North Atlantic Reduced Lateral Separation "RLatSM" Trials

On or soon after 12 November 2015, Gander and Shanwick OCA’s will begin trials of a 25NM lateral separation minimum in the Nat Organized Track System, NAT OTS system portions of the Gander and Shanwick Oceanic Control Areas, OCAs’. This trial is part of the NAT Datalink Mandate implementation and has been widely published via NOTAM, Aeronautical Information Circular and operational bulletins.

Track spacing for MNPS-approved aircraft is currently one degree of latitude, which equates approximately to 60NM. This change will reduce lateral separation for to 25NM by establishing tracks that are spaced by one-half-degree of latitude. This track spacing initiative is referred to as Reduced Lateral Separation Minimum “RLatSM” is the abbreviation that will be used for this procedure.

Planners and Dispathcers Should Note

RLatSM will be implemented using a phased approach. The first portion of the airspace to implement this one-half-degree spacing will be in the OTS between the two core tracks from flight level FL 350 to FL 390 inclusive. All RLatSM tracks and FLs will be uniquely identified in note 3 of the OTS track message.  Other published OTS flight levels are not part of the RLatSM trial. Phase 2 will expand the implementation throughout the entire NAT OTS at a later date.

Operators eligible to flight plan RLatSM tracks IF the three elements below are approved and operational:


As you might expect, any failure or malfunction of GPS, ADS-C, or CPDLC equipment is required to be reported to Air Traffic Control as soon as it becomes apparent.

ATC will use Field 10 (Equipment) and Field 18 (Other Information) of the standard ICAO flight plan to identify an aircraft’s data link and navigation capabilities. Operators should insert the following items into the ICAO flight plan for RNP 4 authorized and FANS-1/A or equivalent aircraft:

  • 'J5' to indicate CPDLC FANS1/A SATCOM (Inmarsat) and/or
  • “J7” to indicate CPDLC FANS1/A SATCOM (Iridium)
  • 'D1' to indicate ADS with FANS 1/A capabilities.
  • 'PBN/' followed by 'L1' for RNP 4.
  • If a flight experiences a failure of an RLatSM required system on the ground or anytime prior to entering RLatSM airspace, the flight should flight plan so as to remain clear of NAT RLatSM tracks between FL 350-390. If a flight experiences a failure of an RLatSM required system while operating in RLatSM airspace, ATC must be immediately advised. Such flights should expect to be re-cleared and exit RLatSM airspace, but consideration will be given to allowing the flight to remain in the airspace, based on specific ATC “Tactical” considerations.

