NAT HLA Airspace and the Track System - ATPL 070 — Operational Procedures

The previous lesson promised that the two-navigation-system requirement would be priced in detail; this lesson pays, and prices the airspace around it. The NAT HLA, the North Atlantic High Level Airspace, is where the two-navigation-system entry ticket applies, where the organised tracks are laid each day, and where a formal approval stands between an operator and the busiest oceanic airspace in the world. The vocabulary alone earns a section: the region speaks in acronyms, and the exam expects them fluent.

The shape of the NAT HLA

Vertically, the NAT HLA runs from FL 285 to FL 420 inclusive. Laterally, it is built from Oceanic Control Areas: Reykjavik, Shanwick excluding its SOTA and BOTA corners, Gander, Santa Maria Oceanic, Bodo Oceanic, and New York Oceanic East north of 27N. For most of a crossing there is no direct controller-pilot VHF and no radar; CPDLC serves suitably equipped aircraft, ADS-B surveillance covers much of the airspace, and safety rests on demanding horizontal and vertical navigation performance and operating discipline. Most traffic flies FL 290 to FL 410, and RVSM applies throughout the region. Operation in the HLA rests on a formal approval by the State of Registry or the State of the Operator, confirming that the aircraft meets the NAT HLA standards and that crew procedures and training are in place; unrestricted operation needs two fully serviceable long-range navigation systems. An aircraft may fly above the HLA without the approval, though its crew is expected to understand the procedures and any active track structure below, and a non-approved aircraft may be cleared to climb or descend through the HLA, traffic permitting.

The region's vocabulary, worth learning as a list because the exam asks it as one:

LRNS: Long Range Navigation System; MASPS: Minimum Aircraft System Performance Specifications.
NAT HLA: North Atlantic High Level Airspace; OCA: Oceanic Control Area; OTS: Organised Track System.
PRM: Preferred Route Message, the operator's routing wish filed into the track design process; in the North Atlantic this abbreviation does not mean a precision runway monitor.
RVSM: Reduced Vertical Separation Minimum; SLOP: Strategic Lateral Offset Procedures, whose flying rules a later lesson owns.
WATRS: West Atlantic Route System, resident in New York Oceanic West and the Miami and San Juan oceanic airspaces.

Routes for the less equipped

The two-LRNS ticket has exceptions drawn on the map. Older editions of the manual named the special routes for aircraft with only one serviceable LRNS the Blue Spruce Routes, stepping-stone routings via Iceland and Greenland flown with State approval; the same editions kept short-stage routes, G3 and G11, where aircraft with normal short-range navigation equipment alone, VOR, DME and ADF, could meet the track-keeping standard. The current edition redraws the picture without changing its logic. The Tango routes link Northern Europe with the Spain, Canaries and Lisbon FIRs, T9, T290, T13, T213 and T16, with T25 between the Azores and the Lisbon FIR; the asterisked members may be flown with short-range equipment plus at least one fully operational LRNS, and T9 and T290 may be flown by approved aircraft with ADS-B out, VHF and RNP 2 capability. Alongside them run the NAT corridors, Iceland-Greenland, Azores and Bodo, where ATS surveillance and VHF coverage let reduced fits cross legally: within the Iceland-Greenland corridor ADS-B is required for all IFR flights.

Watching the performance

Why is the horizontal and vertical navigation performance of NAT operators monitored on a continual basis? Because the separation standards are built on assumed performance, and the assumption is audited. ICAO has established systematic and periodic monitoring of the performance actually achieved, fed by formal reports of specified deviations from track or level. The NAT Central Monitoring Agency, operated on behalf of the NAT Systems Planning Group by the United Kingdom's NATS, collects, analyses and disseminates the data on operational errors and navigation performance in both planes; when the summary statistics show the target level of safety exceeded in either plane, the planning group is told and acts. Every operational error is reviewed in detail and the lessons flow back to operators and ATS authorities. Operators carry their own share: every lateral deviation of 10 NM or greater is investigated, which is why complete in-flight records are kept.

