Separation Methods and Minima

What are the ways of keeping aircraft apart?

The big picture

Who gets separated, and the three ways to do it. Tap a box with + to open it.

• Separation vertical, horizontal or composite ATC keeps controlled traffic apart by vertical or horizontal separation, or by composite separation that combines the two, in the en-route, arrival and departure phases.
  • Who is separated Whether a flight must be given vertical or horizontal separation depends on the airspace class and on whether the flights are IFR, VFR or special VFR.
    • Class A and B all flights In Class A and B airspace every flight is given vertical or horizontal separation from every other flight.
    • Class C, D, E all IFR flights IFR flights are separated from each other in Class C, D and E. In Class C, IFR flights are also separated from VFR flights.
    • Special VFR from IFR and from each other Special VFR is kept apart from IFR traffic. Special VFR flights are also kept apart from one another when the ATS authority says so.
    • Own separation daylight VMC, Class D and E On the pilot's request, after ATC gets the other pilot's agreement.
  • Vertical 1,000 or 2,000 ft A nominal 1000 ft below FL290 and 2000 ft at or above FL290, except 1000 ft when both aircraft are RVSM compliant in RVSM airspace.
    • Below FL 290 1,000 ft Below FL290 the vertical separation minimum is a nominal 1000 ft.
    • Above FL 290, non-RVSM 2,000 ft At or above FL290 the minimum is a nominal 2000 ft, unless both aircraft are RVSM compliant in designated RVSM airspace.
    • RVSM aircraft 1,000 ft If both aircraft are approved for RVSM and are inside RVSM airspace, 1000 ft is enough even at FL290 and above.
    • Vacated level cleared on the vacating report In severe turbulence, or with both aircraft in the same holding, the clearance waits until the first aircraft reaches a separated level.
    • No cruise climb in Indian FIRs A cruise climb may not be flown anywhere in the Indian FIRs.
  • Horizontal track, time or distance Three kinds: lateral (side by side), longitudinal (one behind the other), and minima that rely on radar or other ATS surveillance.
    • Lateral different tracks Aircraft are kept over different geographic locations, or on specified tracks separated by a minimum amount suited to the navigation aid used.
      • VOR 15° at 15 NM The two aircraft fly radials that spread apart by 15° or more, and one of them is already 15 NM or further from the VOR.
      • NDB 30° at 15 NM The two aircraft fly NDB tracks (inbound or outbound) that spread apart by 30° or more, and one of them is already 15 NM or further out.
      • GNSS 15 or 23 NM by level Each aircraft is locked onto a GNSS track between two waypoints, with no offset. The tracks cross at 15° to 135°, and one aircraft is 15 NM (FL010-190) or 23 NM (FL200-600) away from the crossing point.
      • RAIM outage no GNSS separation GNSS track separation is not applied when the pilot reports a RAIM outage.
    • Longitudinal same route, time or distance Aircraft along a route are spaced by time or distance so that the gap between their estimated positions never falls below a prescribed minimum.
      • By time 15, 10, 5, 3 min Same level, same track: 15 minutes, or 10 minutes when navigation aids give frequent position fixes; 5 minutes if the leader is 20 kt faster, 3 minutes if 40 kt faster.
      • By distance DME or GNSS, 20 or 10 NM Same level: 20 NM on the same track, or on crossing tracks under 90 degrees; 10 NM when the leader is 20 kt or more faster. Readings come from the same DME or waypoint.
      • Mach number technique 10, 9 or 8 min Jets on the same track normally keep 10 minutes apart. If the one in front is quicker by 0.02 Mach the gap stays 10 minutes, by 0.03 Mach it can be 9, and by 0.04 Mach 8.
      • Performance-based 50 NM or 30 NM by RNP For aircraft on the same track, or crossing at less than 90 degrees: 50 NM with RNP 10, or 30 NM with RNP 4, with RCP 240 and RSP 180.
    • Composite vertical plus horizontal Each part (vertical and horizontal) can be smaller than usual, but never less than half its normal figure.
  • Surveillance radar Horizontal minima based on radar, ADS-B and/or MLAT under AIP India: 5 NM under surveillance coverage, 3 NM only where specifically authorised.
    • General 5 NM 5 NM is the horizontal minimum anywhere within airspace under surveillance coverage.
    • Major terminal radars 3 NM 3 NM only where it has been specially approved: the Delhi, Mumbai, Bengaluru, Kolkata and Shamshabad radars.
    • Essential traffic told when not separated Controlled traffic that should be separated from you but is not (or soon will not be) kept apart by the normal minimum. ATC tells you about it.
    • FMS for conventional procedures if the AFM permits When the Aircraft Flight Manual authorises it, FMS/RNAV may fly conventional procedures, monitored on the basic display within raw-data tolerances.

Takeaway: Aircraft are kept apart by height, by track, by time or distance along a route, or on radar.

How far apart must two aircraft be?

Minima

Three dials and a board: height, time, Mach and radar distance.

Separation keeps aircraft a minimum distance apart: by height, by time or distance along the route, or on radar.

