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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
that required for VHF, although the secondary area
obstacle clearance requirements are based upon distance from the facility and location of the obstacle
relative to the inside boundary of the secondary area.
Figure 3-9. Designated Mountainous Areas.
Puerto Rico
Mountainous Area
67° 66°30' 66°
20°
25°
67° 66°30' 66°
30°
35°
40°
45°
75°
85°
95° 105°
115°
125°
45°
40°
35°
30°
25°
20°
125°
115°
105°
95°
85°
75°
LEGEND
WASHINGTON
OREGON
MONTANA
IDAHO
NEV ADA
WYOMING
UTAH
COLORADO
CALIFORNIA
ARIZONA
NEW MEXICO
NORTH
DAKOTA
SOUTH
DAKOTA
NEBRASKA
KANSAS
OKLAHOMA
TEXAS
LA
MS
FL
GEORGIA
SC
MD
DE
RI
ME
VT
NH
MA
CT
NEW YORK
PA
WV
VIRGINA
MI
IN
OHIO
WISCONSIN
MINNESOTA
IOWA
ILLINOIS
MISSOURI
ARKANSAS
KENTUCKY
NJ
NO CAROLINA
TENNESSEE
AL
(NOT DRAWN TO SCALE)
Mountainous
Areas
Nonmountainous
Area
Mountainous
Area
Primary Area
Secondary Area
500 Feet
Total Width
of Secondary
Area
Distance from
Obstacle to
Outer Edge of
Secondary Area
Figure 3-10. Obstacle Clearance - Secondary Area.
When a VHF airway or route terminates at a NAVAID or
fix, the primary area extends beyond that termination
point. Figure 3-11 and its inset show the construction of
the primary and secondary areas at the termination
point. When a change of course on VHF airways and
routes is necessary, the en route obstacle clearance
turning area extends the primary and secondary
obstacle clearance areas to accommodate the turn
radius of the aircraft. Since turns at or after fix passage may exceed airway and route boundaries, pilots
are expected to adhere to airway and route protected
airspace by leading turns early before a fix. The
turn area provides obstacle clearance for both turn
anticipation (turning prior to the fix) and flyover
protection (turning after crossing the fix). This does
not violate the requirement to fly the centerline of the
airway. Many factors enter into the construction and
application of the turning area to provide pilots with
adequate obstacle clearance protection. These may
include aircraft speed, the amount of turn versus
NAVAID distance, flight track, curve radii, MEAs,
and minimum turning altitude (MTA). A typical protected airspace is shown in Figure 3-11. Turning area
system accuracy factors must be applied to the most
adverse displacement of the NAVAID or fix and the
airway or route boundaries at which the turn is made.
If applying nonmountainous en route turning area criteria graphically, depicting the vertical obstruction
clearance in a typical application, the template might
appear as in Figure 3-12 on page 3-10.
　
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