Airways and ATS Routes

An airway is a corridor of controlled airspace along which IFR traffic is routed between navigation aids or waypoints. ICAO and SERA define it as a control area, or a portion of one, established in the form of a corridor; older editions added "equipped with radio navigation aids", and some exam questions still quote that wording. Airways are the best-known kind of ATS route, the wider term for any published route used to channel traffic for air traffic services, which also includes advisory routes, flight information routes, RNAV routes, SIDs and STARs.

The network lets controllers predict where traffic will be and separate it with fixed geometry, and lets crews file, clear and fly a route with a few designators. It is changing: RNAV routes now supplement the VOR-to-VOR airways, and across Europe free route airspace is replacing parts of the upper network. The underlying ideas of a designator, significant points and minimum altitudes along a centreline still carry most of the exam questions.

What an airway is

Because airways are controlled airspace, their class decides the service given. In the United States, federal airways and low-altitude RNAV routes are Class E corridors extending, unless otherwise specified, from 1,200 ft above the ground up to but not including 18,000 ft MSL, and a federal airway extends 4 NM either side of its centreline. The chart shows only the centreline. Above them, from 18,000 ft MSL up to FL600, the airspace is generally Class A. UK airways are typically 10 NM wide, 5 NM either side of the centreline.

The width is linked to navigation accuracy. With a possible VOR bearing error of 5.5°, the 1 in 60 rule shows that an aircraft 54.5 NM from a station may be 5 NM off the radial. A UK airway 5 NM each side therefore needs its VORs no more than about 109 NM apart. A pilot must fly along the centreline of an airway, as 14 CFR 91.181 requires in the United States, and lead the turns at course changes: the AIM notes that an aircraft at 400 kt TAS turning at 25° of bank through more than 40° can leave a 4 NM airway if it starts the turn at the fix.

Each airway segment carries minimum altitudes. The minimum en-route altitude (MEA) guarantees obstacle clearance, navigation signal coverage and, in the ICAO definition, ATS communications. The minimum obstruction clearance altitude (MOCA) gives the same obstacle clearance, but assures VOR reception only within 22 NM of the station in the US. A minimum reception altitude (MRA) allows a fix to be identified, a minimum crossing altitude (MCA) must be reached before a fix where the MEA rises, and a maximum authorised altitude (MAA) protects against interference from another station on the same frequency. The cruising level is then chosen by magnetic track from the SERA table: on a track of 275° with an MEA of 6,500 ft the lowest usable IFR level is FL080 (see cruising levels and minimum heights and minimum safe altitudes).

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ATS route designators

Every ATS route has a designator, published in the AIP and used in flight plans and clearances. Under ICAO Annex 11 the basic designator is a letter followed by a number, such as A1 or B6, and the letters are read with the phonetic alphabet. A prefix and a suffix may add information, so that UB3 and UG1 are upper routes:

The United States uses its own families: V for Victor airways, J for jet routes, and Q, T and Y for RNAV routes. SIDs and STARs have designators of their own, built from a significant point, a validity number and a route letter, such as BELUS 2C. In a flight plan, an aerodrome not connected to an ATS route is linked to its first route by "DCT" and the joining point.

Lower and upper airways

In Europe the division between the flight information region (FIR) and the upper information region (UIR) is generally at FL195, and routes above it are upper airways, marked by the U prefix. Traffic in the UIR is essentially in transit, while lower airways also serve arriving and departing traffic. European en-route ATS routes may be designated RNAV 5, which requires a total system error within 5 NM for 95% of the flight time. Basic RNAV (B-RNAV), with the same 5 NM accuracy, was made mandatory in Eurocontrol airspace for aircraft with 30 or more passenger seats.

In parts of the upper airspace the route network has given way to free route airspace (FRA), in which operators plan their own tracks between published significant points instead of following airways. In the French UIR, a flight overflying an FRA cell plans from an entry point (E) to an exit point (X), with intermediate points (I) where cross-border planning allows; departures plan from a point (D) and arrivals to a point (A). Segments between points are filed as DCT, with no distance limit inside the cells. Level changes are allowed only at significant points, levels follow the table of cruising levels, and flights plan round reserved areas as published in the airspace use plans.

Victor airways and jet routes

The FAA's AIM describes three fixed route systems: the federal airways, the jet routes and the RNAV routes, aligned to overlie one another where possible.

Victor airways are VOR airways, identified by V and a number, such as V12, and drawn on low-altitude en-route charts. They extend from 1,200 ft above the surface, or higher, up to but not including 18,000 ft MSL. Where several airways share a segment, it carries all their numbers. Outside Alaska they are based solely on VOR or VORTAC stations; in Alaska some segments use low and medium frequency beacons and are charted in brown. The federal airway system also includes L/MF airways based on non-directional beacons, charted in brown and named by colour and number, such as Amber One: Green and Red airways run east and west, Amber and Blue north and south.

Jet routes run from 18,000 ft MSL to FL450 inclusive, are identified by J and a number, such as J12, and appear on high-altitude charts. Like Victor airways, they are based on VOR or VORTAC stations outside Alaska. Above FL450 flights may be conducted point to point. Reporting points are designated on both systems.

RNAV Q-routes and T-routes

US RNAV routes are charted in blue and identified by Q, T or Y and a number, such as Q13, T205 and Y280. They are RNAV 2 routes, a 2 NM accuracy requirement, unless charted as RNAV 1.

Y-routes, mostly in US offshore airspace, require GPS and RNAV 2 performance. T-routes, the low-altitude counterpart of Q-routes, let GPS-equipped aircraft route through or around busy terminal airspace, often with lower MEAs than the Victor airways. RNAV route charts give a magnetic reference bearing between waypoints for reference, while the navigation system flies the true course. Unpublished direct RNAV routes are also allowed, but need radar monitoring by ATC unless a GNSS-equipped aircraft is cleared via published waypoints recallable from its database (see area navigation).

Significant points

A significant point is a specified geographical location used to define an ATS route or an aircraft's flight path. It may be a radio navigation aid, an intersection of radials, or a waypoint defined by coordinates and given a five-letter name-code, such as TADAN or SURAS, listed in section ENR 4.4 of the AIP. RNAV waypoints are either fly-by, where the turn starts before the point, or flyover, where it starts once the point has been passed.

Significant points serve as reporting points. Compulsory reporting points are shown on FAA charts by a solid triangle and on-request points by an open triangle. A position report gives identification, position, time, level, the next position with its estimated time, and the name of the following point. A revised estimate is passed when the estimate for the next point proves wrong: the FAA requires one for an error of more than 2 minutes, and of 3 minutes or more in the North Atlantic. In the United States, once ATC reports radar contact, routine reports stop until radar contact is lost or radar service ends.

Changeover points

On an airway between two VORs, the changeover point (COP) is where the pilot changes from the station behind to the station ahead. It is normally midway between the stations on a straight segment, or at the intersection of the radials where the route doglegs; an 80 NM segment with no COP charted is changed over at 40 NM. Where signal coverage, terrain screening or interference requires a different position, the chart shows the COP with the distance to each station.

COPs prevent a loss of navigation guidance, avoid interference from other facilities and ensure that aircraft in the same airspace use the same stations, so that all stay within the protected airway. Changing early or waiting for the flag of a fading station defeats that purpose. The same logic explains the MAA, set where a higher aircraft would receive a second station on the same frequency. An aircraft navigating by GNSS has nothing to retune there, but the COP remains on the chart and in the exam syllabus (see VOR).

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.