Should an error in excess of ±4° be indicated through use of
a ground check, or ±6° using the airborne check, IFR flight
shall not be attempted without first correcting the source of
the error. No correction other than the correction card figures
supplied by the manufacturer should be applied in making
these VOR receiver checks.
If a dual system VOR (units independent of each other except
for the antenna) is installed in the aircraft, one system may
be checked against the other. Turn both systems to the same
VOR ground facility and note the indicated bearing to that
station. The maximum permissible variation between the two
indicated bearings is 4°.
Distance Measuring Equipment (DME)
When used in conjunction with the VOR system, DME makes
it possible for pilots to determine an accurate geographic
position of the aircraft, including the bearing and distance TO
or FROM the station. The aircraft DME transmits interrogating
radio frequency (RF) pulses, which are received by the DME
antenna at the ground facility. The signal triggers ground
receiver equipment to respond to the interrogating aircraft. The
airborne DME equipment measures the elapsed time between
the interrogation signal sent by the aircraft and reception of the
reply pulses from the ground station. This time measurement is
converted into distance in nautical miles (NM) from the station.
Some DME receivers provide a groundspeed in knots by
monitoring the rate of change of the aircraft’s position relative
to the ground station. Groundspeed values are accurate only
when tracking directly to or from the station.
DME Components
VOR/DME, VORTAC, ILS/DME, and LOC/DME
navigation facilities established by the FAA provide course
and distance information from collocated components under
a frequency pairing plan. DME operates on frequencies
in the UHF spectrum between 962 MHz and 1213 MHz.
Aircraft receiving equipment that provides for automatic
DME selection assures reception of azimuth and distance
information from a common source when designated VOR/
DME, VORTAC, ILS/DME, and LOC/DME are selected.
Some aircraft have separate VOR and DME receivers, each
of which must be tuned to the appropriate navigation facility.
The airborne equipment includes an antenna and a receiver.
The pilot-controllable features of the DME receiver include:
Channel (Frequency) Selector
Many DMEs are channeled by an associated VHF radio, or
there may be a selector switch so a pilot can select which
VHF radio is channeling the DME. For a DME with its own
frequency selector, use the frequency of the associated VOR/
DME or VORTAC station.
ON/OFF/Volume Switch
The DME identifier is heard as a Morse code identifier with
a tone somewhat higher than that of the associated VOR or
LOC. It is heard once for every three or four times the VOR
or LOC identifier is heard. If only one identifier is heard about
every 30 seconds, the DME is functional, but the associated
VOR or LOC is not.
Mode Switch
The mode switch selects between distance (DIST) or distance
in NMs, groundspeed, and time to station. There may also be
one or more HOLD functions that permit the DME to stay
channeled to the station that was selected before the switch
was placed in the hold position. This is useful when you make
an ILS approach at a facility that has no collocated DME,
but there is a VOR/DME nearby.
Altitude
Some DMEs correct for slant-range error.
Function of DME
A DME is used for determining the distance from a ground
DME transmitter. Compared to other VHF/UHF NAVAIDs,
a DME is very accurate. The distance information can be
used to determine the aircraft position or flying a track that
is a constant distance from the station. This is referred to as
a DME arc.
DME Arc
There are many instrument approach procedures (IAPs) that
incorporate DME arcs. The procedures and techniques given
here for intercepting and maintaining such arcs are applicable
to any facility that provides DME information. Such a facility
may or may not be collocated with the facility that provides
final approach guidance.
As an example of flying a DME arc, refer to Figure 9-17 and
follow these steps:
1. Track inbound on the OKT 325° radial, frequently
checking the DME mileage readout.
2. A 0.5 NM lead is satisfactory for groundspeeds of
150 knots or less; start the turn to the arc at 10.5
miles. At higher groundspeeds, use a proportionately
greater lead.
3. Continue the turn for approximately 90°. The roll-out
heading is 055° in a no wind condition.
4. During the last part of the intercepting turn, monitor
the DME closely. If the arc is being overshot (more
than 1.0 NM), continue through the originally planned
roll-out heading. If the arc is being undershot, roll-out
of the turn early.
055°
OKT VORTAC
°
°
10 DME arc
Figure 7-16. DME arc interception.
Lead points 10.5 NM
Lead points 9.5 NM
Figure 9-17. DME arc interception.
The procedure for intercepting the 10 DME when outbound
is basically the same, the lead point being 10 NM minus 0.5
NM or 9.5 NM.
When flying a DME arc with wind, it is important to keep a
continuous mental picture of the aircraft’s position relative to
the facility. Since the wind-drift correction angle is constantly
changing throughout the arc, wind orientation is important.
In some cases, wind can be used in returning to the desired
track. High airspeeds require more pilot attention because of
the higher rate of deviation and correction.
Maintaining the arc is simplified by keeping slightly inside
the curve; thus, the arc is turning toward the aircraft and
interception may be accomplished by holding a straight
course. When outside the curve, the arc is “turning away”
and a greater correction is required.
To fly the arc using the VOR CDI, center the CDI needle upon
completion of the 90° turn to intercept the arc. The aircraft’s
heading is found very near the left or right side (270° or 90°
reference points) of the instrument. The readings at that side
location on the instrument give primary heading information
while on the arc. Adjust the aircraft heading to compensate
for wind and to correct for distance to maintain the correct
arc distance. Recenter the CDI and note the new primary
heading indicated whenever the CDI gets 2°– 4° from center.
