Figure 3-6 depicts numerous arrowed, single direction jet routes on this excerpt from a NACO en route high altitude chart, effective at and above 18,000 feet MSL up to and including FL 450. Notice the MAAs of 41,000 and 29,000 associated with J24 and J193, respectively. Additionally, note the BAATT, NAGGI, FUMES, and MEYRA area navigation (RNAV) waypoints. Waypoints are discussed in detail later in this chapter.


All published routes in the NAS are based on specific obstacle clearance criteria. An understanding of en route obstacle clearance areas helps with situational awareness 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 figure 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. Figure 3-7 depicts the primary and secondary obstacle clearance areas.


The primary obstacle clearance area has a protected width of 8 nautical miles (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.

If the distance from the NAVAID to the changeover 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 Part 95 IFR altitude is 1,000 feet over the highest obstacle. [Figure 3-8]

Mountainous areas for the Eastern and Western U.S. are designated in Part 95, as shown in Figure 3-9 on page 3-8. Additional mountainous areas are designated for Alaska, Hawaii, and Puerto Rico. With some exceptions, the protected en route area minimum obstacle clearance over terrain and manmade obstacles in mountainous areas is 2,000 feet. Obstacle clearance is sometimes reduced to not less than 1,500 feet above terrain in the designated mountainous areas of the Eastern U.S., Puerto Rico, and Hawaii, and may be reduced to not less than 1,700 feet in mountainous areas of the Western U.S. and Alaska. Consideration is given to the following points before any altitudes providing less than 2,000 feet of terrain clearance are authorized:

  • Areas characterized by precipitous terrain.
  • Weather phenomena peculiar to the area.
  • Phenomena conducive to marked pressure differentials.
  • Type of and distance between navigational facilities.
  • Availability of weather services throughout the area.
  • Availability and reliability of altimeter resetting points along airways and routes in the area.

Altitudes providing at least 1,000 feet of obstacle clearance over towers and/or other manmade obstacles may be authorized within designated mountainous areas if the obstacles are not located on precipitous terrain where Bernoulli Effect is known or suspected to exist.
Bernoulli Effect, atmospheric eddies, vortices, waves, and other phenomena that occur in conjunction with disturbed airflow associated with the passage of strong winds over mountains can result in pressure deficiencies manifested as very steep horizontal pressure gradients. Since downdrafts and turbulence are prevalent under these conditions, potential hazards may be multiplied.