Pilots Face SOP Issues

  • Track spacing for RLatSM involve the use of waypoints consisting of half-degree coordinates verification of waypoint degrees and minutes takes on new importance. FMS and NAV display limitations result in truncation (READ: “Abbreviation”) of waypoints consisting of latitude/longitude to a maximum of seven characters. Minutes of latitude are not displayed.  For example: “N50W020” could represent a whole degree 50.00 North or a half degree 50.30 North of latitude. Only thru full 13-character representations of latitude/longitude waypoints, expanded and reviewed via the FMC display can this discrepancy be resolved.
  • To mitigate the possibility for gross navigation errors resulting from incorrect waypoint insertion, procedures must be used to display and verify the DEGREES and MINUTES loaded into the Flight Management Computer,FMC for the “un-named” (Lat/Long) waypoints defining the route contained in the oceanic clearance. Regardless of FMC waypoint format and entry method, crew procedures should require each pilot independently display and verify the degrees and minutes loaded into the FMC for each oceanic waypoint defining the cleared route of flight.
  • Approaching an oceanic waypoint, one pilot should verify the full latitude and longitude coordinates of that waypoint in the FMC, the NEXT and NEXT +1 waypoints, while the other pilot crosschecks the latitude and longitude coordinates against the master flight plan/oceanic clearance. It is strongly recommended that pilot pre-flight and in-flight procedures call for the pilot to compare the track and distance between waypoints shown on the Computer Flight Plan, CFP to those displayed by the FMC. Pilots should be aware that waypoint insertion errors of ½degree of latitude might in some cases result in only small differences in track and distance. The currency of magnetic variation tables loaded into aircraft navigation databases and the point at which the track is measured affect the track displayed on the FMC by as much as +/-3 degrees.
  • Crew training programs, operations manuals and strong crew resource management techniques prevent opportunities for error occurring as a result of confirmation bias Training and SOPs must incorporate guidance to enable pilots to understand map and FMC displays of ½ _degree vs. whole degree waypoints and how to mitigate misinterpretation of waypoint coordinates during flight.
  • Pilot’s following the Flight Plan in-stead-of the CLEARANCE is historically the #1 cause of Large Height Deviations,LHD and Gross Navigation Errors,GNE in the NAT. Close crew adherence to proven and well established oceanic SOPs is the best Risk  Mitigation tool available to crews in this situation. If pilots are notified that ATC indicates the aircraft is not flying the cleared route, pilots are expected to immediately display the full degrees and minutes loaded into the FMC for the NEXT and NEXT + 1 waypoints, and verify against the cleared route. Pilots should also be advised that if ATC reroutes flights onto RLatSM identified tracks containing ½ degree coordinates, this will only be transmitted via CPDLC using Uplink Message UM79, UM80 or UM83. Aircraft will therefore not be re-routed onto ½ degree OTS tracks if ARINC 623 data link or voice is used for the issuance of the oceanic clearance.

Operational Contingencies, Diverts and Weather Deviations

  • Pilots must be aware that when crossing adjacent tracks without an ATC clearance, the potential vertical separation provided by the In-flight Contingency Procedure is 500 ft. Weather Deviation Procedures call for a climb or descent of 300 ft. based on direction of flight and direction of deviation. RLatSM separation and ½ degree track spacing significantly reduces the distance and time a diverting aircraft has to maneuver when executing a diversion, turn-back or weather deviation without an ATC clearance before approaching adjacent tracks. An aircraft deviating from track can be in the proximity of aircraft on an adjacent track within approximately 4 minutes, depending on the angle of deviation from cleared track. It also reduces the time that a pilot has to obtain an ATC clearance. Pilot knowledge and preparation to execute the published in ICAO Doc 4444, paragraph 15.2 and (paragraph 15.2.3) need to be emphasized in training and checking programs.
  • TCASII/ACAS provides a valuable situational awareness tool and safety net for pilots. Pilot contingency procedures and ADS-C monitoring capabilities should mitigate the occurrence of lateral deviations related to ½ _degree waypoints, The carriage and operation of TCASII/ACAS provides and additional Risk Mitigation in RLatSM airspace should lateral deviations occur. TCASII/ACAS provides a situational awareness tool that will enable pilots to be better prepared to safely execute weather deviation and contingency procedures necessitating lateral deviations.
  • For further reading on this subject please review the notices listed below

ICAO-North Atlantic Systems Planning Group, NAT-SPG

NAT Operations Bulletin Serial Number: 2015-003 Subject: RLatSM Special Emphasis Items Effective: 1 June 2015


Aeronautical Information Circular 18/15, 25JUN15

Aeronautical Information Circular 23/15


Aeronautical Information Circular Y 062/2015, 6AUG15

Aeronautical Information Circular P 059/2015, 9JUL15


Domestic/International Notices to Airmen
Part 3, Section 2 , Page 3-INTL-20, Guidance For U.S. Operators: Special Emphasis Items For Operation In North Atlantic Reduced Lateral Separation Minimum (RLatSM) Trials


  • 1. RNP-4, ADS-C and CPDLC are becoming more "Needed" to cross the NAT.
  • 2. Space between the NAT OTS tracks are less than half of what have known
  • 3. Oceanic Waypoints are coded/abbreviated in a confusing manner
  • 4. "Expanding the Coordinates" needs to be part of your SOP
  • 5. Contingency adherence and execution are VERY important