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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)
class facilities and are depicted on en route high
altitude charts.
Figure 3-11. Turning Area, Intersection Fix, NAVAID Distance less than 51 NM.
Radii
Center
Secondary
Arcs
Primary
Arcs
Primary
Indexes
"Outside"
"Inside"
Termination
Areas
CENTERLINE
CENTERLINE
En Route
Facility Facility
Providing
Intersection
Radial
Fix
Displacement
Area
4.5°
3.6°
3-9
3-10
• For operations that are off established airways
from 18,000 feet MSL to FL 450, use aids not
more than 260 NM apart. These are High Altitude
(H) class facilities and are depicted on en route
high altitude charts.
• For operations that are off established airways
below 18,000 feet MSL, use aids not more than
80 NM apart. These are Low Altitude (L) class
facilities and are shown on en route low altitude
charts.
• For operations that are off established airways
between 14,500 feet MSL and 17,999 feet MSL
in the conterminous United States, use H-class
facilities not more than 200 NM apart.
The use of satellite based navigation systems has
increased pilot requests for direct routes that take the
aircraft outside ground based NAVAID service volume
limits. These direct route requests are approved only in
a radar environment, and approval is based on pilot
responsibility for staying on the authorized direct route.
ATC uses radar flight following for the purpose of aircraft separation. On the other hand, if ATC initiates a
direct route that exceeds NAVAID service volume limits, ATC also provides radar navigational assistance as
necessary. More information on direct route navigation
is located in the En Route RNAV Procedures section
later in this chapter.
NAVIGATIONAL GAPS
Where a navigational course guidance gap exists,
referred to as an MEA gap, the airway or route segment
may still be approved for navigation. The navigational
gap may not exceed a specific distance that varies
directly with altitude, from zero NM at sea level to 65
NM at 45,000 feet MSL and not more than one gap may
exist in the airspace structure for the airway or route
segment. Additionally, a gap usually does not occur at
any airway or route turning point. To help ensure the
maximum amount of continuous positive course guidance available when flying, there are established en
route criteria for both straight and turning segments.
Where large gaps exist that require altitude changes,
MEA “steps” may be established at increments of not
less than 2,000 feet below 18,000 feet MSL, or not less
than 4,000 feet at 18,000 MSL and above, provided that
a total gap does not exist for the entire segment within
the airspace structure. MEA steps are limited to one
step between any two facilities to eliminate continuous
or repeated changes of altitude in problem areas. The
allowable navigational gaps pilots can expect to see
Secondary Area
Primary Area
500 Feet
500 Feet
Figure 3-12. Turning Area Obstruction Clearance.
Airway Route Boundary
　
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