Airspace — Page 266, Lesson 341

Let's pick up with Required Navigation Performance — RNP. This is a concept you'll see constantly in airspace design, so I want you to really understand it from the ground up. First, the definition. RNP is a numerical representation of the navigational accuracy required within ATS airspace of a State. Let me unpack that. ATS means Air Traffic Services. So each State — each country — decides how accurately an aircraft must be able to navigate within its airspace, and that required accuracy is expressed as a number. That number is prescribed on the basis of regional air navigation agreements, which we call RANs. So it's not arbitrary — it's agreed regionally. Now, the key part: the number is based on a 95% containment factor. What does that mean? It means an aeroplane will be within the required RNP for a period of not less than 95% of the time the aircraft is within the airspace concerned. Alternatively, you can think of it as: not less than 95% of the aircraft flying in a given airspace will be navigated within the stated RNP factor. So either way you look at it — by time or by proportion of aircraft — 95% is the containment standard. And the State is responsible for specifying the RNP value for its own airspace. Let me give you the theory behind the numbers. The RNP factor relates to navigational accuracy relative to the aircraft's plotted position, and it's expressed in nautical miles. So, for example, RNP4 implies that the aircraft will be within 4 nautical miles of the plotted position for 95% of the time the aircraft is within the airspace concerned. The plotted position is where the navigation system thinks the aircraft is — and RNP tells you how tightly it must stay around that point. Now, the applicable RNP factors are: RNP1, RNP4, RNP10, RNP12.6, and RNP20. All ATS airspace is classified for RNP — every piece of airspace gets an RNP value. Here's an interesting wrinkle. Where VOR/DME is used for airways or RNAV navigation, the RNP specified is RNP5. But within the classification of RNP, RNP5 does not actually exist as a formal factor. Let me explain the history. It was envisaged that VOR/DME would cease to be used for RNAV by the year 2005, and would be replaced by more accurate systems offering at least RNP4. That didn't happen — VOR/DME at RNP5 will continue for the foreseeable future. So RNP5 is a de facto value used in practice, even though it's not in the formal classification list. Now, how do different navigation aids relate to RNP? The use of radar permits RNP1. GPS theoretically offers a relative value of RNP0.3 — that's the most accurate of the ones we're discussing. And the value of RNP12.6 is derived from the historical accuracy of multiple IRS — Inertial Reference Systems — used for transatlantic navigation. So each RNP value traces back to a real navigational capability. Let me show you how this is applied in practice, because this is where it becomes concrete. A great example is the track spacing used for the NAT tracks — North Atlantic Tracks — in the MNPSA, which is the Minimum Navigation Performance Specification Airspace of the North Atlantic Oceanic regions. That airspace is classified as RNP20. So aircraft flying those routes will be within 20 nautical miles of the plotted position for not less than 95% of the flight time. Now here's the clever part — how that translates into track spacing. Because the airspace reserved to a NAT track must be 20 nautical miles either side of the specified route, the additional safety buffer is equal to the RNP itself. So the track spacing works out to 20 plus 20 plus 20, which equals 60 nautical miles. Let me walk you through that: you have 20 nautical miles of reserved airspace on one side of the track, 20 nautical miles on the other side, and then the 20 nautical mile buffer — that gives you 60 nautical miles between adjacent tracks. That's the whole point of RNP — it directly determines how tightly you can pack routes together safely. Now let's move to the establishment of airways. The corridors linking CTAs — Control Areas — are called airways. These have evolved by a process of demand, much like the road structure in the UK. They weren't designed from a blank sheet; they grew where traffic needed them. The airways carry the en route traffic, so they're primarily concerned with traffic in the cruise rather than manoeuvring traffic. The ATC problem here is relatively simple — it involves separating traffic heading in opposite directions. That's the core of airway design: keeping opposing streams apart. So to tie it all together: RNP is the accuracy requirement, expressed in nautical miles with a 95% containment factor, and it directly drives how much airspace you must reserve around a route. And airways are the corridors that link control areas, built up by demand, carrying cruise traffic that simply needs opposing-direction separation.