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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
Airway Route Boundary
Turning
Fix
Early Turn
Turn at or after
Fix Passage
Figure 3-13. Adhering to Airway/Route Turning Area.
3-11
are determined, in part, by reference to the graph
depicted in Figure 3-14. Notice the en route chart
excerpt depicting that the MEA is established with a
gap in navigation signal coverage northwest of the
Carbon VOR/DME on V134. At the MEA of 13,000,
the allowable navigation course guidance gap is
approximately 18.5 NM, as depicted by Sample 2. The
navigation gap area is not identified on the chart by
distances from the navigation facilities.
CHANGEOVER POINTS
When flying airways, pilots normally change frequencies midway between navigation aids, although there
are times when this is not practical. If the navigation
signals cannot be received from the second VOR at the
midpoint of the route, a changeover point (COP) is
depicted and shows the distance in NM to each NAVAID,
as depicted in Figure 3-15 on page 3-12. COPs indicate
the point where a frequency change is necessary to
receive course guidance from the facility ahead of the
aircraft instead of the one behind. These changeover
points divide an airway or route segment and ensure
continuous reception of navigation signals at the prescribed minimum en route IFR altitude. They also
ensure that other aircraft operating within the same portion of an airway or route segment receive consistent
azimuth signals from the same navigation facilities
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Instrument Procedures Handbook (IPH)仪表程序手册上(68)
