The letters are daily track designators in the North Atlantic’s Organized Track System. A always names the northernmost westbound track; Z always names the southernmost eastbound track. The sequence works inward from each anchor — I is skipped westbound and O eastbound.
Each day’s publication carries a Track Message Identification number tied to the Julian date, so a letter alone is meaningless without that day’s message.
One letter, three digits, and a time window. That is the entire shorthand for a piece of sky that did not exist the day before.
Twice a day, controllers on both sides of the Atlantic redraw the corridors that carry westbound and eastbound traffic between North America and Europe. Each fresh version is labeled as if it had always been there.
The labels look simple: a letter, a few named fixes, a handful of coordinates. But the system only makes sense once you see what holds it together. The letters are anchored, not arbitrary. And the whole structure carries a date stamp, so a label from one day cannot quietly stand in for the next.
For a Western traveler, this is not an abstraction. It explains why a westbound crossing often feels like a different operation from the eastbound return — and why the route your flight followed last month is not the route it follows today. If you have ever watched a transatlantic departure slip, or wondered why the two legs of the same trip never seem to match, the answer usually traces back here.
A starts at the top, Z anchors the bottom
Inside the Organized Track System, the letters are not scenic decoration. They are directional anchors. On the westbound side, A is assigned to the northernmost track at its point of origin. From there the alphabet runs in order, moving inward — B, C, D, and so on — skipping only I.
The eastbound side runs the same logic in reverse. Z is permanently pinned to the southernmost eastbound track, and the sequence climbs backward toward the middle of the alphabet, skipping only O.
Why I and O? The operational publications confirm the omissions and then go quiet. Neither the FAA’s material nor the wider guidance reviewed for this piece gives a stated reason.
It is tempting to blame readability — I and O look like 1 and 0 — but that remains unverified.
When the track count changes, the letters just stop being handed out
The anchors do not move. A day with fewer tracks simply uses fewer letters. A busier day hands out more, always working inward from the fixed endpoint.
What the documentation does not publish is any separate reassignment algorithm. Each day’s message simply lists its own tracks, and the letters are counted off from the fixed endpoint in the order those tracks appear — anything finer than that is unverified.
One letter names the track, three digits date it
Here is the usual place people tangle two different things. The track letter and the Track Message Identification number do separate jobs.
The letter identifies a route within a single day’s publication: the corridor, its entry and exit fixes, its coordinate points. The TMI number dates the publication itself. It is keyed to the Julian calendar day, so a pilot and a controller can each be certain they are reading the same day’s track data.
Spotting a label in the FAA’s present-day track service makes the gap concrete. A track message may open with a compact identifier like TMI 237 — the three digits fix the day, the letter fixes the corridor within it.
Most passengers will never see either one, but the shorthand travels with every flight plan that follows a designated track from end to end.
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The map gets torn up twice a day
Jet streams move, and the corridors are rebuilt around them. A track optimized for this morning’s wind is not the track for tonight’s.
What actually controls the operation is not the sort of notice a pilot skims before departure.
The FAA’s North Atlantic track display data are, in the agency’s own user-guide wording, “not actual NOTAMs” — the authoritative publication is the NAT Track Message, issued by the responsible Oceanic Control Centre and distributed through the Aeronautical Fixed Telecommunication Network.
Each published message carries three elements: the oceanic entry point, the latitude/longitude coordinates along the route, and the oceanic exit point. That is the full geography of the corridor, expressed in fixes rather than letters.
The letters and coordinates only matter during a narrow validity window. Westbound aircraft crossing 30 degrees West are typically valid between 11:30 and 19:00 UTC. Eastbound aircraft crossing the same longitude are typically valid between 01:00 and 08:00 UTC.
| Direction | Anchor letter | Anchor position | Sequence direction | Omitted letter | Typical 30W crossing window (UTC) | Relationship to TMI |
|---|---|---|---|---|---|---|
| Westbound | A | Northernmost track at its point of origin | Alphabetical, moving inward from A | I | 11:30–19:00 | The letter identifies the track; the TMI identifies the effective daily track message. |
| Eastbound | Z | Southernmost track at its point of origin | Reverse alphabetical, moving inward from Z | O | 01:00–08:00 | The letter identifies the track; the TMI identifies the effective daily track message. |
| Both | Varies by track count | Anchors remain fixed even when the number of tracks changes | Sequential assignment from the applicable anchor | I westbound; O eastbound | Windows are tied to the 30W crossing and the published message | TMI is a three-digit identifier linked to the Julian effective date. |
| Source: SKYbrary Aviation Safety; Federal Aviation Administration (FAA) | ||||||
Where the letters came from — and where the trail stops
Organized North Atlantic tracks were in occasional aircraft use by 1961. Daily publication became routine by 1965, pushed by rising airline traffic and the growing need for procedural separation.
