Aviation Theory

Representative sections of the enroute charts might be available in the future. In the meantime, it would be a good idea for you to have a chart handy so that you can follow the narrative and identify the features on your charts.

INTRODUCTION

Being able to quickly and accurately read the various charts associated with IFR flight is a trait that will make your flights easier and less pressured. You should become familiar with all the symbols contained in the legend so that you do not have to continually look up the symbols.

There is a tremendous amount of information contained on the charts concerning radio navigation aids, airways, safe altitudes, airports, airspace, and communications. Like in the IAP charts, you have a choice between the NOS and Jeppesen charts. Although more expensive, the new series Jeppesen charts often seem more popular.

In this lesson, we will talk about the Low Altitude Enroute charts (used for navigation up to but not including 18,000 ft MSL), the High Altitude Enroute charts (used for navigation at 18,000 MSL and above) and the Area charts, which give a more detailed view of certain terminal areas. Although not all information discussed is contained on each of the charts, the charts themselves are very similar and once you can read one type, very little additional study is required to use the others.

AIRPORTS

Airports are shown on the enroute charts along with some basic information such as elevation. More complete information is contained in the Airport/Facility Directory (A/FD) and in the Jeppesen manuals. Additional information is also found on the IAPs for the specific airports having a published instrument approach.

NAVIGATIONAL AIDS

The basis for the formation of airways, the VOR and VORTAC, are shown on the enroute charts. DME information is also shown. NDBs are also shown when the form a part of the ATC enroute structure.

Localizer front courses and back courses are shown on the NOS charts if they form a part of the ATC enroute structure, such as an intersection.

ROUTES

The airway structure is controlled airspace in the form of a corridor between navigation aids, usually VORTACs. Below 18,000 ft MSL (Mean Sea Level), the airways are know as Victor Airways and are designated by the letter "V" and a number, e.g. V 64. Information shown on the airways includes the magnetic course, distance, Minimum Enroute Altitude (MEA) and Minimum Obstacle Clearance Altitude (MOCA). Courses are in degrees magnetic and distances are in nautical miles. They extend down to 1200 ft MSL in most cases.

Intersections (published fixes) are shown as breaks, marked with a triangle, in the airway.  Mileage break points may also be indicated by an "X" and exist where an intersection is not published. DME distances to the relevant VORTAC may also be shown.

You may notice the some airways have a compound number, such as V 9-64. These are segments where the route is common to more than one airway. For flight planning, you only need to indicate the airway you are using. Alternate airways may be shown with a letter (N,S,E, or W) following the airway designation. For example, V 64W would be an alternate west of V 64.

From 18.000 ft MSL up to and including FL 600 (class A airspace) flight is restricted to IFR only. Airways from 18.000 ft MSL up to and including FL 450 are know as Jet routes and are shown on the High Altitude Enroute charts designated by the letter "J" and a number. These routes are shown on the High Altitude Enroute charts.

MINIMUM ENROUTE ALTITUDE (MEA)

The MEA is listed as a number, e.g. 6500, along the airway and is the lowest altitude between radio fixes that assures navigational signal coverage (not necessarily two way voice communication) and meets obstacle clearance requirements between the fixes. The clearance is 1000 ft in non mountainous areas, 2000 ft in mountainous areas, and includes the airspace within plus or minus 4 nm of the airway centerline.

Altitudes for IFR flight usually are in accord with the hemispherical rule of odd altitudes for magnetic courses of 0° - 179° and even altitudes for magnetic courses of 180°-359°. An easy way to remember the rule is WEEO - West evens; East odds. ATC may assign altitudes that do not correspond to the rule, however.

Some route may have a gap in the navigation signal. This is designated as MEA GAP on NOS charts and by a broken bar on the Jepp charts. MEAs may also be directional, based on the travel from high ground to low or low to high. A bar crossing the airway at an intersection indicates a change in MEA. You are not required to increase your altitude until crossing the intersection, unless a minimum crossing altitude (MCA) is specified.

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MINIMUM CROSSING ALTITUDE

The MCA is the lowest altitude allowed at certain fixes. While the MCA is usually due to obstacle clearance requirements, it may be related to navigational signal reception. The MCA is indicated by a flagged X on NOS charts and as an airway number and altitude on the Jepps (e.g. V 64 12000S means a MCA of 12,000 fr MSL when flying south on V 64). The climb to the MCA should be commenced before reaching the intersection so that the MCA is not violated.

MINIMUM OBSTRCTION CLEARANCE ALTITUDE (MOCA)

The MOCA is the lowest altitude in effect between fixes which meets the obstacle clearance requirements for the entire segment. Navigational signals are assured only within 22 nm of the VOR. If the MOCA is different than the MEA, it is indicated by an asterisk on NOS charts (*3500) and a "T" on the Jepps (3500T). If the MOCA and MEA are the same, no indication is made on the charts.

MINIMUM RECEPTION ALTITUDE (MRA)

The MRA is the lowest altitude at which an intersection may be identified. At altitudes lower than the MRA, one or more of the navaids forming the intersection may not be received. The MRA is indicated by a flagged R on the NOS charts and MRA on the Jepps (e.g. MRA 6000).

CHANGEOVER POINTS (COPs)

The changeover point is that point between two navaids where you would switch from the last navaid to the next navaid. The COP is normally at the midway point between the two. If the COP is not at the midway point, it will be designated on the chart and the appropriate DME distance is shown. The purpose of the COP may be to assure reception or to prevent interference from stations on the same frequency.

MAXIMUM AUTHORIZED ALTITUDE (MAA)

The MAA is the highest usable altitude at which adequate reception on navigational signals is assured. Above the MAA, there may be interference from other navaids.

AIRSPACE

The different classes of airspace are indicated on the enroute charts. For a discussion of the airspace restrictions, see the Lesson on Airspace.

On the NOS Low Altitude charts, the airspace up to, but not including, 18,000 ft MSL is shown as follows: Class B - light blue shading with a solid blue border Class C - light blue shading with a dashed blue border Class C and D airports and indicated by the appropriate letter in a box Class E - open white space Class G - brown shaded areas Mode C (altitude encoding altimeter required) - striped blue tint lines Special use airspace is indicated by either blue or brown vertical dashed line boxes.

The Jepps show airspace classification with the appropriate letter in parentheses, e.g. (B). Class G airspace is indicated by a gray screened area.

COMMUNICATIONS

FSS (Flight Service Station) frequencies are shown near navaid boxes. On the NOS chart, the full frequency is indicated, e.g. 122.45. The Jepps omit the "12" and only list the frequency as 2.45 since the first two digits are always "12". If a frequency is receive only, it is marked with R on the NOS and G (guard) on the Jepps.

The emergency frequency of 121.5 MHz is international and is not indicated on the charts.

The boundaries of the ARTCC (Centers) responsible for an area is shown by a ragged line on the NOS and a dotted line on the Jepps, running across the chart, with the appropriate center name on either side. The frequencies for the centers are shown in boxes at several places throughout the area of coverage.

IFR AREA CHARTS

The area charts have many of the same features as the enroute charts, but, the scale is smaller, allowing more detail. The NOS area charts are one single chart with San Francisco, Los Angeles, Denver, Minneapolis/St. Paul, Dallas/Fort Worth, Kansas city, Chicago/Milwaukee, Detroit, St. Louis, Atlanta, Jacksonville, and Miami areas. The New York area is shown on the corresponding Low Altitude chart. Jeppesen area charts are interleaved with the IAPs.

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