ATPL · Extra · Operational Procedures Atpl Ground Training… lesson — Page 126, Lesson 199

I want to walk you through the North Atlantic traffic system, starting with the core concept of MNPSA — Minimum Navigation Performance Specification Airspace. This is a designated area over the North Atlantic where aircraft must meet very precise navigation standards to operate safely, because there is very little radar coverage and the traffic is heavily concentrated along a few routes. Let's look at those routes. The most desirable paths across the North Atlantic are called Minimum Time Tracks, or MTTs. These run from major European departure points — Paris, London, and central Europe — and terminate at destinations like New York, Chicago, Atlanta, Montreal, and Boston. Generally speaking, an MTT is a great circle track that takes advantage of the most favourable wind conditions. If you look at a chart of the North Atlantic, you will see that most of the routes to these destinations follow very similar paths. That means the traffic is concentrated: the majority of aircraft crossing the North Atlantic region end up flying on just a few flight levels, usually adjacent to the tropopause, and on virtually the same track. Now, the situation gets more complicated because of time differences, which create what we call the 'tidal flow'. Think about the typical passenger's desire: people want to leave Europe in the morning and arrive in North America around midday local time. For example, a 7-hour flight to New York departing London at 9am, which is 0900Z, would arrive at 11am local time, which is 1600Z. Concorde used to arrive before it had taken off — that's a fun historical note, because of the time zone difference. For the return journey, daytime flying is wasteful. A 6-hour flight from Boston to London departing at 10am, which is 1500Z, would land at 9pm London time, which is 2100Z. To overcome this inefficiency, the majority of flights to Europe from North America depart early in the evening so they arrive first thing in the morning. For instance, the 6pm Virgin flight from Orlando, which is 2300Z, lands at Gatwick at 7am the next morning, which is 0700Z. So we have all these flights going the same direction at approximately the same time. To accommodate them safely, a set of roughly parallel tracks is established. The lateral separation between these tracks is based on the MNPS RNP — that's Minimum Navigation Performance Specification Required Navigation Performance. In simple terms, it means each aircraft must be able to stay within a very tight lateral corridor, so the tracks can be placed closer together than would otherwise be possible. The flight levels allocated to these tracks are RVSM levels — Reduced Vertical Separation Minima. That means the vertical spacing between aircraft is reduced from the standard 2,000 feet down to 1,000 feet, as long as the aircraft and crew are certified for it. To make even better use of the vertical airspace, the tracks are made effectively 'one way only'. Both the eastbound and the westbound semi-circular RVSM levels are allocated to the track direction — so all aircraft on that track are flying the same way, which eliminates the risk of head-on encounters and allows maximum use of the available airspace.