What the record does not contain is a date for the alphabet itself. No reviewed source names the decision-maker who introduced the A-through-Z convention, or points to a separate decision establishing those fixed anchors. The letters turn up in operational material without an origin story attached.
The same gap sits in the archives. Anyone hoping to cross-reference old track messages against their coordinate sets will find no openly accessible public repository linking them. The FAA’s current retrieval service exists and treats its data as distinct from NOTAMs, but a unified historical archive is not readily available — a negative claim the documentation supports practically rather than absolutely.
What this means for a passenger
Most of this system is invisible from the cabin. But its asymmetry shows up in the block time on your ticket. Jet-stream winds run west to east, and the two directions are planned around that: eastbound corridors are built to use the flow, while westbound corridors are routed clear of its strongest core.
The resulting ground-speed gap is why the same transatlantic city pair usually runs longer westbound — no specific route or exact minute difference is verified here, just the mechanism.
Departure times cluster for the same reason. The published validity windows funnel eastbound crossings through the early-UTC hours and westbound crossings through the late morning and afternoon UTC.
The track structure was last checked against current guidance on September 23, 2026, with the standard windows still in place and no verified seasonal variation.
Key terms
- Organized Track System
- The Organized Track System is the daily set of designated flight paths that aircraft follow across the North Atlantic, maintained separately for westbound and eastbound traffic. Its tracks are anchored rather than numbered arbitrarily: A marks the northernmost westbound track at its point of origin and Z the southernmost eastbound track, with the remaining letters assigned inward from those two ends. Because the whole set is rebuilt rather than fixed, the same letter can point to a different corridor from one period to the next, which is why a designator only means something alongside that period’s Track Message Identification number.
- NAT Track Message
- The NAT Track Message is the operational publication that sets out the North Atlantic tracks in effect for a given period. It is issued by the responsible Oceanic Control Centre and reaches the parties that need it through the Aeronautical Fixed Telecommunication Network. The FAA’s North Atlantic track display service shows the same material, but the agency’s user guide states that what the service returns is not an actual NOTAM — the message remains the document that governs the crossing.
- Track Message Identification
- The Track Message Identification is the three-digit number that dates a NAT Track Message. It is linked to the Julian date of the effective message, so a pilot and a controller can each confirm they are reading the same day’s track data. Two aircraft following the same letter on different days are following different corridors, and the TMI is the part of the label that tells them apart.
- Aeronautical Fixed Telecommunication Network
- The Aeronautical Fixed Telecommunication Network is the communications channel through which the NAT Track Message is circulated. It is named in the operational guidance as the route by which the finalized track message reaches interested parties. For anyone reading a track label off a display, that is the practical point: the authoritative text travels over the network, not through the interface that happens to show it.
Questions? Answers.
Is the North Atlantic track data published as a NOTAM?
No. The FAA’s North Atlantic track display data are explicitly not actual NOTAMs. The authoritative publication is the NAT Track Message, distributed through the Aeronautical Fixed Telecommunication Network.
Why are the letters I and O skipped in NAT track designators?
I is omitted from westbound track lettering, and O is omitted from eastbound lettering. The operational sources reviewed do not explain the reason for either omission.
What happens if a flight only uses part of an organized track?
The abbreviated NAT-plus-track-letter notation only applies when a flight is planned along the entire organized track from its oceanic entry point to its oceanic exit point. A partial-track operation must file the complete route details instead.
When did the North Atlantic track system begin, and when did the A-through-Z lettering convention start?
Occasional use of organized North Atlantic tracks began in 1961, and daily publication began in 1965. No dated decision specifically establishing the A-through-Z lettering convention has been verified.
