Reading IFR En-Route Charts

An IFR en-route chart is the map of the airway system. Between the departure and arrival procedures, it is the chart from which an instrument pilot chooses a route and a level, tracks from one navaid to the next, knows where to report and which unit to call, and finds a safe altitude when something goes wrong. Almost every figure on it is a promise about something: terrain clearance, navigation signal, communications. Reading it well means knowing which promise each figure makes.

The principles are the same in every State, because the charts follow ICAO Annex 4 and the route rules of Annex 11, but the symbols differ between the national charts of an AIP, the FAA's charts and commercial charts such as Jeppesen's. This article reads the chart element by element; the route system itself is explained in airways and ATS routes, and the full family of minimum altitudes in minimum safe altitudes.

IFR en-route charts

A State publishes its en-route charts in the ENR part of its AIP. France, for example, publishes separate en-route charts for the lower and the upper airspace at 1:2,000,000, and area charts where the network around a busy terminal area is too dense for that scale. The FAA divides by altitude: IFR en route low altitude charts cover the airspace below 18,000 ft MSL, with the Victor airways and the RNAV T-routes, and high altitude charts cover 18,000 ft MSL and above, with the jet routes and Q-routes. Commercial charts such as Jeppesen's redraw State data in one uniform style, but the AIP remains the authoritative source (see aeronautical charts).

Aeronautical data change on the 28-day AIRAC cycle, and charts are reissued to match. Before use, check the effective date; changes between editions arrive by NOTAM.

Airways and designators

The chart draws only the centerline of an airway. Its width is a rule, not a line: a US federal airway extends 4 NM either side of the centerline, from 1,200 ft above the surface up to but not including 18,000 ft MSL, and UK airways are typically 5 NM either side. The obstacle clearance of the MEA applies across that whole width.

Each route carries its designator. Under Annex 11 it is a letter and a number, such as A1 or B6, read with the phonetic alphabet. A prefix U marks an upper route, K a helicopter low-level route and S a supersonic route; a suffix F marks an advisory route and G a flight information route. The FAA uses V for Victor airways and J for jet routes, both drawn in black, and Q, T and Y for RNAV routes, drawn in blue. Airway segments based on low or medium frequency beacons, in Alaska and Canada, are charted in brown, as are the colored L/MF airways such as Amber One.

Along each segment the chart prints its designator, its track, its distance in nautical miles and its minimum altitudes. RNAV routes show a magnetic reference bearing between waypoints. Some segments carry level rules: on Jeppesen charts an E beside the altitude means even levels are flown in the direction of the arrow, overriding the usual odd and even scheme, and PPR beside an arrow means prior permission from ATC is required to fly that way.

The cruising level then comes from the semicircular table, at or above the minimum altitude. On a magnetic track of 275° with an MEA of 6,500 ft and a QNH of 1013 hPa, the lowest usable IFR level is FL080 (see cruising levels and minimum heights). A low QNH lowers every flight level, and very cold air makes the true altitude lower than indicated, so the lowest usable level may have to rise.

Minimum altitudes: MEA, MOCA, MORA, MRA and MCA

Each figure guarantees something different:

The FAA's MEA clears obstacles by 1,000 ft, or 2,000 ft in designated mountainous areas. The MOCA gives the same clearance without the signal; under 14 CFR 91.177 a pilot may fly below the MEA down to the MOCA, but when navigating by VOR only within 22 NM of the station. An MRA above the MEA does not raise the floor. Cruising at an MEA of 4,000 ft toward an intersection with an MRA of 6,000 ft, the airplane is clear of terrain but cannot identify the fix by its cross-radial; it climbs, or fixes the position by DME or GNSS. An MCA, by contrast, is about terrain: the climb must be complete before the fix, because the ground close beyond it requires the higher altitude. Where a turn at a fix, at the higher speeds flown at and above 10,000 ft MSL, needs more obstacle clearance than the MEA gives, the FAA charts a minimum turning altitude with the MCA flag and a note.

Off-route altitudes serve planning, diversions and doubt about position. Jeppesen's route MORA and Grid MORA clear terrain and obstacles by 1,000 ft where these are at or below 5,000 ft, and by 2,000 ft where they are higher; the Grid MORA is printed in hundreds of feet. The FAA's OROCA uses 1,000 ft, or 2,000 ft in designated mountainous areas. The ICAO counterpart is the area minimum altitude (AMA), which France's legend also prints in hundreds of feet.

