Separation Standards Notes (Vertical, RVSM, Horizontal, Longitudinal)

Vertical Separation

  • Definition: Vertical separation exists when the vertical distance between aircraft is never less than the prescribed minimum.
  • What are the minima?
    • Up to and including FL290: 1000\ \text{ft} (i.e., 1000 feet).
    • Above FL290:
    • Non-RVSM airspace: 2000\ \text{ft}.
    • RVSM airspace (RVSM compliant aircraft): 1000\ \text{ft}.
  • How vertical separation is obtained:
    • By requiring aircraft using prescribed altimeter setting procedures to operate at different levels expressed in terms of flight levels or altitudes.
    • Reference framework: Commission Implementing Regulation (EU) 2017-373; ICAO Doc 4444.
  • How vertical separation is assessed by controllers:
    • Observing secondary radar Mode C responses.
    • Obtaining level reports from pilots.
  • RVSM relevance (summary): Vertical separation minima and equipment requirements change above FL290 when RVSM is in effect.

RVSM (Reduced Vertical Separation Minima)

  • What is RVSM?
    • From FL290 to FL410, the standard separation is 1000\ \text{ft} for RVSM-compliant aircraft.
    • Non-RVSM airspace requires 2000\ \text{ft} separation above FL290.
  • Equipment and performance:
    • MASPS: Minimum Aircraft Systems Performance Specification governs RVSM compliance.
    • Equipment categories:
    • Standard Altimeter Equipment
    • Enhanced Altimeter Equipment
    • RVSM compliance conditions are set by MASPS; non-compliant aircraft require 2000 ft separation above FL290.
  • Documentation and reference:
    • See DOC 9574 for more information on RVSM, including equipage of A/C and approval process.
  • RVSM separation specifics:
    • RVSM separation of 1000 ft is maintained up to FL410 when both aircraft are RVSM compliant.
    • Levels are allocated according to a single Alternate Flight Level Orientation System (FLOS).
  • Single Alternate FLOS (levels from FL290 to FL410):
    • FL410, FL400, FL390, FL380, FL370, FL360, FL350, FL340, FL330, FL320, FL310, FL300, FL290
  • RVSM flight planning:
    • Aircraft intending to fly within RVSM airspace (FL290–FL410) shall:
    • Request a Single Alternate FLOS level on the flight plan.
    • Show RVSM status on the flight plan.
  • RVSM Status Qualifiers (FPS):
    • RVA = RVSM approved
    • RVN = Not RVSM compliant
    • RVU = RVSM compliance unknown
    • RVX = RVSM exempt
  • Flight Progress Strips (FPS) indicators:
    • W = the aircraft is RVSM equipped.
    • If not RVSM equipped, Box B (the level box) will show RVN, RVX or RVU as appropriate.
  • Loss of MASPS compliance:
    • If a pilot declares Unable RVSM (due to equipment or turbulence), controller provides horizontal or 2000 ft vertical separation and clears aircraft out of RVSM airspace.
    • If an aircraft cannot maintain RVSM status, the controller is required to clear the aircraft out of RVSM airspace unless in an emergency or such clearance would endanger the aircraft, because en-route safety cases assume only RVSM-approved or exempt aircraft operate in the sectors.
  • RVSM summary (key questions to consider):
    • What are the RVSM minima?
    • How are equipment requirements specified?
    • What is Single Alternate FLOS?
    • How do the attention getters appear on FPS strips?
    • What happens if an aircraft becomes non-compliant?
    • What is the associated phraseology?
  • Practical notes:
    • RVSM airspace is a constrained regime; compliance and accurate reporting of RVSM status are essential for safe reduced separation.

Vertical Separation Rules (General)

  • Climb/descent rules:
    • During climb or descent, an aircraft may be cleared to a level previously occupied by another aircraft after the latter has reported vacating it, except when:
    • Severe turbulence is known to exist.
    • The higher aircraft is effecting a cruise climb.
    • The difference in performance may result in less than the applicable minima; in such cases clearance shall be withheld until the vacating aircraft has reported at or passing another level separated by the prescribed minima.
  • VMC climbs or descents:
    • Applies to Classes D or E airspace.
    • A/C must be in VMC and during daylight hours.
    • Pilot of A/C climbing/descending agrees to maintain own separation from other A/C, whose pilot also agrees to the maneuver.
    • Clearance will be for a portion of flight at or below FL100.
    • If VMC may become impractical, an IFR flight will be issued with alternative instructions.
    • The pilot of an IFR flight shall inform ATC before entering IMC and will comply with the alternative clearance.
    • Essential traffic information shall be passed.

Summary (Vertical)

  • Recap of Vertical Separation Minima, RVSM, and Rules for changing levels; VMC climbs and descents.

