Separation

Lateral separation proves that protected route areas do not overlap. The proof may come from positive position reports, divergent tracks from a common navigation aid or independently protected navigation paths.

Geographical separation

Aircraft are laterally separated when position reports positively place them over different geographical locations whose protected areas do not overlap. The locations may be determined visually or by reference to a navigation aid.

Divergence from the same facility

GNSS distance before separation exists

For a divergence between 15 and 135 degrees, Bali uses 15 NM from the common point from FL 010 through FL 190, and 23 NM from FL 200 through FL 600. These are ground distances. When DME supplies distance, the controller accounts for slant range.

Integrity and offsets

Before applying GNSS track separation, ATC confirms that the aircraft is navigating by GNSS and is not applying an authorised strategic lateral offset. GNSS separation is not used after a reported RAIM outage. When aircraft use different aids or RNAV methods, their protected airspaces or applicable RNP containment areas must not overlap.

Longitudinal separation protects aircraft one behind another or approaching a common point. Before choosing a time or distance minimum, the controller classifies the angular relationship between the tracks.

Track definitions

Longitudinal separation is applied so that the estimated positions of the aircraft never become closer than the prescribed minimum. It may be established by assigning departure times, requiring aircraft to pass a point at stated times, holding an aircraft over a point, assigning an appropriate level change or maintaining a stated distance.

Time-based information

Time separation can use positions and estimates received by voice, CPDLC or ADS-C. Every calculation is cross-checked against the traffic geometry, because a correct subtraction applied to the wrong order of aircraft is still unsafe.

The basic procedural time interval is 15 minutes. It reduces only when position and speed can be checked more frequently, or when the leading aircraft has enough speed advantage and the geometry meets the stated conditions.

Aircraft at the same cruising level

Aircraft climbing or descending

A clearance transmitted through a third party or by CPDLC includes the restriction needed to preserve the timing condition. The controller does not assume that a level change began at the instant the clearance was issued.

Distance-based separation replaces estimates with simultaneous measured distances. The measurements must refer to the same on-track DME station, the same waypoint or a collocated DME and waypoint.

Common-reference conditions

Both aircraft may use DME, both may use GNSS, or one may use each system. Direct controller-to-pilot VHF communication is maintained. Measurements are obtained simultaneously and often enough to confirm that the minimum will not be infringed. When area-navigation aircraft are involved, the controller specifically requests GNSS-derived distance.

When GNSS distance cannot be used

Loss of GNSS integrity, inadequate airborne equipment or loss of the GNSS input to an integrated navigation system requires another form of separation. A GNSS distance from an integrated system is acceptable only when the system provides the required integrity.

ATS surveillance minima in India

Bali states a normal horizontal surveillance minimum of 5 NM. A 3 NM minimum may be specifically authorised within suitable coverage for Delhi, Mumbai, Bengaluru, Kolkata and Shamshabad radars. When ADS-B or MLAT is combined with radar, the applicable minimum is the radar minimum. MLAT used alone follows the minimum prescribed before implementation.

Mach number technique stabilises relative speed between turbojet aircraft on the same track where surveillance is limited. Performance-based minima then connect navigation, communication and surveillance capability to a permitted spacing.

Mach number technique

Each aircraft agrees to maintain the Mach number assigned by ATC for the specified segment, normally to within Mach 0.01. No speed change is made without clearance unless an emergency requires it. Position reports are supplied at compulsory points by the available long-range communication system.

The exact value depends on the applicable Doc 4444 and FIR procedure. On suitable designated RNAV or VOR-defined routes, an 80 NM RNAV distance minimum with Mach number technique may replace the 10-minute interval.

RNP, RCP and RSP

RNP specifies navigation containment. RCP specifies communication transaction performance, including the time for an instruction to reach the aircraft and its acknowledgement to return. RSP specifies surveillance-delivery performance from the service-provider interface to the ATS processing and display system. Performance-based communication and surveillance use these specifications with the aircraft, ground system and operating procedures as one complete chain.

These minima apply only when every required approval, capability and reporting condition is satisfied. If a required performance element is lost, ATC establishes a different valid minimum.

A lifting wing sheds counter-rotating vortices from rotation until touchdown. The strongest operational hazard is produced by a heavy aircraft flying slowly, and the following aircraft is most vulnerable when it is lighter, lower and close behind.

Wake categories

Procedural time spacing

For a SUPER aircraft, the current AIP and controller procedure provides the applicable time minimum. The category word SUPER or HEAVY is included with the call sign on initial contact where required.

Surveillance wake spacing on approach and departure

A separation minimum is a live condition, not a one-time calculation. The controller monitors the geometry, predicts where the current method will cease to work and establishes the next valid method before that point.

Loss or change of the current method

If the navigation aid, communication, surveillance source, aircraft performance or route geometry needed for a minimum is lost, ATC acts before the minimum is infringed. It may stop a level change, issue vectors, assign another level, change speed, increase longitudinal spacing or revert to a larger procedural minimum.

Transfer and pilot-maintained separation

Transfer of control between ATS units does not create a gap in separation responsibility. The agreed transfer condition specifies the aircraft position, level, clearance and any restriction still in force. Responsibility changes only at the coordinated point or time. A pilot accepts responsibility for separation from one named aircraft only under an authorised visual-separation procedure and after reporting the traffic in sight or agreeing to the stated VMC condition.

Essential traffic information

Essential traffic is controlled traffic to which separation applies but which is not, or will not be, separated from another controlled flight by the required minimum. ATC gives both flights the information needed to understand the conflict, normally including direction, aircraft type and wake category when relevant, level and the estimated position or time of closest approach. Information does not by itself establish separation.

Operational sequence

Common exam traps

• Do not use 1,000 ft above FL 290 unless RVSM conditions are satisfied.
• Do not reduce a time minimum unless every speed, reporting and common-point condition is met.
• Do not mix distances from unrelated DME stations.
• Do not confuse traffic information with ATC separation.
• Do not apply a smaller surveillance minimum when wake turbulence requires more.
• Do not authorise cruise climb in an Indian FIR.