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straight line. There are cautions,
however. Placing our round earth on
a flat piece of paper causes distortions, particularly on long east-west
routes. If your route is 180 degrees
or 360 degrees, there is virtually no
distortion in the course line.
About the only way you can confidently avoid protected airspace is by the use of some
type of airborne database, including a graphic display
of the airspace on the long-range navigation system
moving map, for example. When not using an airborne
database, leaving a few miles as a buffer helps ensure
that you stay away from protected airspace.
In Figure 3-33 on page 3-28, a straight line on a magnetic course from SCRAN intersection of 270 degrees
direct to the Fort Smith Regional Airport in Arkansas
will pass just north of restricted area R-2401A and B,
and R-2402. Since the airport and the restricted areas
are precisely plotted, there is an assurance that you will
stay north of the restricted areas. From a practical
standpoint, it might be better to fly direct to the Wizer
NDB. This route goes even further north of the
restricted areas and places you over the final approach
fix to Runway 25 at Fort Smith.
One of the most common means for you to fly direct
routes is to use conventional navigation such as VORs.
When flying direct off-airway routes, remember to
apply the FAA distance limitations, based upon
NAVAID service volume.
RANDOM RNAV ROUTES
Random RNAV routes may be an integral solution in
meeting the worldwide demand for increased air traffic system capacity and safety. Random RNAV routes
are direct routes, based on RNAV capability. They are
typically flown between waypoints defined in terms of
latitude and longitude coordinates, degree and distance
fixes, or offsets from established routes and airways at
a specified distance and direction. Radar monitoring by
ATC is required on all random RNAV routes.
With IFR certified RNAV units (GPS or FMS), there are
several questions to be answered, including “Should I
fly airways or should I fly RNAV direct?” One of the
considerations is the determination of the MIA. In most
Note - Only B-747 and DC-10 operations authorized in these areas.
AUTHORIZED AREAS OF
EN ROUTE OPERATION
LIMITATIONS, PROVISIONS,
AND REFERENCE PARAGRAPHS
The 48 contiguous United States
and the District of Columbia
Note 1
Canada, excluding Canadian MNPS
airspace and the areas of magnetic
unreliability as established in the
Canadian AIP
Note SPECIAL REQUIREMENTS:
Note 1 737 Class II navigation operations with a single long-range system
is authorized only within this area of en route operation.
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Figure 3-32. Excerpt of Authorized Areas of En Route Operation.
3-28
places in the world at FL 180 and above, the MIA is not
significant since you are well above any terrain or obstacles. On the other hand, a direct route at 18,000 feet from
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
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Instrument Procedures Handbook (IPH)仪表程序手册上(80)