• Separation keeps aircraft a minimum distance apart: by height, by time or distance along the route, or on radar.
• Levels
• How much height between two aircraft?
• Drag to a flight level. FL 290 is the dividing line.
• low
• high
• 1,000 ft
• below FL 290, everyone
• Below FL 290 the minimum is a nominal 1,000 ft, for every aircraft.
• 2,000 ft, or 1,000 ft RVSM
• at or above FL 290
• At FL 290 and above: normally 2,000 ft, but 1,000 ft if both aircraft are approved for RVSM and are inside RVSM airspace.
• Speed
• Same level, same track: how many minutes?
• Drag to how much faster the leader is flying than the aircraft behind (true airspeed).
• same speed
• much faster
• 15 or 10 minutes
• 10 with frequent fixes
• With no big speed difference the basic minimum is 15 minutes, or 10 minutes where navigation aids permit frequent position and speed fixes.
• 5 minutes
• leader 20 kt or more faster
• Same track, leader at least 20 kt faster.
• 3 minutes
• leader 40 kt or more faster
• Same track, leader at least 40 kt faster.
• Mach
• Mach number technique
• Turbojets on the same track. Drag to how much faster, in Mach, the leader is.
• same Mach
• faster
• 10 minutes
• normal, and up to 0.02 faster
• Example: A at M0.84 ahead of B at M0.82 needs 10 min. Turbojets on the same track normally get 10 min, level, climbing or descending. RNAV may use 80 NM instead.
• 9 minutes
• leader 0.03 Mach faster
• Jets on the same track normally keep 10 minutes apart. If the one in front is quicker by 0.02 Mach the gap stays 10 minutes, by 0.03 Mach it can be 9, and by 0.04 Mach 8.
• 8 minutes
• leader 0.04 Mach or more faster
• Radar
• Where 3 NM is allowed
• 5 NM anywhere under surveillance cover; 3 NM only at these five radars. Tap a row.
• Delhi: 3 NM
• 3 NM authorised
• Delhi, Mumbai, Bengaluru, Kolkata and Shamshabad.
• DELHI
• Mumbai: 3 NM
• MUMBAI
• Bengaluru: 3 NM
• BENGALURU
• Kolkata: 3 NM
• KOLKATA
• Shamshabad: 3 NM
• SHAMSHABAD
• Everywhere else: 5 NM
• 5 NM under surveillance coverage
• With radar and/or ADS-B or MLAT, 5 NM applies within airspace under surveillance coverage.
• ELSEWHERE

Takeaway: 1,000 ft below FL 290, 2,000 ft above unless RVSM. Same track: 15 or 10 min, 5 if 20 kt faster, 3 if 40 kt. Mach: 10, 9, 8 min. Radar 5 NM, 3 NM at five.

When may you take a level someone has just left?

Level changes

Vacated levels, pilots' own separation and VMC climbs. Tap + to open each.

This lesson is part of the Exam-Ready plan. See plans

How far apart must two tracks diverge?

Lateral

Both aircraft fly away from the same aid. Separation exists once both are 15 NM out.

This lesson is part of the Core plan. See plans

Same, crossing or reciprocal?

Track geometry

Put your track at the top and see where the other aircraft's track falls.

This lesson is part of the Core plan. See plans

How do different nav aids keep tracks apart?

Lateral methods

Same aid, different angles. Also: position reports over different places, or protected airspaces that don't overlap.

This lesson is part of the Exam-Ready plan. See plans

Which time minimum applies?

Which time minimum?

Read each case and press the stamp that applies.

This lesson is part of the Core plan. See plans

How many minutes when climbing through another's level?

Climb through

Same or crossing track, no vertical separation yet. Tap each marker.

This lesson is part of the Exam-Ready plan. See plans

When can opposite traffic climb through each other's level?

Passing head-on

Two aircraft on reciprocal tracks use the same DME or fix. Fly the steps.

This lesson is part of the Core plan. See plans

How does DME or GNSS distance separation work?

DME and GNSS

Find the situation, read the minimum and the conditions.

This lesson is part of the Exam-Ready plan. See plans

How does the Mach number technique keep aircraft apart?

Mach technique

Two jets on one oceanic track, the faster one ahead. Tap each number.

This lesson is part of the Exam-Ready plan. See plans

What do RNP, RCP and RSP numbers mean?

RNP, RCP, RSP

One row of the performance table. Tap each part.

This lesson is part of the Exam-Ready plan. See plans

Which longitudinal minimum for which performance?

Performance minima

Every row needs RCP 240 and RSP 180. Read across.

This lesson is part of the Exam-Ready plan. See plans

What separation do radar, ADS-B and MLAT give in India?

Radar minima

Surveillance minima from the AIP. Tap + to open each.

This lesson is part of the Exam-Ready plan. See plans

Numbers to know

Numbers

The figures examiners like, in one list.

This lesson is part of the Core plan. See plans

Pairs that catch students out

Don't mix up

Look-alike rules side by side, and how to tell them apart.

This lesson is part of the Core plan. See plans

Test yourself

Ten questions from this chapter.

This lesson is part of the Core plan. See plans