The organised track system

Passenger demand, time zones and noise restrictions concentrate the ocean's traffic into two alternating one-way flows: a westbound flow leaving Europe in the morning and an eastbound flow leaving North America in the evening, peaking across 30W between 1130 and 1900 UTC and between 0100 and 0800 UTC respectively. The organised track system serves those peaks: a set of parallel tracks re-built twice daily from the minimum time tracks, the airlines' preferred routes and the day's airspace restrictions. The day-time westbound OTS is built by Shanwick at Prestwick, the night-time eastbound OTS by Gander, each coordinating with the neighbouring centres, and the flight levels typically associated with the system run FL 340 to FL 400 inclusive. The operators feed the design through collaborative decision making, opening with the Preferred Route Message, filed not later than 1900 UTC for the following day-time system and 1000 UTC for the night-time one, and the proposed tracks are posted for comment before the design is fixed. When a strong jetstream hugs the great circle, Shanwick may publish a split westbound structure either side of it.

The hours of validity of the two systems are those peak windows at 30W, 1130-1900 UTC westbound and 0100-0800 UTC eastbound, adjustable by agreement between Gander and Shanwick and stated in each track message. Between the systems sit the changeover periods, 0801-1129 UTC and 1901-0059 UTC, when flights are often coordinated individually and cleared levels may not match the plan; a level-critical flight is advised to ask the initial oceanic centre about level availability before filing. And a fact the exam loves: use of the OTS is not mandatory. Aircraft may plan random routes clear of it, or routes that join, leave or cross it, accepting that re-routes and level changes are likely during the traffic periods.

Reading the track message

The agreed system is promulgated by the NAT track message, sent via the AFTN, typically at 2200 UTC for the day-time system and 1400 UTC for the night-time one. It carries the full coordinates of each track, the flight levels expected in use on each, and usually the associated domestic entry and exit routings. The lettering runs from the edges: in the westbound system the most northerly track at its origin is Track A, then B and so on southward; in the eastbound system the most southerly is Track Z, then Y northward. The remarks section closes the message with the day's essentials, including the Track Message Identification: a three-digit number equal to the Julian calendar date on which the system is effective, so the OTS effective on 1 February is TMI 032. Any amendment touching the entry and exit points, the coordinates or the level allocation re-issues the whole message with an alphabetic suffix, TMI 032A and onward, while changes to items such as the North American Routes do not revise the TMI. Every NAT flight at or above FL 290, even one crossing above the HLA or clear of the tracks, must carry a copy of the current track message including amendments.

Around the edges

The system needs joints where the ocean meets the continents. The North American Routes, the NARs, are a numbered series of predetermined routes interfacing NAT oceanic airspace with North American domestic airspace, designed around the major North American airports; their details live in the United States Chart Supplement and the Canada Flight Supplement on the 56-day AIRAC cycle. On the eastern side, parts of the Shanwick OCA are carved into transition areas: the Shannon Oceanic Transition Area and the Northern Oceanic Transition Area, both served by Shannon Control, with the Brest Oceanic Transition Area below them under Brest Control, and the Gander Oceanic Transition Area performing the same office in the west under Gander Centre. The fine print differs by area: NOTA's block from FL 055 upward contains NAT HLA levels, while SOTA and BOTA are not part of the NAT HLA at all, though a flight transiting them into Shanwick between FL 285 and FL 420 must still meet the HLA requirements. Further south, the West Atlantic Route System of the acronym list connects North America with the Caribbean.

Planning with and against the flow

Between the significant points of the previous lesson's grid, all flights should plan to operate on great-circle tracks joining successive waypoints: the grid names the points, the geodesic joins them. During the hours of validity of the OTS, operators are encouraged to plan in one of three ways: in accordance with the OTS; along a route to join or leave an outer track of it; or on a random route that remains clear of it, laterally or vertically. Flying against the pending flow adds a courtesy with teeth: an eastbound flight crossing 30W after 1029 UTC, or a westbound one after 2359 UTC, less than an hour before the opposing system takes effect, should plan to remain clear of the pending structure.

Levels follow the same split. A flight planned along an OTS track for its entire length during the validity period may plan any of the levels available on that track, the ones the track message publishes for it. Random flights, and any flight outside the OTS periods, may plan any flight level, weighing feasibility against the track system and the traffic flows; planned climbs may be filed, but every level change in flight must still be requested from ATC and granted only as traffic permits, which prudent fuel planning takes into account. That is also the answer to the cruising-level question: away from the ocean a long-range IFR flight sits on the standard cruising levels appropriate to its direction, but on the North Atlantic OTS the direction rule gives way to the track message, whose levels are allocated to each one-way track for the period of its validity.