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 CPU uses tune inputs to tune the frequency synthesizer. The CPU give a signal to the transmitter to send interrogation pulses. The transmit pulses go through a circulator then to the DME antenna.

The transmitter sends a signal to the suppression circuits. During a transmission, the suppression circuit in the DME 1 interrogator sends a suppression pulse to these units:

· DME 2 interrogator

· ATC 1 and 2 transponders (XPNDR)

· TCAS computer.

The suppression pulse stops receiver operation in the other LRUs to prevent damage to internal circuits.

 

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

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:

· The LRU status and control fail LEDs come on red for two seconds.

· The LRU status LED changes to green.

· All LEDs go off for two seconds.

· The applicable LEDs come on to show the LRU status.

The red CONTROL FAIL LED shows when the tuning source is invalid.

The LRU STATUS LED shows interrogator test results. The LRU STATUS LED shows green for a good test or it shows red for a test failure.

 

 

 

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