The NAT Track Message and TMI Number Explained
Every day the North Atlantic track system is published in a NAT Track Message. A short number at the top of it, the TMI, is how crews and controllers confirm they are all working from the same version.
What the message contains
- The coordinates of each track of the OTS, with the entry and exit points.
- The flight levels expected to be in use on each track.
- A header giving the hours during which that OTS is valid.
- A remarks section. For the eastbound OTS, the remarks also give clearance delivery frequencies based on the oceanic entry position.
The TMI
Each NAT Track Message is identified by a three-digit Track Message Identification (TMI) number based on the Julian calendar date for which that OTS is effective. In practice that is the day of the year: 16 January is 016 and 5 March is 064 in a normal year. In 2026, 22 October is day 295. Use the TMI calculator to check a date, and always confirm against the date in the message itself.
Re-issued messages
If anything affecting the entry or exit points, the routes or the flight level allocation changes, the whole message is re-issued. A letter is added to the TMI, so a re-issue of 032 becomes 032A.
How the TMI is used
The TMI lets crews and controllers check that everyone is using the same tracks. When a clearance for an organised track is read back in abbreviated form, the readback includes the TMI of the current message. If the TMI is not available or is incorrect, air traffic services issue a full route clearance and ask for a full readback. Dispatchers and crews must therefore make sure they are working from the current message.
The message is the authority. This guide explains its structure only.
FAQ
Is the TMI the same as the date?
It is based on the Julian calendar date, which means the day number in the year, written with three digits.
What does a letter after the TMI mean?
The track message was re-issued. The first re-issue adds A, the next B, and so on.
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For training and reference only. Check official sources before any real flight.