Horizontal Separation

Lateral Separation

  • Definition:
    • Lateral separation shall be applied so that the distance between those portions of the intended routes for which the aircraft are to be laterally separated is never less than an established distance plus a buffer.
    • The buffer is determined by the appropriate authority and included in the lateral separation minima as an integral part.
  • How lateral separation is achieved:
    • By operating on different routes or in different geographical locations (visual observation, navigation aids, or RNAV).
    • If navigation equipment fails or deteriorates below performance requirements, ATC shall apply alternative separation methods or minima.
  • Criteria and minima:
    • Achieved by reference to the same or different geographic locations, by position reports indicating different locations, or by the same navigational aid/method with tracks separated by the required minima.
  • Track separation examples (typical minima):
    • VOR/VOR: radials diverging by at least 15^{\u00b0} and at least one aircraft at a distance of 28\ \text{km} \ (15\ \text{NM}) or more from the facility.
    • NDB: diverging tracks by at least 30^{\circ} and at least one aircraft at 28\ \text{km} \ (15\ \text{NM}) from the facility.
  • Different navigational aids and RNAV: ensure that protected airspaces do not overlap when using different nav aids or RNAV equipment; RNAV requires protected airspace separation that does not overlap.
  • Attachments and guidance:
    • Annex 11, Attachments A and B provide material relating to routes defined by VOR and RNAV/RNP usage.
    • Guidance material includes the Air Traffic Services Planning Manual (Doc 9426), the methodology doc (Doc 9689), and the Performance-based Navigation Manual (Doc 9613).
  • Summary of methods:
    • Differing Geographical Locations
    • The same Navigational Aids/Method
    • Different Navigational Aids/Methods

Track Separation (Horizontal) – Practical Notes

  • Parallel tracks: separation is achieved with the use of different nav aids or RNAV; ensure protected airspaces do not overlap.
  • Diverging tracks: ensure the angular divergence and distance from facilities provide adequate protection.
  • Converging tracks: apply appropriate minima for convergence geometry.

Lateral Separation - Key Figures and Rules

  • VOR/VOR: 15° divergence; 28 km (15 NM) or more from facility.
  • NDB/NDB: 30° divergence; 28 km (15 NM) or more from facility.
  • RNAV/RNP: 15° divergence; protected airspaces do not overlap; angular difference drives lateral protection distance.

Horizontal Separation (continued) – Additional Notes

  • The ICAO Doc 4444 standard 5.4 sections provide the baseline minima; states may establish other minima under safe conditions if safety is maintained.
  • Details on track spacing for parallel routes can be found in Annex 11, Attachments A and B; RNAV/RNP considerations are included.
  • Other guidance materials cited include Doc 9426, Doc 9689, and Doc 9613.

Summary (Horizontal)

  • Lateral separation is achieved via different routes, navigational aids, RNAV, or protection of different navigational airspaces.
  • Remember the typical track-based minima for common aids (VOR, NDB) and the RNAV/RNP considerations.

Longitudinal Separation

Definition

  • Longitudinal separation shall be applied so that the spacing between the estimated positions of the aircraft being separated is never less than a prescribed minimum.
  • Separation minimums can be based on time or distance.

Time-based longitudinal separation

  • Can be used for en-route and related operations; time-based separation may rely on position information and estimates from voice reports, CPDLC, or ADS-C.
  • Common time-based minima:
    • Same cruising level & same track:
    • 15\ \text{minutes} or 10\ \text{minutes} if nav aids permit frequent determination of position and speed.
    • If TAS differences exist: 5 minutes if preceding aircraft is maintaining true airspeed of at least 20 knots more than the following aircraft; i.e., ext{TAS}{ ext{preceding}} - ext{TAS}{ ext{following}} \ge 20\ \text{kt}.
    • 3 minutes if the preceding airspeed exceeds following by at least 40\ \text{kt}; i.e., ext{TAS}{ ext{preceding}} - ext{TAS}{ ext{following}} \ge 40\ \text{kt}.
  • Climb/descend on the same track:
    • 5 minutes at the time levels are crossed, provided that the level change is commenced within 10\ \text{minutes} of the time when the second aircraft reported over the same ERP.
    • If nav aids permit, this may be increased to 10\ \text{minutes} or even 15\ \text{minutes} as applicable.
  • Crossing tracks: 15\ \text{minutes} or 10\ \text{minutes} if nav aids permit.
  • Reciprocal tracks: longitudinal separation to be provided for at least 10\ \text{minutes} prior to and after the estimated passing time; once passing has occurred, the minimum need not apply.
  • En-route and holding: longitudinal separation while en-route aircraft are within the published holding path or area requires at least 5\ \text{minutes} flying time of holding aircraft’s flight path.

Time-based longitudinal separation – En-route example

  • Departing aircraft, en-route, or en-route with other operations require appropriate time-based spacing to maintain safety margins.

Longitudinal Separation – Summary of Time-based rules

  • Time separations apply to:
    • Departing
    • En Route and Departing
    • En Route
    • En Route and Holding
  • Time-based separations complement distance-based methods where applicable.

Longitudinal Separation – Distance-based (DME/GNSS)

  • Longitudinal separation can be based on DME/GNSS references by comparing cross-checks with common reference points.
  • Methods include:
    • Same on-track DME station
    • One aircraft using DME and the other GNSS
    • Both using GNSS and same waypoint
  • Separation checks are performed by obtaining simultaneous DME and/or GNSS readings at frequent intervals.

