An operator may plan any route they want across the North Atlantic — Page 126, Lesson 200

I want to walk you through how the North Atlantic airspace is structured for high‑density traffic, because it’s a really clever system that keeps things safe when hundreds of jets are crossing the ocean every day. First, the key idea: the tracks across the North Atlantic are made effectively one way only. That means on a given track, all the traffic is going in the same direction. To make that work, both the eastbound and the westbound semi‑circular RVSM levels — those are the standard altitude assignments you’d normally use based on magnetic track — are allocated to that single track direction. So instead of splitting the altitudes between opposite directions, all the available flight levels on that track go to traffic going one way. These tracks are called organized tracks, and the overall concept is the Organized Track System, or OTS. Now, even though this sounds very formal and structured, using the OTS is not mandatory. An operator may plan any route they want across the North Atlantic. A route that does not comply with the existing OTS is called a random route, and operators planning random routes are asked to observe certain restrictions — we’ll cover those later in the book. Let’s move to OTS track designation. To accommodate the tidal nature of the transatlantic flow — meaning the traffic shifts direction depending on the time of day — two separate OTS are established. There’s a daytime OTS westbound and a night‑time OTS eastbound. Each track in the OTS gets an individual identifier, or designator. For the daytime OTS, the tracks are lettered starting from “A” for the most northerly track at the start point, and then sequentially lettered in a southerly direction. For the night‑time OTS, it’s the opposite: the most southerly track is “Z”, and the next track to the north is “Y”, and so on. Now, the really important part is the OTS changeover. At some point during the day, the eastbound OTS gets replaced with the westbound OTS, and vice versa. This has to be carefully organized, or chaos would result. Let me give you the example from the book: if the eastbound OTS finished at 1000Z and the westbound started at 1001Z, it would be perfectly possible for a flight to join the eastbound OTS at 0959Z at FL 310 — that’s flight level 310, or 31,000 feet — and then, at some point in the flight, conflict with a flight joining the westbound OTS at 1001Z, also at FL 310, but going the opposite way. That’s a head‑on conflict at the same altitude. To overcome this, the OTS period is defined at 30W — that’s the 30° West meridian, which is approximately the midpoint for most traffic across the North Atlantic. An aircraft flying the OTS must plan to cross 30W during the period of the OTS to be able to fly the entire route as a NAT track. But that alone doesn’t totally solve the problem. Consider an aircraft crossing 30W at one minute before the end of the OTS — it’s still a very tight situation. So, to make it as safe as possible, a changeover (buffer) period is also established. This buffer exists from the end of one OTS until the start of the OTS in the reverse direction. This allows an aircraft that crosses 30W at the very end of the OTS to complete its route before an aircraft flying the reverse direction would be permitted to join the route. That buffer period is the safety margin that prevents those opposite‑direction conflicts at the same altitude.