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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
Figure 3-6. VHF Jet Routes.
3-7
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.
4.5°
4.5°
51
51
4.5°
4.5°
4 NM
4 NM
4 NM
4 NM
2 NM
2 NM
6.7°
6.7°
6.7°
6.7°
51
51
Primary Obstacle Clearance Area
Secondary Obstacle Clearance Area
Figure 3-7. VHF En Route Obstacle Clearance Areas.
1,000 Feet Above
Highest Obstacle
Primary En Route
Obstacle Clearance Area
Nonmountainous Area
Figure 3-8. Obstacle Clearance - Primary Area.
3-8
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.
SECONDARY AREA
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 midpoint. In all areas, mountainous and nonmountainous,
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
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 3-10]
The obstacle clearance areas for LF airways and routes
are different than VHF, with the primary and secondary
area route widths both being 4.34 NM. The accuracy
lines are 5.0° in the primary obstacle clearance area and
7.5° in the secondary area. Obstacle clearance in the
primary area of LF airways and routes is the same as
　
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