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Without an RMI, orientation is more difficult since there is
no direct azimuth reference. However, the procedure can be
flown using the OBS and CDI for azimuth information and
the DME for arc distance.
Intercepting Lead Radials
A lead radial is the radial at which the turn from the arc to the
inbound course is started. When intercepting a radial from
a DME arc, the lead will vary with arc radius and ground
speed. For the average general aviation aircraft, flying arcs
such as those depicted on most approach charts at speeds
of 150 knots or less, the lead will be under 5°. There is no
difference between intercepting a radial from an arc and
intercepting it from a straight course.
With an RMI, the rate of bearing movement should be
monitored closely while flying the arc. Set the course of the
radial to be intercepted as soon as possible and determine
the approximate lead. Upon reaching this point, start the
intercepting turn. Without an RMI, the technique for radial
interception is the same except for azimuth information,
which is available only from the OBS and CDI.
The technique for intercepting a localizer from a DME arc
is similar to intercepting a radial. At the depicted lead radial
(LR 223 or LR 212 in Figures 7-19, 7-20, and 7-21), a
pilot having a single VOR/LOC receiver should set it to the
localizer frequency. If the pilot has dual VOR/LOC receivers,
one unit may be used to provide azimuth information and the
other set to the localizer frequency. Since these lead radials
provide 7° of lead, a half-standard rate turn should be used
until the LOC needle starts to move toward center.
DME Errors
A DME/DME fix (a location based on two DME lines of
position from two DME stations) provides a more accurate
aircraft location than using a VOR and a DME fix.
DME signals are line-of-sight; the mileage readout is the
straight line distance from the aircraft to the DME ground
facility and is commonly referred to as slant range distance.
Slant range refers to the distance from the aircraft’s antenna
to the ground station (A line at an angle to the ground
transmitter. GPS systems provide distance as the horizontal
measurement from the WP to the aircraft. Therefore, at 3,000
feet and 0.5 miles the DME (slant range) would read 0.6 NM
while the GPS distance would show the actual horizontal
distance of .5 DME. This error is smallest at low altitudes
and/or at long ranges. It is greatest when the aircraft is closer
to the facility, at which time the DME receiver will display
altitude (in NM) above the facility. Slant range error is
negligible if the aircraft is one mile or more from the ground
facility for each 1,000 feet of altitude above the elevation of
the facility.
Area Navigation (RNAV)
Area navigation (RNAV) equipment includes VOR/DME,
LORAN, GPS, and inertial navigation systems (INS). RNAV
equipment is capable of computing the aircraft position,
actual track, groundspeed, and then presenting meaningful
information to the pilot. This information may be in the form
of distance, cross-track error, and time estimates relative to
the selected track or WP. In addition, the RNAV equipment
installations must be approved for use under IFR. The Pilot’s
Operating Handbook/Airplane Flight Manual (POH/AFM)
should always be consulted to determine what equipment is
installed, the operations that are approved, and the details of
equipment use. Some aircraft may have equipment that allows
input from more than one RNAV source, thereby providing
a very accurate and reliable navigation source.
7-20
Figure 7-19. An aircraft is displayed heading southwest to intercept the localizer approach, using the 16 NM DME Arc off of ORM.
7-21
Figure 7-20. The same aircraft illustrated in Figure 7-19 shown on the ORM radial near TIGAE intersection turning inbound for the
localizer.
7-22
Figure 7-21. Aircraft is illustrated inbound on the localizer
course.
Figure 7-22. RNAV Computation.
7-23
Figure 7-23. Onboard RNAV receivers have changed significantly.
Originally, RNAV receivers typically computed combined data
from VOR, VORTAC, and/or DME. That is generally not the case
now. Today, GPS such as the GNC 300 and the Bendix King KLS
88 LORAN receivers compute waypoints based upon embedded
databases and aircraft positional information.
VOR/DME RNAV
VOR RNAV is based on information generated by the present
VORTAC or VOR/DME system to create a WP using an
airborne computer. As shown in Figure 7-22, the value of
side A is the measured DME distance to the VOR/DME. Side
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