Technical and operational mitigations for air risks - Specific category

When preparing for all operation types in the ‘specific’ category, the operator must familiarise themselves with the operational volume and the likelihood of manned aircraft in the volume before starting the operation. The density of manned aviation in the operational volume and the surrounding traffic observation area impact the air risk class (ARC) applicable in the area. When reviewing air risk, the initial risk class based on the characteristics of the volume must be determined first. The flight geography and contingency volume that make up the operational volume must be determined for the assessment of air risk.

Flight geography is the volume in which the UAV is operated normally, e.g., according to a programmed trajectory, or the normal operational volume in manual control, or a point-form location where the UAV is elevated to hover. An area of at least ten meters must be calculated around the nominal trajectory, point or operational volume, or if the speed of the UAV exceeds 5 m/s, a distance corresponding to a flight time of two seconds. Operations are normal and according to plan in the flight geography.

Contingency volume is determined outside the flight geography. The size of the contingency volume is a minimum of ten meters, or if the speed of the UAV is 5 m/s, the minimum distance corresponding to a flight time of two seconds of the UAV. If the UAV ends up in the contingency volume, contingency procedures must be initiated immediately to bring the UAV back into the flight geography. The performance of the planned task itself is interrupted until the UAV is returned to the flight geography, or if returning is impossible, the UAV must be landed. 

Operational volume consists of the flight geography and contingency volume.
 

The air risk classes are as follows:

• ARC-a, airspace where the risk of a collision between a UAS and a manned aircraft is acceptable without the addition of any tactical mitigation. These airspace volumes include

1. Separate volumes reserved for unmanned aircraft operation.
2. Danger or restricted area, or an unmanned operation at a very low flight altitude (30 m AGL, 15 m above an obstacle).
3. In the immediate vicinity of an existing obstacle, either natural or manmade (up to 50 m horizontal distance).

• ARC-b, volume of airspace where the probability of encountering another manned aircraft is elevated and low integrity tactical risk mitigations are required. An example of such a volume is an uncontrolled airspace, airspace class G, in a sparsely populated environment and at an elevation of up to 150 m from the surface of the earth.
• ARC-c, volume of airspace where the probability of encountering another manned aircraft is clearly elevated, the operation takes place in the same airspace where manned aircraft operate normally. Requires medium integrity tactical risk mitigations. These airspaces include controlled airspace outside the restrictive UAS geographical zones of aerodromes, controlled or uncontrolled airspace above densely populated environments.
• ARC-d, volume of airspace in the area of aerodromes, with manned aircraft operating low, e.g. take-off and landing. The operations require high-level risk mitigations or operating procedures clearly agreed with the ATS provider. 
   

Due to the ultimate residual risk level of the currently available UAV equipment and operations, the maximum acceptable air risk class is ARC-b.

Initial air risk can be mitigated by employing strategic air risk mitigations. Strategic mitigations are not mandatory, if the air risk of the operations is sufficiently low, but several forms of operations require risk mitigation. Based on the table above, the very typical operations type will be classified as ARC-c in air risk assessment, e.g. when the operations occur in densely populated environments.

Air risk can be mitigated by restricting the flight altitude used during the operation. According to Traficom’s interpretation, air risk can be mitigated from ARC-c to ARC-b by using the same flight altitude in the ‘specific’ category that is allowed in the ‘open’ category in the operational volume in question. E.g. by decreasing flight altitude 150 m AGL to 120 m AGL in urban areas. Operating outside restrictive UAS geographical zones is also considered in air risk assessment and it is deemed a strategic mitigation.

Operators also have other means of demonstrating that the air risk of a planned operation in the location in question is actually smaller than illustrated in the above table, and the operator may use this as a strategic air risk mitigation. These include operating in a restricted area in the vicinity of an aerodrome based on an advance agreement with the aerodrome or the party providing air navigation services, where no manned aircraft are flying, or by changing the time of unmanned aircraft operation to a time when there is no manned aviation in the operational volume. 

In addition to the air risk class and mitigations, the operator must also observe the tactical mitigations, i.e. mitigations executed during the operation itself. Tactical mitigation performance requirements (TMPRs) have been determined for each air risk class. These requirements specify the UAV system air traffic observation and deconflicting ability requirements in accordance with the different air risk classes, which proportion of the traffic approaching the operational volume must the operator be able to observe and how the UAV must perform in deconflicting a manned aircraft. The detection distance of manned aircraft is connected to all TMPRs. The detection distance is greatly dependent on detected traffic, especially the speed of traffic, and the UAV’s ability to deconflict has an impact on it. No specific dimensions of the detection distance can be given due to changing factors.

• ARC-a, no TMPRs. As the risk of collision in the airspace in question is very low, no observation or deconflicting ability is required.
• ARC-b, low integrity TMPR. In this air risk class, the UAV system must be able to observe 50% of manned aircraft entering the observation area. As a deconflicting measure, the UAV must be able to decrease its altitude to 20 metres in less than 60 seconds after the deconflicting order is issued, or otherwise deconflict the manned aircraft with a clear margin.
  • Technical systems that can be used for observing manned aircraft can include:
    • web-based general air service monitoring sites
    • an ADSB-in/FLARM/Pilot Aware receiver
    • U-space dynamic geofence
    • monitoring the aviation radio frequency.
• ARC-c, medium integrity TMPR. In this air risk class, the UAV systems must be able to observe 90% of manned aircraft entering the observation area. This level of performance is not addressed in this training material in more detail, as the residual SAIL risk of the air risk class in question is high.
• ARC-d, the TMPR for the system complies with requirements RTCA SC-228 or EUROCAE WG-105 MOPS/MASPS or similar, and will not be addressed in more detail in this training material.

Flights based on visual line of sight, VLOS, meet the TMPR on the ability to observe and deconflict without separate technical systems.