirements in this step #6 since the risk is already considered to be at an acceptable level and the applicant can directly proceed to step #7, whereas for all other residual risk categories (ARC-b to ARC-d), objectives to be met vary from low to high levels of robustness. The TMPR is the total performance required by all tactical mitigations combined and must also take into account the interactions between them. In some cases, combining mitigations does not improve effectiveness because interdependencies can occur and produce negative consequences on the final outcome. Concretely, tactical mitigations are to be applied after take-off to reduce the risk of collision by following a feedback loop. The principle of a feedback loop is that the situation is dynamically re-evaluated based on real-time information and as a result corrective actions can be taken to maintain safety throughout the operation. In VLOS operations, tactical mitigations follow the “See and Avoid” principle. Since the remote pilot always keeps visual contact with the aircraft (s)he can scan the surrounding airspace and take action if necessary, for example when another aircraft is detected in the vicinity. VLOS is generally sufficient mitigation to comply with low, medium and high levels of robustness unless the competent authority decides otherwise. Additionally, when VLOS is used as a means to meet the safety objectives, it is important that the operator develops a complete deconfliction strategy that covers the complexity of the operation and in which all methods, criteria, procedures, training, and phraseology should be described. In BVLOS operations, the situation is different and also more complex since the remote pilot is not able to maintain direct visual contact with the aircraft but uses an alternate means to human vision instead. These alternate means are described as “Detect and Avoid” (DAA) and cover both ground-and air-based detect and avoid systems. They can include for example a collaboration with ATC services, TCAS, UTM, or U-Space, and may integrate a wide range of sensors and detection systems. Generally, various means of mitigation are combined in BVLOS operations to fulfill the required TMPR together. All details about the criteria and robustness levels for tactical mitigations can be found in the SORA Annex D.

The complete air risk process described above can be summarized as follows: Initial ARC determination ⇨ strategic mitigations ⇨ residual ARC ⇨ tactical mitigations.

If you enjoyed reading this article and want to learn about the other steps of the SORA, have a look at my other articles in this series: they will provide you with an overview of the whole process!

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