Distance measuring equipment - introduction Purpose
The distance measuring equipment (DME) system supplies slant range (line of sight) distance measurement between the airplane and the ground station.
Abbreviations and Acronyms
AC - alternating current
ACP - audio control panel
ADF - automatic direction finder
alt - alternate
altn - alternate
app - approach
ARINC - Aeronautical Radio, Inc.
ATC - air traffic control
ATE - automatic test equipment
auto - automatic
B - both
BITE - built-in test equipment
BL - buttock line
CAPT - captain
CDU - control display unit
circ - circulator
CPU - central (control) processing unit
DC - direct current
DEU - display electronics unit
DME - distance measurement equipment
EFIS - electronic flight instrument system
F/O - first officer
FCC - flight control computer
FDAU - flight data acquisition unit
FMC - flight management computer
FMCS - flight management computer system
freq - frequency
fwd - forward
grd - ground
I/C - intercom
ILS - instrument landing system
LCD - liquid crystal display
LED - light emitting diode
LRU - line replacable unit
maint - maintenance
MHz - megahertz
MKR - marker beacon
nav - navigation
NCD - no computed data
ND - navigation display
NORM - normal
PFD - primary flight display
R - range
rcvr - receiver
REU - remote electronics unit
RF - radio frequency
rly - relay
R/T - receive/transmit
spkr - speaker
sta - station
TCAS - traffic alert and collision avoidance system
TFR - transfer
TX - transmitter
V - voice
V - volts
VHF - very high frequency
xfr - transfer
xmtr - transmitter
xpndr - transponder
Dme system - general description
General
The DME system has two DME interrogators and two antennas.
Description
The interrogators get manual tune inputs and flight management computer system (FMCS) autotune inputs from the navigation control panel. If the NAV control panel tune inputs fail, the interrogators get autotune inputs directly from the FMC.
The DME system sends data to the display electronics units to show on the primary flight displays (PFDs) and navigation displays (NDs).
The DME system sends data to these units:
Flight control computers (FCC)
Flight management computer System (FMCS)
Flight data acquisition unit (FDAU)
Remote Electronics Unit (REU).
The FCCs use DME data as an input to calculate the VOR capture point in the autopilot VOR mode. The DME data is also used in the VOR mode to find when over station is sensed (OSS) for the VOR ground station.
The FMCS uses DME data to calculate FMC position updates. The flight data acquisition unit receives DME data, formats it, and sends it to the flight data recorder.
The REU receives audio from the DME station and sends it to the flight deck headsets and speakers.
The DME system sends and receives a suppression pulse between these units:
DME
ATCs
TCAS.
The DME interrogator receives the station audio identifiers and sends them to the remote electronics unit (REU).
Dme system - component location - flight compartment General
These are the components in the flight compartment that have an interface with the DME system:
FMC source select switch
Left inboard and outboard display units
Left and right EFIS control panels
Right inboard and outboard display units
Captain’s and first officer’s NAV control panel
Captain’s and first officer’s audio control panel.
DME SYSTEM - COMPONENT LOCATION - ELECTRONIC EQUIPMENT COMPARTMENT AND ANTENNAS
DME Component Locations
These components are in the electronics equipment
compartment:
DME 1 interrogator
DME 2 interrogator.
These components are on the exterior of the airplane:
DME 1 antenna
DME 2 antenna.
DME SYSTEM - ARINC 429 INTERFACES
Navigation Control Panel
The navigation control panels supply a manual tune frequency input to the DME interrogator. They also send four auto tune frequency inputs from the Flight Management Computer (FMC). The control panels send tune and test data on two output data buses. One output data bus goes to the MMR receiver and one output data bus goes to both the DME interrogator and VOR receiver.
DME Outputs
Each DME interrogator has two output buses. One output bus supplies data to the on-side flight control computer (FCC). DME 1 sends data to FCC A and DME 2 sends data to FCC B. the other Output bus supplies data to these units:
Flight management computer system (FMCS).
Display electronics units 1 and 2
Flight data acquisition unit (FDAU).
The FMCS uses DME distance to calculate position updates.
The DEUs use DME distance for displays.
The FDAU receives DME distance and formats it for the flight data recorder.
Flight Management Computer
The FMC supplies auto tune commands through the navigation control panel to the DME interrogator. If the navigation control panel has a failure, the DME gets auto tune signals directly from the FMC.
DME SYSTEM - POWER, ANTENNA, ANALOG, AND DISCRETE INTERFACE
Power Interface
Each DME interrogator receives 115v ac from the DME circuit breakers.
DME Antenna Interface
The DME antennas transmit and receive DME signals. The antennas transmit signals to the ground stations. They then receive the reply signals from the DME ground station and send them to the interrogator.
Source Select Discrete
The source select discrete controls the ARINC 429 receive ports in the DME interrogator. With the source select open, the DME interrogator gets manual and auto tune commands from the navigation control panel. If the navigation control panel has a failure, the source select discrete goes to ground. The DME then gets auto tune signals directly from the FMC.
PSEU
The proximity switch electronics unit (PSEU), supplies an air/ ground discrete inputs to the DME interrogators to set flight leg count. The air/ground discrete also goes to the navigation control panels to inhibit test commands in the air.
Default Scan Inhibit Discrete
The default scan inhibit discrete locks the DME interrogator so it can not make a scan of all the DME frequencies. It tunes to five channels controlled by the navigation control panel and the flight management computer.
DME/ATC/TCAS Suppression
These systems operate in the same frequency band:
DME
Air traffic control (ATC)
Traffic alert and collision avoidance system (TCAS).
When a DME interrogator, an ATC transponder, or the TCAS computer transmits, it sends a suppression pulse through the suppression lines. This pulse stops the reception of the other four units. This prevents damage to the receiver circuits of the other LRUs.
DME Audio
The DME interrogator sends the DME station audio identifier to the remote electronics unit (REU). The audio goes to the headsets and the flight interphone speakers.
DME SYSTEM - INTERROGATOR
General
The DME interrogator tunes to 252 channels and calculates distance information for all the channels in the DME range. There are 200 DME channels for the VHF NAV frequencies. The other 52 channels are for military TACAN functions. The DME receive frequency is 63 MHz above or below the transmit frequency.
Purpose
These are the purposes of the DME interrogator:
Interrogate DME stations
Receive the station replies
Receive audio identifiers
Calculate slant range distances.
Frequencies
These DME tuning frequencies are in the VHF band:
108.00 to 117.95 MHz for DME stations with VOR or ILS
133.30 to 135.95 MHz for DME only stations.
Front Panel
The front panel of the DME interrogator has these features:
LED status indicators
A self test switch.
DME SYSTEM – ANTENNA
General
The L-band DME antenna transmits the interrogator output signal and receives the ground station reply and identification signals.
Physical Description
The antenna has an o-ring moisture seal and attaches to the airplane with four screws. The DME and the ATC antennas are the same and are interchangeable.
Training Information Point
Extra force on the antenna may be necessary to break the weatherproofing seal. To prevent damage to aircraft skin or electrical cable at the antenna base, carefully pry around the antenna with a sealant removal tool.
DME SYSTEM - NAVIGATION CONTROL PANEL
General
The navigation (NAV) control panels supply frequency inputs and test commands to these navigation radios:
DME
ILS
VOR.
When you put a paired VOR or ILS frequency into the navigation control panel, it also goes to the DME interrogator. The DME tunes a DME frequency in the range of 108MHz to 117.95MHZ. Set the frequency on the navigation control panel for these audio outputs:
VOR and DME station identification
ILS and DME station identification.