DISTANCE MEASURING EQUIPMENT - INTRODUCTION

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

· ACP - audio control panel · ADF - automatic direction finder · alt - alternate

DME SYSTEM - GENERAL DESCRIPTION

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…

DME SYSTEM - COMPONENT LOCATION - FLIGHT COMPARTMENT

General

· FMC source select switch · Left inboard and outboard display units · Left and right EFIS control panels

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

· Flight management computer system (FMCS). · Display electronics units 1 and 2 · Flight data acquisition unit (FDAU).

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

· DME · ILS · VOR.

Operation

The transfer switch is a momentary action switch. It transfers the frequency in the standby frequency display window to the active frequency display… The frequency select control is a continuous rotary knob. There is an inner… At power up, the frequency displays show the last frequency entry before power down.

Test

When you push the test switch, the NAV control panel sends a test command out on it’s output bus. If a VOR frequency shows in the active frequency window, the test command goes to the VOR receiver. IF an ILS frequency shows in the active frequency window, the control panel sends a test command to the ILS receivers. If there is a DME frequency that is paired with the VOR or ILS frequency, a test command also goes to the DME interrogators.

When you do a test of the master dim and test system, the NAV control panel shows 188.88. The display shows for two seconds on then one second off until the test is complete.

 

 

DME SYSTEM - OPERATION - CONTROLS

General

The audio control panels (ACP) permit the crew to hear the DME station identification signals. The identification signals are 1350 Hz.

The EFIS control Panel mode selector switch selects the NAV display modes that show DME distance.

Audio Controls

Set these controls on the audio control panel to listen to DME audio:

· Push ON the NAV receiver volume control (NAV 1 for DME 1 and NAV 2 for DME 2)

· Select B (both) or R (range) on the voice range filter switch

· Set the NAV receiver volume control.

The voice/range selector permits you to hear DME audio. You will hear the DME audio when the voice/range selector is in the R or B position.

EFIS Controls

The EFIS control panel mode selector switch must be in the VOR/ILS position to show DME distance on the top right of the ND.

You put the VOR/ADF 1 switch to VOR 1 to show DME 1 in the lower left corner of the ND. DME 2 shows in the lower right corner of the ND when the VOR/ADF 2 switch is put to VOR 2.

 

 

 

DME SYSTEM - DISPLAY

Normal PFD Display

The left PFD shows DME 1 data. The right PFD shows the DME 2 data. The DME distance display shows in white letters and numbers.

Normal RDMI Display

The radio distance magnetic indication (RDMI) on the captain’s primary electronic flight instrument (EFIS) display shows DME distance. The first officer’s RDMI on the secondary EFIS shows the DME distance.

The RDMI show DME 1 distance above the DME1 legend on the left side and DME 2 distance on the right side above the DME2 legend.

The DME distance display shows in white letters and numbers.

Normal ND Display

DME distance shows on the top right corner of these ND displays:

· Expanded and centered VOR

· Expanded and centered ILS.

The DME also shows in the lower left (VOR 1) and the lower right (VOR 2) corner of the ND if you select VOR on the EFIS control panel.

DME NCD or Failed Displays

Amber dashes replace numbers when the DME distance is no computed data (NCD). If the DME has a failure, an amber DME flag replaces the DME distance. The NCD and fail flag show in amber.

DME NCD and Fail Displays

White dashes replace the DME distance when the DME distance data is no computed data (NCD). If the DME has a failure, the DME amber flag replaces the DME distance.

 

DME SYSTEM - FUNCTIONAL DESCRIPTION

Normal Tune Input

The navigation control panel is the normal source for tune inputs. The flight management computer (FMC) sends up to four channels of auto tune signals to the navigation control panel. The navigation control panel adds one manual channel and sends five tune channels to the DME interrogator.

Alternate Tune Input

If the navigation control panel has a failure, the FMC sends auto signals directly to the DME interrogator.

Discrete Inputs

When the navigation control panel has a failure, it grounds a source select discrete to the DME central processing unit (CPU). The CPU changes the input from the navigation control panel to the FMC.

The proximity switch electronics unit (PSEU)supplies an air/ ground discrete to prevent a DME test when the aircraft is in the air. The discrete also supplies flight leg data.

Operation

The transmitter sends a signal to the suppression circuits. During a transmission, the suppression circuit in the DME 1 interrogator sends a… · DME 2 interrogator · ATC 1 and 2 transponders (XPNDR)

Receive

The circulator sends the RF pulse pairs it receives from the antenna to the receiver. The receiver sends the pulse pair to the CPU. The CPU calculates the slant range distance. It uses the time it takes to transmit pulse pairs and get a reply from the ground station. When another L-band system transmits, a suppression pulse stops the receiver operation.

Interrogator Output

After the CPU calculates slant range distance, the CPU sends it to two ARINC 429 transmitters. One ARINC 429 transmitter sends the range data to the DEUs for the flight deck displays and to other systems. A second ARINC 429 transmitter sends range data to the flight control computer. The CPU sends the pulse pairs to the pulse pair decoder. The decoder sends the DME audio to the REU.

 

Bite Module

The built-in test equipment (BITE) in the CPU monitors the circuits in the DME interrogator for faults. The fault memory in the DME keeps the number of faults per flight. Shop personnel can read the fault memory contents.

Test

The CPU does a test of the interrogator when it receives a test command from the navigation control panel. You can also push the test switch on the front panel of the interrogator. The LEDs on the front panel of the interrogator do not show during the navigation control panel test.

 

 

DME SYSTEM - SELF TEST

Push the test switch on the DME interrogator to start a DME selftest. Front Panel Displays The LED status indicators go through this sequence:

DME SYSTEM - TEST DISPLAY

General

· The DME fail condition for two seconds · The DME NCD condition for the next two seconds · The DME normal condition to end the test.