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Where nothing is published, the fallback differs. SERA.5015 requires 1,000 ft above the highest obstacle within 8 km of the estimated position, or 2,000 ft over high terrain or in mountainous areas; 14 CFR 91.177 measures 4 NM from the course instead.

Changeover points and navaid boxes

On an airway between two VORs, the changeover point (COP) is where the pilot changes from the station behind to the station ahead. It lies midway between them on a straight segment, or at the intersection of the radials where the route doglegs, and is not charted there: on an 80 NM segment with no COP shown, the change is made at 40 NM. Where signal coverage, terrain screening or interference puts it elsewhere, the chart shows the COP with the distance to each navaid. COPs exist to prevent a loss of navigation guidance and interference from other stations, and so that aircraft in the same airspace use the same facilities; changing early, or waiting for a warning flag, defeats that.

Each navaid has a navaid box with its name, frequency and identifier, often its Morse code and coordinates. On FAA charts the box also gives the class of a VOR (T, L or H), and an underlined frequency means no voice is transmitted on it. In ICAO States a navaid's frequency is protected from interference only within its published designated operational coverage (DOC); outside it, the receiver may lock onto another station on the same frequency and show plausible but false indications.

Compulsory and on-request reporting points

A reporting point is a specified geographical location in relation to which an aircraft's position can be reported. A solid triangle marks a compulsory reporting point, an open triangle an on-request point. France's legend uses the same two categories.

A compulsory point requires a report unless ATC dispenses with it, as it may where a surveillance service is provided; an on-request point needs one only when ATC asks. In the United States, once ATC reports radar contact, compulsory reports stop until it advises radar contact lost or radar service terminated, and over oceanic, non-radar routes every point used in the flight plan is reported, whatever its symbol. A report gives the aircraft identification, position, time, level, the next position with its estimated time, and the ensuing significant point. If the estimate for the next reporting point, FIR boundary or destination turns out to be more than 2 minutes wrong, a revised estimate is passed under SERA.8020 (see position reports).

Airspace and frequency data

The chart also shows the airspace the routes run through: FIR and UIR boundaries, controlled airspace, and prohibited, restricted and danger areas with their identifiers. In Europe the boundary between FIR and UIR is generally at FL195; in the United States, Victor airways are Class E airspace and the airspace from 18,000 ft MSL up to FL600 is generally Class A.

Communication data come with it. An area control center divides its airspace into sectors, each with its own frequency, and the pilot changes frequency from sector to sector. France's legend has symbols for an ACC sector boundary, the sector's name and frequency, flight information sector boundaries and airspace delegated to an approach unit. On FAA charts, airports with a published instrument approach are drawn in blue or green and those without in brown, unlike the sectional, where color shows whether there is a control tower.

Charts also show holding patterns at the fixes most often used. When ATC clears an aircraft to hold at a charted pattern without giving full instructions, the pilot holds as depicted. A maximum holding speed other than the standard is printed in parentheses inside the racetrack, such as (175), and patterns between 6,001 ft and 14,000 ft restricted to 210 KIAS are marked by an icon. On Jeppesen charts a shaded square marks a speed limiting point (SLP), from which a maximum indicated airspeed applies, and in French controlled airspace IFR flights below 10,000 ft AMSL on ATS routes, standard departure and arrival routes or radar vectors are limited to 250 kt IAS unless ATC clears otherwise.

Jeppesen and FAA differences

The two chart families describe the same airspace with different conventions. Jeppesen draws every State's routes in one style; the FAA charts the United States in low and high altitude series. Exam questions often turn on which family a symbol belongs to:

The GNSS MEA lies below the MRA of the land-based navaid that defines the segment, and is usable with an approved GPS/WAAS receiver; it still guarantees standard obstacle clearance and two-way communications. The AIM also states that only holding patterns depicted on US government charts, or on commercial charts meeting FAA requirements, should be used.

Whatever the publisher, the method is the same: identify the route and its designator, read the MEA and any MOCA, MRA, MCA or MAA, find the changeover points and reporting points, note the frequencies and airspace, and know the off-route altitude for every part of the route. The figures are barometric, so in very cold air they need correcting before they can be trusted for terrain clearance.

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.