Airspace — Page 266, Lesson 343

Let’s pick this up right where the airspace chapter is taking us into the detail of airways and ATS routes. I want to walk you through the design and designation of these routes, because this is where the regulations get very specific and very examinable. First, the big picture. An airway is a defined corridor of controlled airspace, and its upper and lower limits are set for very particular reasons. The upper limit of an airway may be defined as the lower limit of the UIR — that’s the Upper Information Region, the airspace above the airway structure where upper air routes operate. So the airway tops out exactly where the UIR begins. The lower limit, though, is dictated by three things: airspace restrictions, terrain avoidance considerations, and the needs of other air users, for example the military. So the bottom of the airway is not arbitrary — it’s pushed up by restricted areas, by the need to stay clear of high ground, and by the demands of other traffic like military operations. Now, historically, airways served the purpose of linking CTAs with CAS. CTA is a Control Area, and CAS is Controlled Airspace. So the original job of the airway was to connect one control area to another through controlled airspace. But here’s the modern reality: in today’s environment of increasing traffic density, airways create as many problems as they solve. Let me list those problems, because they’re a classic exam point. Airways create choke points, they reduce flexibility, they create delays, they increase transit times, and they reduce fuel efficiency. That’s five distinct drawbacks. And because of that, the use of airways is declining. With the use of smaller aeroplanes and many regional airports, traffic — especially in the UK — now flies off-route, making use of military radar facilities. So instead of being funneled along an airway, aircraft fly direct and get radar service from the military. Under the ‘open skies’ policy in Europe, and the ‘gate-to-gate’ operations philosophy encouraged by Eurocontrol, the use of airways will continue to decline. And the consequence is significant: effectively, all airspace above about 6000 ft will become CAS — Controlled Airspace. So the trend is that the airway structure is dissolving, and the airspace above 6000 feet is becoming controlled airspace in its own right. Now let’s look at designation, because this is where the terminology gets precise. Historically, airways were ‘beacon hopping’ routes — that means they went from VOR to VOR. VOR is a VHF Omni-directional Range, a ground-based radio beacon. So the old airway was a straight line from one VOR beacon to the next. During the 70s and 80s, advances were made in area navigation — that’s RNAV — and RNAV airways using waypoints based on VOR/DME information were established. DME is Distance Measuring Equipment. So instead of hopping from beacon to beacon, the RNAV airway uses waypoints, which are defined points in space, positioned using VOR and DME data. Now, beacon hopping routes — which we call non-RNAV routes — still exist. But nearly all the airways introduced in the latter part of the 20th century were RNAV routes. So the modern airway is an RNAV route. Routes are also classified in two other ways. They’re either regional or non-regional. A regional route is one which exists between states in one ICAO region — so it crosses borders but stays within a single ICAO region. A non-regional route is one which does not extend beyond the borders of a state — it stays entirely inside one country. So when you combine these two classifications with the RNAV question, you get four options. Let me give you all four: Regional non-RNAV routes, Regional RNAV routes, Non-regional non-RNAV routes, and Non-regional RNAV routes. That’s the full matrix — regional or non-regional, crossed with RNAV or non-RNAV. Now, beyond airways, there are other ATS routes. ATS stands for Air Traffic Services. These other ATS routes include SIDs, STARs, and low level helicopter routes. SID is a Standard Instrument Departure — the published route out of an airport. STAR is a Standard Terminal Arrival Route — the published route into an airport. And then there are low level helicopter routes. All of these are given specific designators, which can be referred to in ATC communications and in FPs — that’s Flight Plans. So the designator is the shorthand that lets ATC and the flight plan refer to a route precisely. Now, the route designator itself — this is the core of the designation system. An airway is given a ‘designator’ which does three jobs. It defines the type of airway, it gives it a unique number, and it provides additional information about the type of route. So the specific route designator indicates what type of route is defined, and in addition, a unique number from 1 to 999. So every airway gets a number in that range. On top of that, a prefix can be added. There are three prefixes. ‘U’ means Upper air route — that’s a route in the upper airspace. ‘S’ means Supersonic transport route — a route designated for supersonic aircraft. And ‘K’ means Helicopter low level route — a route for helicopters at low level. And then, additionally, suffixes may be applied. There are four suffixes. ‘F’ means Advisory route, and that’s in Class F airspace. ‘G’ means FIS route, and that’s in Class G airspace — FIS is Flight Information Service. Then we have two very specific ones that involve RNP. ‘Y’ means an RNP1 route at and above FL200 where turns between 30° and 90° are to be made within the allowable RNP tolerance of a tangential arc defined by a radius of 22.5 NM. Let me unpack that. RNP is Required Navigation Performance — a measure of how accurately the aircraft can navigate. RNP1 means the aircraft must stay within 1 nautical mile of the route centreline. FL200 is Flight Level 200, which is 20,000 feet. So the ‘Y’ suffix applies to an RNP1 route at or above FL200, and it specifies how turns between 30 and 90 degrees are to be flown — within the allowable RNP tolerance of a tangential arc with a radius of 22.5 nautical miles. So the turn is not a sharp corner; it’s a smooth arc, and the aircraft must stay within its RNP tolerance while flying that arc. And the ‘Z’ suffix is the same idea but at a lower level. ‘Z’ means an RNP1 route at and below FL190 — that’s Flight Level 190, 19,000 feet — where turns between 30° and 90° are to be made within the allowable RNP tolerance of a tangential arc defined by a radius of 15 NM. So the difference between Y and Z is purely the altitude and the radius: Y is at or above FL200 with a 22.5 NM arc radius, Z is at or below FL190 with a 15 NM arc radius. Same turn geometry requirement, different altitude band, different radius. So to tie it all together: the designator tells you the route type, gives it a unique number from 1 to 999, and then the prefix tells you whether it’s upper, supersonic, or helicopter low level, and the suffix tells you about the airspace class or the RNP turn requirement. That’s the complete designation system for airways and ATS routes.