With an RMI, in a no wind condition, pilots should
theoretically be able to fly an exact circle around the facility
by maintaining an RB of 90° or 270°. In actual practice,
a series of short legs are flown. To maintain the arc in
Figure 9-18, proceed as follows:
1. With the RMI bearing pointer on the wingtip reference
(90° or 270° position) and the aircraft at the desired
DME range, maintain a constant heading and allow the
bearing pointer to move 5°– 10° behind the wingtip.
This causes the range to increase slightly.
2. Turn toward the facility to place the bearing pointer
5°– 10° ahead of the wingtip reference, and then
maintain heading until the bearing pointer is again
behind the wingtip. Continue this procedure to
maintain the approximate arc.
3. If a crosswind causes the aircraft to drift away from
the facility, turn the aircraft until the bearing pointer is
ahead of the wingtip reference. If a crosswind causes
the aircraft to drift toward the facility, turn until the
bearing is behind the wingtip.
4. As a guide in making range corrections, change the
RB 10°– 20° for each half-mile deviation from the
desired arc. For example, in no-wind conditions, if the
aircraft is ½ to 1 mile outside the arc and the bearing
pointer is on the wingtip reference, turn the aircraft
20° toward the facility to return to the arc.
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 varies with arc radius and groundspeed.
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 is 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.
° Radial
10°
10°
20°
20°
11 DME arc
9 DME arc
Desired arc 10 DME
OBS
21 15
NAV
GS
30 24
12 6
HDG
24 21
6 3
S W
HDG
HDG
Figure 7-17. Using DME and RMI to maintain arc.
Instrument view is from
the pilot’s perspective,
and the movable card is
reset after each turn
Figure 9-18. Using DME and RMI to maintain an arc.
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 9-19, 9-20, and 9-21 ), 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 displays 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.
INSET MAP DCLTR
NAV1 108.00 113.00
NAV2 108.00 110.60
134.000 118.000 COM1
123.800 118.000 COM2
GS 120KT XTK 8.15NM ETE 08:28 ESA 3100FT
MAP - NAVIGATION MAP
S E - 3 , 3 1 A U G 2 0 0 6 t o 2 8 S E P 2 0 0 6
IAP
Figure 7-18a. Localizer interception from DME arc. Figure 9-19. An aircraft is displayed heading southwest to intercept the localizer approach, using the 16 NM DME arc off of ORM.
INSET PFD CDI DCLTR
NAV1 108.00 113.00
NAV2 108.00 110.60
134.000 118.000 COM1
123.800 118.000 COM2
GS 120KT XTK 8.15NM ETE 08:28 ESA 3100FT
MAP - NAVIGATION MAP
Figure 7-18b. Localizer interception from DME arc.
Figure 9-20. The same aircraft illustrated in Figure 9-19 shown on the ORM radial near TIGAE intersection turning inbound for the
localizer.
ENGINE MAP DCLTR
NAV1 108.00 113.00
NAV2 108.00 110.60
134.000 118.000 COM1
123.800 118.000 COM2
GS 120KT XTK 8.15NM ETE 08:28 ESA 3100FT
MAP - NAVIGATION MAP
Figure 7-18c. Localizer interception from DME arc.
Figure 9-21. Aircraft is illustrated inbound on the localizer course.
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.
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 9-22, the value of
side A is the measured DME distance to the VOR/DME. Side
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
°
°
0 10 20 30 40 50 60 70
ANGLE 3
ANGLE 1
ANGLE 2
VOR/DME
Figure 7-19. RNAV computation.
Waypoint
Figure 9-22. RNAV computation.
Figure 9-23. 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.
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 9-23] 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 9-24 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
OBS
N E
S W
24 21
TO
Waypoint
°
°
OBS
E S
15 12
TO
33 30
2I
I5 I2
12 NM
OBS
21 15
NAV
GS
Instrument view is
from the pilot’s
perspective, and
the movable card is
reset after each turn
Figure 9-25. Aircraft/DME/waypoint relationship.
E C - 1 ,
E C - 1 ,
Figure 7-21. VOR/DME RNAV Rwy 25 approach (excerpt).
16 DEC 2010 to 13 JAN 2011
16 DEC 2010 to 13 JAN 2011
Figure 9-24. VOR/DME RNAV RWY 25 approach (excerpt).
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.
To fly to a WP using RNAV, observe the following procedure
[Figure 9-25]:
1. Select the VOR/DME frequency.
2. Select the RNAV mode.
3. Select the radial of the VOR that passes through the
WP (225°).
4. Select the distance from the DME to the WP (12 NM).
5. Check and confirm all inputs, and center the CDI needle
with the TO indicator showing.
6. Maneuver the aircraft to fly the indicated heading plus or
minus wind correction to keep the CDI needle centered.
7. The CDI needle indicates distance off course of 1 NM
per dot; the DME readout indicates distance in NM
from the WP; the groundspeed reads closing speed
(knots) to the WP; and the time to station (TTS) reads
time to the WP.
VOR/DME RNAV Errors
The limitation of this system is the reception volume.
Published approaches have been tested to ensure this is not
a problem. Descents/approaches to airports distant from the
VOR/DME facility may not be possible because, during
the approach, the aircraft may descend below the reception
altitude of the facility at that distance.
Advanced Technologies
Global Navigation Satellite System (GNSS)
The Global Navigation Satellite System (GNSS) is a
constellation of satellites providing a high-frequency signal
that contains time and distance that is picked up by a receiver