En Route Operations (Part Five)

IFR En Route High Altitude Chart

En route high altitude charts provide aeronautical information for navigation under IFR conditions at and above FL 180. [Figure 2-31] High altitude charts include the following information:

Jet route structure
RNAV Q-routes
VHF radio aids to navigation (frequency, ID, channel, geographic coordinates)
Selected airports
Reporting points
Navigation reference system (NRS) waypoints [Figure 2-32]

Figure 2-31. IFR en route high altitude chart. [click image to enlarge]

Figure 2-32. Navigation reference system (NRS) waypoints.

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Jet routes are depicted in black with a “J” identifier followed by the route number (e.g., “J12”) and are based on VOR or VORTAC NAVAIDs. [Figure 2-33] RNAV “Q” Route MEAs are shown when other than 18,000 feet. [Figure 2-34] MEAs for GNSS RNAV aircraft are identified with a “G” suffix. MEAs for DME/DME/IRU RNAV aircraft do not have a “G” suffix. All RNAV routes and associated data is charted in aeronautical blue and magnetic reference bearings are shown originating from a waypoint, fix/reporting point, or NAVAID. When joint Jet/RNAV routes are depicted, the route identification boxes are located adjacent to each other with the route charted in black. [Figure 2-35] With the exception of “Q” routes in the Gulf of Mexico, GNSS or DME/DME/IRU RNAV equipment is required along with radar monitoring capabilities. For aircraft that have DME/DME/IRU RNAV equipment, refer to the CS for specific DME information.

Figure 2-33. High altitude jet routes. [click image to enlarge]

VHF Airways

Victor airways are a system of established routes that run along specified VOR radials, from one VOR station to another. The purpose is to make flight planning easier and they help ATC to organize and regulate the air traffic flow. Almost all commercial flights are routed along these airways but they are available for use by any pilot provided that the proper altitudes are employed.

Victor Airway Navigation Procedures

The procedure for getting established on a victor airway is to either fly directly to a nearby VOR or to intercept an airway radial along the route of flight. Once the pilot is established on an airway, it is important to follow the procedures and guidelines put in place to ensure air traffic separation and optimal safety on the airway. When using victor airways for navigation, procedures do not allow the pilot to jump from one VOR to another, but must navigate from one to the next by using the alternating outbound/ inbound procedure of linking VORs. For example, when departing from Zanesville VOR on V-214, the pilot selects the 090° radial with a FROM indication on the course deviation indicator (CDI) and should correct as necessary to continuously maintain track on the centerline of the airway. [Figure 2-36] The pilot should continue on this course until it is time to change over to the inbound course to the Bellaire VOR.

Figure 2-36. Zanesville VOR/Victor Airway 214. [click image to enlarge]

LF/MF Airways

The basic LF/MF airway width is 4.34 nautical miles (NM) on each side of the centerline; the width expands by five degrees when the distance from the facility providing course guidance is greater than 49.66 NM. [Figure 2-37]

Figure 2-37. LF/MR airway width. [click image to enlarge]

En Route Obstacle Clearance Areas

All published routes in the NAS are based on specific obstacle clearance criteria. An understanding of en route obstacle clearance areas helps with SA and may help avoid controlled flight into terrain (CFIT). Obstacle clearance areas for the en route phase of flight are identified as primary, secondary, and turning areas.

The primary and secondary area obstacle clearance criteria, airway and route widths, and the ATC separation procedures for en route segments are a function of safety and practicality in flight procedures. These flight procedures are dependent upon the pilot, the aircraft, and the navigation system being used, resulting in a total VOR system accuracy factor along with an associated probability factor. The pilot/aircraft information component of these criteria includes pilot ability to track the radial and the flight track resulting from turns at various speeds and altitudes under different wind conditions. The navigation system information includes navigation facility radial alignment displacement, transmitter monitor tolerance, and receiver accuracy. All of these factors were considered during development of en route criteria. From this analysis, the computations resulted in a total system accuracy of ±4.5° 95 percent of the time and ±6.7° 99 percent of the time. The 4.5° value became the basis for primary area obstacle clearance criteria, airway and route widths, and the ATC separation procedures. The 6.7° value provides secondary obstacle clearance area dimensions.

Primary and Secondary En Route Obstacle Clearance Areas

The primary obstacle clearance area has a protected width of 8 NM with 4 NM on each side of the centerline. The primary area has widths of route protection based upon system accuracy of a ±4.5° angle from the NAVAID. These 4.5° lines extend out from the NAVAID and intersect the boundaries of the primary area at a point approximately 51 NM from the NAVAID. Ideally, the 51 NM point is where pilots would change over from navigating away from the facility, to navigating toward the next facility, although this ideal is rarely achieved. [Figure 2-38]

Figure 2-38. Primary obstacle clearance area. [click image to enlarge]

If the distance from the NAVAID to the change-over point (COP) is more than 51 NM, the outer boundary of the primary area extends beyond the 4 NM width along the 4.5° line when the COP is at midpoint. This means the primary area, along with its obstacle clearance criteria, is extended out into what would have been the secondary area. Additional differences in the obstacle clearance area result in the case of the effect of an offset COP or dogleg segment. For protected en route areas, the minimum obstacle clearance in the primary area, not designated as mountainous under 14 CFR Part 95—IFR altitude, is 1,000 feet over the highest obstacle. [Figure 2-39] The secondary obstacle clearance area extends along a line 2 NM on each side of the primary area. Navigation system accuracy in the secondary area has widths of route protection of a ±6.7° angle from the NAVAID. These 6.7° lines intersect the outer boundaries of the secondary areas at the same point as primary lines, 51 NM from the NAVAID. If the distance from the NAVAID to the COP is more than 51 NM, the secondary area extends along the 6.7° line when the COP is at mid-point. [Figure 2-40] In all areas, mountainous and non-mountainous, obstacles that are located in secondary areas are considered as obstacles to air navigation if they extend above the secondary obstacle clearance plane. This plane begins at a point 500 feet above the obstacles (natural or man-made) upon which the primary obstacle clearance area is based, and slants upward at an angle that causes it to intersect the outer edge of the secondary area at a point 500 feet higher. [Figure 2-41]

Figure 2-39. Non-mountainous obstacle clearance in the primary area.

Figure 2-40. Secondary obstacle clearance area. [click image to enlarge]

Figure 2-41. Primary and secondary obstacle clearance area.

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