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requirements, although adequate communication at the
MEA is not guaranteed.
MINIMUM OBSTRUCTION
CLEARANCE ALTITUDE
The minimum obstruction clearance altitude (MOCA)
is the lowest published altitude in effect between fixes
on VOR airways, off-airway routes, or route segments
that meets obstacle clearance requirements for the entire
route segment. This altitude also assures acceptable navigational signal coverage only within 22 NM of a VOR.
The MOCA seen on the NACO en route chart, may have
been computed by adding the required obstacle clearance (ROC) to the controlling obstacle in the primary
area or computed by using a TERPS chart if the controlling obstacle is located in the secondary area. This figure
is then rounded to the nearest 100 - foot increment, i.e.,
2,049 feet becomes 2,000, and 2,050 feet becomes 2,100
feet. An extra 1,000 feet is added in mountainous areas,
in most cases. The MOCA is based upon obstacle clearance over the terrain or over manmade objects, adequacy
of navigation facility performance, and communications
requirements.
ATC controllers have an important role in helping pilots
remain clear of obstructions. Controllers are instructed
to issue a safety alert if the aircraft is in a position that, in
their judgment, places the pilot in unsafe proximity to
terrain, obstructions, or other aircraft. Once pilots inform
ATC of action being taken to resolve the situation, the
controller may discontinue the issuance of further alerts.
A typical terrain/obstruction alert may sound like this:
“Low altitude alert. Check your altitude immediately.
The MOCA in your area is 12,000.”
MINIMUM VECTORING ALTITUDES
Minimum vectoring altitudes (MVAs) are established
for use by ATC when radar ATC is exercised. The MVA
provides 1,000 feet of clearance above the highest
obstacle in nonmountainous areas and 2,000 feet above
the highest obstacle in designated mountainous areas.
Because of the ability to isolate specific obstacles, some
MVAs may be lower than MEAs, MOCAs, or other
minimum altitudes depicted on charts for a given
location. While being radar vectored, IFR altitude
assignments by ATC are normally at or above the
MVA.
Controllers use MVAs only when they are assured an
adequate radar return is being received from the aircraft. Charts depicting minimum vectoring altitudes
are normally available to controllers but not available
to pilots. Situational awareness is always important,
especially when being radar vectored during a climb
into an area with progressively higher MVA sectors,
similar to the concept of minimum crossing altitude.
Except where diverse vector areas have been established, when climbing, pilots should not be vectored
into a sector with a higher MVA unless at or above the
next sector’s MVA. Where lower MVAs are required
in designated mountainous areas to achieve compatibility with terminal routes or to permit vectoring to an
instrument approach procedure, 1,000 feet of obstacle
clearance may be authorized with the use of Airport
Surveillance Radar (ASR). The MVA will provide at
least 300 feet above the floor of controlled airspace.
The MVA charts are developed to the maximum radar
range. Sectors provide separation from terrain and
obstructions. Each MVA chart has sectors large
enough to accommodate vectoring of aircraft within
the sector at the MVA. [Figure 3-17 on page 3-14]
MINIMUM RECEPTION ALTITUDE
Minimum reception altitudes (MRAs) are determined by FAA flight inspection traversing an entire
route of flight to establish the minimum altitude the
navigation signal can be received for the route and
for off-course NAVAID facilities that determine a fix.
When the MRA at the fix is higher than the MEA, an
MRA is established for the fix, and is the lowest altitude at which an intersection can be determined.
MINIMUM CROSSING ALTITUDE
A minimum crossing altitude (MCA) is the lowest altitude at certain fixes at which the aircraft must cross when
proceeding in the direction of a higher minimum en route
IFR altitude, as depicted in Figure 3-18 on page 3-14.
MCAs are established in all cases where obstacles intervene to prevent pilots from maintaining obstacle clearance
during a normal climb to a higher MEA after passing a
point beyond which the higher MEA applies. The same
protected en route area vertical obstacle clearance requirements for the primary and secondary areas are considered
in the determination of the MCA. The standard for determining the MCA is based upon the following climb
gradients, and is computed from the flight altitude:
• Sea level through 5,000 feet MSL—150 feet per
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Instrument Procedures Handbook (IPH)仪表程序手册上(70)
