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B, the distance from the VOR/DME to the WP, and angle 1
(VOR radial or the bearing from the VORTAC to the WP)
are values set in the flight deck control. The bearing from
the VOR/DME to the aircraft, angle 2, is measured by the
VOR receiver. The airborne computer continuously compares
angles 1 and 2 and determines angle 3 and side C, which is
the distance in NMs and magnetic course from the aircraft
to the WP. This is presented as guidance information on the
flight deck display.
VOR/DME RNAV Components
Although RNAV flight deck instrument displays vary among
manufacturers, most are connected to the aircraft CDI with a
switch or knob to select VOR or RNAV guidance. There is
usually a light or indicator to inform the pilot whether VOR
or RNAV is selected. [Figure 7-23] The display includes the
WP, frequency, mode in use, WP radial and distance, DME
distance, groundspeed, and time to station.
Most VOR/DME RNAV systems have the following airborne
controls:
1. Off/On/Volume control to select the frequency of the
VOR/DME station to be used.
2. MODE select switch used to select VOR/DME mode,
with:
a. Angular course width deviation (standard VOR
operation); or
b. Linear cross-track deviation as standard (±5 NM
full scale CDI).
3. RNAV mode, with direct to WP with linear cross-track
deviation of ±5 NM.
4. RNAV/APPR (approach mode) with linear deviation
of ±1.25 NM as full scale CDI deflection.
5. WP select control. Some units allow the storage of more
than one WP; this control allows selection of any WP
in storage.
6. Data input controls. These controls allow user input
of WP number or ident, VOR or LOC frequency, WP
radial and distance.
While DME groundspeed readout is accurate only when
tracking directly to or from the station in VOR/DME mode,
in RNAV mode the DME groundspeed readout is accurate on
any track.
Function of VOR/DME RNAV
The advantages of the VOR/DME RNAV system stem from
the ability of the airborne computer to locate a WP wherever it
is convenient, as long as the aircraft is within reception range
of both nearby VOR and DME facilities. A series of these
WPs make up an RNAV route. In addition to the published
routes, a random RNAV route may be flown under IFR if it is
approved by air traffic control (ATC). RNAV DPs and standard
terminal arrival routes (STARs) are contained in the DP and
STAR booklets.
VOR/DME RNAV approach procedure charts are also
available. Note in the VOR/DME RNAV chart excerpt shown
in Figure 7-24 that the WP identification boxes contain the
following information: WP name, coordinates, frequency,
identifier, radial distance (facility to WP), and reference facility
elevation. The initial approach fix (IAF), final approach fix
(FAF), and missed approach point (MAP) are labeled.
To fly a route or to execute an approach under IFR, the RNAV
equipment installed in the aircraft must be approved for the
appropriate IFR operations.
In vertical navigation (VNAV) mode, vertical guidance is
provided, as well as horizontal guidance in some installations. A
WP is selected at a point where the descent begins, and another
WP is selected where the descent ends. The RNAV equipment
computes the rate of descent relative to the groundspeed; on
some installations, it displays vertical guidance information
on the GS indicator. When using this type of equipment
during an instrument approach, the pilot must keep in mind
that the vertical guidance information provided is not part of
the nonprecision approach. Published nonprecision approach
altitudes must be observed and complied with, unless otherwise
directed by ATC.
7-24
OBS
N
E
S
W
3
33
24
21
15
12
30
6
TO
Waypoint
135°
225°
OBS
N
E
S
W
3
33
24
21
15
12
30
6
TO
33
30
24
2I
I5
I2
6
3
12 NM
OBS
N
E
S
W
3
33
24
21
12
30
6
NAV
GS
View is from the pilots
perspective, and the
movable card is reset
after each turn
Figure 7-25. Aircraft/DME/Waypoint Relationship.
Figure 7-24. VOR/DME RNAV Rwy 25 Approach (Excerpt).
To fly to a WP using RNAV, observe the following procedure
[Figure 7-25]:
1. Select the VOR/DME frequency.
2. Select the RNAV mode.
3. Select the radial of the VOR that passes through the WP
　
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