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Salt Lake City, Utah to Denver, Colorado, means terrain
and obstacles are very important. This RNAV direct
route across the Rocky Mountains reduces your distance
by about 17 NM, but radar coverage over the Rockies at
lower altitudes is pretty spotty. This raises numerous
questions. What will air traffic control allow on direct
flights? What will they do if radar coverage is lost? What
altitudes will they allow when they can’t see you on
radar? Do they have altitudes for direct routes? The easy
answer is to file the airways, and then all the airway
MIAs become usable. But with RNAV equipment, a
direct route is more efficient. Even though on some
routes the mileage difference may be negligible, there
are many other cases where the difference in distance is
significant. ATC is required to provide radar separation
on random RNAV routes at FL 450 and below. It is logical to assume that ATC will clear you at an altitude that
allows it to maintain radar contact along the entire route,
which could mean spending additional time and fuel
climbing to an altitude that gives full radar coverage.
All air route traffic control centers have MIAs for their areas
of coverage. Although these altitudes are not published
anywhere, they are available when airborne from ATC.
OFF ROUTE OBSTRUCTION
CLEARANCE ALTITUDE
An off-route obstruction clearance altitude
(OROCA) is an off-route altitude that provides
obstruction clearance with a 1,000-foot buffer in nonmountainous terrain areas and a 2,000-foot buffer in
designated mountainous areas within the U.S. This
altitude may not provide signal coverage from
ground-based navigational aids, air traffic control
radar, or communications coverage. OROCAs are
intended primarily as a pilot tool for emergencies and
situational awareness. OROCAs depicted on NACO
en route charts do not provide you with an acceptable
altitude for terrain and obstruction clearance for the
purposes of off-route, random RNAV direct flights in
either controlled or uncontrolled airspace. OROCAs
are not subject to the same scrutiny as MEAs, MVAs,
MOCAs, and other minimum IFR altitudes. Since
they do not undergo the same obstruction evaluation,
Figure 3-33. Direct Route Navigation.
3-29
airport airspace analysis procedures, or flight inspection, they cannot provide the same level of confidence
as the other minimum IFR altitudes.
If you depart an airport VFR intending to or needing to
obtain an IFR clearance en route, you must be aware of
the position of your aircraft relative to terrain and
obstructions. When accepting a clearance below the
MEA, MIA, MVA, or the OROCA, you are responsible for your own terrain/obstruction clearance until
reaching the MEA, MIA, or MVA. If you are unable
to visually maintain terrain/obstruction clearance, you
should advise the controller and state your intentions.
[Figure 3-34]
For all random RNAV flights, there needs to be at least
one waypoint in each ARTCC area through which you
intend to fly. One of the biggest problems in creating
an RNAV direct route is determining if the route goes
through special use airspace. For most direct routes,
the chances of going through prohibited, restricted,
or special use airspace are good. In the U.S., all direct
routes should be planned to avoid prohibited or
restricted airspace by at least 3 NM. If a bend in a
direct route is required to avoid special use airspace,
the turning point needs to be part of the flight plan.
Two of the most prominent long range navigation
systems today include FMS with integrated GPS
and stand-alone GPS. The following example is a
simplified overview showing how the RNAV systems
might be used to fly a random RNAV route:
In Figure 3-35 on page 3-30, you are northeast of
Tuba City VORTAC at FL 200 using RNAV (showing
both GPS and FMS), RNAV direct on a southwesterly
heading to Lindbergh Regional Airport in Winslow.
As you monitor your position and cross check your
avionics against the high altitude en route chart, you
receive a company message instructing you to divert
to Las Vegas, requiring a change in your flight plan as
highlighted on the depicted chart excerpt.
Figure 3-34. Off-Route Obstacle Clearance Altitude.
3-30
During your cockpit review of the high and low altitude
en route charts, you determine that your best course of
action is to fly direct to the MIRAJ waypoint, 28 DME
northeast of the Las Vegas VORTAC on the 045° radial.
This places you 193 NM out on a 259° magnetic course
inbound, and may help you avoid diverting north,
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Instrument Procedures Handbook (IPH)仪表程序手册下(76)
