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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.
Turns that begin at or after fix passage may exceed the
protected en route turning area obstruction clearance.
Figure 3-13 on page 3-10 contains an example of a
flight track depicting a turn at or after fix passage,
together with an example of an early turn. Without
leading a turn, an aircraft operating in excess of 290
knots true airspeed (TAS) can exceed the normal airway or route boundaries depending on the amount of
course change required, wind direction and velocity,
the character of the turn fix (DME, overhead navigation aid, or intersection), and pilot technique in making
a course change. For example, a flight operating at
17,000 feet MSL with a TAS of 400 knots, a 25° bank,
and a course change of more than 40° would exceed the
width of the airway or route; i.e., 4 NM each side of
centerline. Due to the high airspeeds used at 18,000
feet MSL and above, additional IFR separation protection for course changes is provided.
NAVAID SERVICE VOLUME
Each class of VHF NAVAID (VOR/DME/TACAN)
has an established operational service volume to
ensure adequate signal coverage and frequency protection from other NAVAIDs on the same frequency.
The maximum distance at which NAVAIDs are usable
varies with altitude and the class of the facility. When
using VORs for direct route navigation, the following
guidelines apply:
• For operations above FL 450, use aids not more
than 200 NM apart. These are High Altitude (H)
　
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