Airspace and Avoiding Collisions

Avoiding collisions when operating Unmanned Aircraft and Airspace issues

Avoiding Collisions

The requirement for avoiding collisions between aircraft, or between aircraft and objects, applies equally to manned and unmanned aviation. Therefore, appropriate steps must be taken to cater for the absence of a pilot within the aircraft. For UAS flights, the methods used to prevent collisions depend on whether the aircraft is being flown within or beyond the 'Line of Sight' of its pilot.

Visual Line of Sight Operations

Visual Line of Sight is termed as being the maximum distance that the flight crew is able to maintain separation and collision avoidance, under the prevailing atmospheric conditions, with the unaided eye (other than corrective lenses). For flights within Line of Sight, the pilot is required to employ the See-and-Avoid principle through continued observation of the aircraft, and the airspace around it, with respect to other aircraft and objects. Within the UK, Visual Line of Sight operations are normally accepted out to a maximum distance of 500 m horizontally, and 400 ft vertically, from the pilot.

Beyond Line of Sight Operations

For operations beyond Line of Sight, it is not possible for the pilot to directly see the unmanned aircraft and avoid other aircraft or objects. Therefore alternative arrangements to prevent collisions must be taken. In these cases, the aircraft must either be fitted with a Sense-and-Avoid system or, in the absence of such a system, it must be operated within Segregated Airspace.

Sense-and-Avoid is a generic term used to describe a system involving one or more sensors, which has the capability to see, sense or detect conflicting traffic or other hazards and take the appropriate action to comply with the applicable rules. In this way, the system acts as a substitute for See-and-Avoid in manned aircraft.

Segregated Airspace, as the name suggests, is a block of airspace specifically allocated for an unmanned aircraft's flight. Collision risks are eliminated by either preventing or strictly controlling entry to this airspace by other aircraft.

The CAA is not currently aware of any Sense-and-Avoid system with adequate performance and reliability, but several areas of segregated airspace have already been established to enable unmanned aircraft to undergo development and testing.


As indicated above, it may be necessary to operate an unmanned aircraft within segregated airspace if the pilot wishes to fly it beyond unaided visual line of sight.

Unmanned aircraft with a mass of more than 7 kg (excluding fuel) must not be flown within controlled airspace, restricted airspace or an Aerodrome Traffic Zone (ATZ) unless permission has been obtained from the relevant ATC unit. More information about contacting ATC can be obtained from the Aeronautical Information Service (AIS).