Distance-based longitudinal separation minima

  • Same cruising level, same track: 20\ \text{NM}; 10 NM if leading aircraft TAS ≥ 20 knots higher than following.
  • Same cruising level, crossing tracks: 20\ \text{NM}; 10 NM if leading TAS difference and relative angle < 90°.
  • Climb/descend on same track: 10\ \text{NM} when levels are crossed; one aircraft maintains level flight while vertical separation temporarily does not exist.
  • Reciprocal tracks: 10 NM separation when cleared to climb/descend through level occupied by the other aircraft after passing.

Longitudinal Separation – Interim Summary

  • Longitudinal separation can be time-based or distance-based depending on track geometry, nav aid availability, and speed differences.
  • Summary categories:
    • Time separations: Departing, En Route/Departing, En Route, En Route/Holding
    • Distance separations: En Route, including same level/same track, same level/crossing tracks, climbing/descending and reciprocal tracks

Track Separation (Horizontal) – Parallel, Diverging, Converging

Track separation overview

  • Track separation considers the relative geometry of aircraft tracks (parallel, diverging, converging) and uses lateral separation minima along with track geometry.
  • Track separation is a component of horizontal separation and interacts with lateral and RNAV-based methods.

Track separation criteria

  • Per ICAO Doc 4444, and ICAO 4444 references, the following concepts apply:
    • Parallel track separation can be achieved using different navigation aids or RNAV methods; ensure protected airspaces do not overlap.
    • Diverging tracks rely on angular separation and distance from navigation facilities to ensure protection.
    • Converging tracks require appropriate laterally-protected airspace minima to maintain safety margins.

Track separation – Practical examples

  • VOR/DME/TACAN: If two aircraft are on radials diverging by at least 15^{\circ} and at least one aircraft is at a distance of 28\ \text{km} \ (15\ \text{NM}) or more from the facility, track separation exists.
  • NDB: Divergence criteria are 30^{\circ} and at least one aircraft is at 28\ \text{km} \ (15\ \text{NM}) from the facility.
  • RNAV/RNP: If both aircraft are on tracks diverging by 15^{\circ} or more, the protected airspace associated with each track must not overlap.

Track separation – Additional notes

  • When using different navigation aids or RNAV, ensure that the derived protected airspaces do not overlap.
  • Lateral separation procedures also apply to aircraft arriving/departing, and to those following parallel/non-intersecting tracks where RNAV/RNP is specified.
  • Transition into airspace with higher lateral separation minima should be managed accordingly.

Track Separation – Summary

  • Track separation uses parallel, diverging, and converging conceptions to maintain appropriate lateral protection.
  • The choice of method (VOR/DME, NDB, RNAV) determines the required angular divergence and distance from facilities to guarantee non-overlapping protected airspaces.

Definitions and Terminology

  • Same track: Same direction and intersecting tracks with less than 45° or more than 315° difference, where protected airspaces may overlap.
  • Reciprocal track: Opposite tracks and intersecting tracks with angular difference greater than 135° but less than 225°, with overlapping protected airspaces.
  • Crossing track: Tracks intersecting at angles not covered by the above definitions.
  • ERP: Estimated Position (term used in longitudinal separation discussions).

Horizontal Separation – Important References and Notes

  • ICAO Doc 4444, Section 5.4: States may establish other minima or additional conditions, provided safety is upheld.
  • Annex 11 Attachments A and B: Materials on track definitions for VOR-based routes and RNAV/RNP usage.
  • Guidance Manuals: Doc 9426 (ATM Planning Manual), Doc 9689 (Airspace Planning Methodology for Separation Minima), Doc 9613 (Performance-based Navigation Manual).

Interim Summary (Horizontal)

  • Lateral (horizontal) separation rests on distances and buffers, track geometry, and the selection of navigation aids.
  • Longitudinal separation rests on time or distance between aircraft along the same route or on intersecting routes.
  • Track separation combines these concepts with proper use of VOR/DME, NDB, and RNAV/RNP to maintain safe spacing.

Summary of the Separation Standards (Key Takeaways)

  • Vertical separation: Minimum as a function of altitude/flight level; RVSM reduces vertical separation from 2000 ft to 1000 ft in RVSM airspace (FL290–FL410) for compliant aircraft.
  • RVSM: Requires MASPS-compliant equipment; Single Alternate FLOS provides a set of LVLs from FL290 to FL410; status on FPS; non-compliance results in reversion to non-RVSM separations above FL290.
  • Horizontal separation: Lateral, track geometry, and the use of different nav aids or RNAV to ensure non-overlapping protected airspaces; guidance on VOR, NDB, RNAV/RNP minima and procedures.
  • Longitudinal separation: Time-based and distance-based minima depending on track relationships; rules for same level/same track, crossing tracks, reciprocal tracks, and en-route/holding situations.
  • Rules for climb/descent and VMC climbs/descents: Specific restrictions and exceptions when applying level changes in climb/descent and VMC conditions.

Any Questions or Comments?

  • (Open prompt from slide; ensure understanding of all concepts discussed and how to apply them in scenarios.)