Separation principles

What is being separated from what?

The airspace class and type of flight determine the service.

Vertical and horizontal

ATC provides separation to the combinations of flights required by the applicable airspace class, including IFR–IFR in controlled airspace and additional IFR–VFR/all-flight combinations in the relevant classes. The pilot still maintains collision vigilance in VMC; a clearance does not remove the obligation to avoid collision.

Vertical separation uses different assigned levels with a common reference. The ordinary minimum is 1,000 ft up to FL410 under the applicable RVSM conditions; above, 2,000 ft is normally used. In non-RVSM conditions the ordinary boundary for 2,000 ft is above FL290, and non-approved aircraft in RVSM airspace need the applicable exception/separation. Wake and special cases can require more.

Lateral and longitudinal

Horizontal separation is lateral, longitudinal or a combination established by an approved method. Lateral separation can use geographical references, different routes or protected tracks from navigation aids; the geometry and navigation accuracy must meet the criterion. Longitudinal separation uses time, distance or another approved surveillance/position method along or relative to tracks.

The ordinary surveillance separation is 5 NM unless an approved reduction or other minimum applies; 3 NM and, under specified final-approach conditions, 2.5 NM are not universal defaults. Mach number technique maintains assigned true Mach relationships and appropriate entry spacing so longitudinal separation remains protected over time.

When separation conditions change

Own separation is a specific authorised arrangement, not a blanket handover.

Sequence and level changes

Aircraft approaching the same holding point normally receive levels correlated with arrival sequence, first at the lowest available level, subject to priorities and ATC action. A clearance to maintain a specified vertical separation during a climb/descent requires the prescribed conditions, pilot agreement, communication and other safeguards; it is not authority to guess the other aircraft’s altitude.

An own-separation clearance in VMC is limited to the specified flight portion, class, altitude and conditions, including the pilots’ agreement and visual ability under the applicable rule. ATC must provide essential traffic information where required. The pilot then carries the accepted responsibility for that particular arrangement.

Essential traffic and approved reductions

Essential traffic is controlled traffic for which the required separation is not provided from the aircraft concerned. Pass its direction, type/wake information, level and position/time information as appropriate so the pilot can understand the conflict. Essential local traffic is traffic on or near the aerodrome relevant to a particular movement.

Reductions in separation require the responsible authority’s prescribed conditions, such as suitable equipment, visibility, visual observation or approved local procedures. A controller’s workload or a pilot’s preference is not itself an approval to reduce a minimum.

Can you apply the rules?

Four original questions. Choose from the facts, then review the explanation.