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regardless of the direction of flight.
Where signal coverage from two VORs overlaps at the
MEA, the changeover point normally is designated at
the midpoint. Where radio frequency interference or
other navigation signal problems exist, the COP is
placed at the optimum location, taking into consideration the signal strength, alignment error, or any other
known condition that affects reception. The changeover
point has an effect on the primary and secondary obstacle clearance areas. On long airway or route segments,
if the distance between two facilities is over 102 NM
and the changeover point is placed at the midpoint,
the system accuracy lines extend beyond the minimum widths of 8 and 12 NM, and a flare or spreading
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90
MEA OF AIRWAY OR
ROUTE SEGMENT
(THOUSANDS OF FEET)
Sample 1: Enter with MEA of 27,000 Feet.
Read Allowable Gap 39 NM
Sample 2: Enter with MEA of 13,000 Feet.
Read Allowable Gap 18.5 NM
ALLOWABLE NAVIGATION COURSE GUIDANCE GAP (NM)
Figure 3-14. Navigational Course Guidance Gaps.
3-12
outward results at the COP, as shown in Figure 3-16.
Offset changeover points and dogleg segments on airways or routes can also result in a flare at the COP.
IFR EN ROUTE ALTITUDES
Minimum en route altitudes, minimum reception altitudes, maximum authorized altitudes, minimum
obstruction clearance altitudes, minimum crossing
altitudes, and changeover points are established by
the FAA for instrument flight along Federal airways,
as well as some off-airway routes. The altitudes are
established after it has been determined that the navigation aids to be used are adequate and so oriented
on the airways or routes that signal coverage is
acceptable, and that flight can be maintained within
prescribed route widths.
For IFR operations, regulations require that pilots operate their aircraft at or above minimum altitudes. Except
when necessary for takeoff or landing, pilots may not
operate an aircraft under IFR below applicable minimum altitudes, or if no applicable minimum altitude is
prescribed, in the case of operations over an area designated as mountainous, an altitude of 2,000 feet above
the highest obstacle within a horizontal distance of 4 NM
from the course to be flown. In any other case, an altitude
of 1,000 feet above the highest obstacle within a horizontal distance of 4 NM from the course to be flown must be
maintained as a minimum altitude. If both a MEA and a
minimum obstruction clearance altitude (MOCA) are
prescribed for a particular route or route segment, pilots
may operate an aircraft below the MEA down to, but not
below, the MOCA, only when within 22 NM of the VOR.
When climbing to a higher minimum IFR altitude (MIA),
pilots must begin climbing immediately after passing the
point beyond which that minimum altitude applies,
except when ground obstructions intervene, the point
beyond which that higher minimum altitude applies must
be crossed at or above the applicable minimum crossing
altitude (MCA) for the VOR.
If on an IFR flight plan, but cleared by ATC to maintain
VFR conditions on top, pilots may not fly below minimum en route IFR altitudes. Minimum altitude rules
are designed to ensure safe vertical separation between
the aircraft and the terrain. These minimum altitude
rules apply to all IFR flights, whether in IFR or VFR
Figure 3-16. Changeover Point Effect on Long Airway or Route Segment.
4 NM
4 NM
2 NM
2 NM
6.7°
4.5°
4.5°
6.7°
70
70
Secondary Areas
Primary Area
Flare
Flare
Figure 3-15. Changeover Points.
22
45
13,000 ft
13,000
V 344
3-13
weather conditions, and whether assigned a specific
altitude or VFR conditions on top.
MINIMUM EN ROUTE ALTITUDE
The minimum enroute altitude (MEA) is the lowest
published altitude between radio fixes that assures
acceptable navigational signal coverage and meets
obstacle clearance requirements between those fixes.
The MEA prescribed for a Federal airway or segment,
RNAV low or high route, or other direct route applies
to the entire width of the airway, segment, or route
between the radio fixes defining the airway, segment,
or route. MEAs for routes wholly contained within
controlled airspace normally provide a buffer above the
floor of controlled airspace consisting of at least 300
feet within transition areas and 500 feet within control
areas. MEAs are established based upon obstacle clearance over terrain and manmade objects, adequacy of
navigation facility performance, and communications
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Instrument Procedures Handbook (IPH)仪表程序手册上(69)
