The aircraft has two distance measuring equipment (DME) systems. The distance measuring equipment (DME) system shows the direct-line distance between the aircraft and a ground station.

The DME system supplies the functions that follow:

• Sets the aircraft position
• Finds the vertical aircraft separation
• Finds the approach to an airport
• Causes the aircraft to stay out of a protected airspace
• Holds the aircraft at a position
• Calculates the ground speed

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The DME transceiver No. 1 is installed in a mounting tray in the LH equipment rack. The DME transceiver No. 2 is installed in a mounting tray in the RH equipment rack.

The DME transceiver has a weight of 5.3 lb (2.3 kg). The DME transceiver has a swivel handle on the front panel to help remove the transceiver. At the rear, the DME transceiver has a connector that connects to the mounting tray receptacle to supply the interface with other aircraft systems.

The DME transceiver is a fully solid-state device. It finds, calculates, and shows the distance between the aircraft and a ground station. The DME transceiver can monitor three ground stations at the same time. The DME system also supplies ground station identification to the audio integrating system through the radio interface unit (RIU).

The DME transceiver operates in the frequency range of 960 to 1215 MHz. There are 252 DME frequency-channels that are related to the VHF navigation frequency-channels. These VHF frequency-channels are in the VHF navigation frequency-range of 108.00 to 117.95 MHz.

The tune increments are 50 KHz and tens of KHz. There are 20 numbered presets (1–20). The letter P can be added to the preset number (optional). With this option, the ranges are108.00 to 117.95 MHz, and 133.00 to 135.95 MHz. The optional DME frequency range is made by the configuration strapping unit (CSU).

The distance data are shown on the pilot and copilot primary flight-displays (PFDs) and the multi-function displays (MFDs). The DME transceiver distance-data are supplied to the PFDs and MFDs through the RIUs and the integrated-avionics processor system (IAPS).

The DME transceiver is tuned with the pilot and copilot MFDs, or with the CDU. Independent DME tune is controlled by the CSU.

The DME transceiver No. 1 is supplied with 28 VDC electrical power from the L MAIN BUS. The DME transceiver No. 2 is supplied with 28 VDC electrical power from the R MAIN BUS.

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The distance measuring equipment (DME) No. 1 transceiver tray is a mounting tray that is installed in the LH equipment rack. The DME No. 2 transceiver tray is installed in the RH equipment rack.

The mounting trays are attached to the equipment racks with four screws. At the rear, the mounting tray has a receptacle that connects with the interface connector of the DME transceiver. Two hold-down clamps on the front of the mounting tray safety the transceiver.

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The DME antennas are installed on the bottom of the fuselage. The DME antenna No. 1 is at FS334.00. The DME antenna No. 2 is at FS419.00. The antenna coaxial cable connects to the front of the DME transceiver.

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DME Operation Modes

The DME system uses a scanning transceiver that can keep full three-channel tracking with identity recognition. DME channel selections are usually entered with the related VOR or ILS frequency. The DME can also tune automatically with the FMS system. The DME has an internal non-volatile memory (NVM) which keeps all the primary data (tuning, valid distance, ground speed, time-to go station, identity, and maintenance data).

The DME starts to operate in a power-on mode. In this mode, the DME does a system initialization test. If the initialization test is satisfactory, the DME starts the normal operation mode.

In the normal mode, if the DME power supply is lost for up to one second, the DME supplies all parameters within 100 milliseconds after a power return-to-service. If the power supply becomes available only after an interruption of one second or more, the DME uses the same frequencies that were used during the tracking just before the power loss to lock on again with the ground stations.

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DME data (distance, identification) shows together with the navigation data that shows on the PFDs and MFDs. The DME functions that can be controlled by the flight crew are:

• The DME identification function
• The DME hold function
• The DME tune function.

The flight crew controls these DME functions, as applicable, with:

• Controls on the audio control panels (ACP)
• MFD/DCP controls and displays
• CDU controls and displays.

ACP Controls

The DME identification tone is heard in the flight crew and observer (when the third audio system option is installed) headsets when the applicable DME 1 or DME 2 audio selection/volume control, on the audio control panel (ACP), is in the pulled-out position.

MFD/DCP Controls and Displays

DME Hold

The DME H pushbutton, on the display control panel (DCP), controls the DME hold (H) function of the DME receiver (when the active navigation (NAV) source is VOR or localizer (LOC)).

The DME H pushbutton, when pushed, locks the current DME frequency (when the NAV frequency is changed, the DME remains locked at the current frequency). When the DME H pushbutton is pushed a second time, it unlocks the current DME frequency (the NAV and the DME frequencies change when the NAV frequency is changed).

When DME H is selected, auto tune is not active. When DME H is deselected, the auto tune function must be reselected.

DME Tune

When the DME hold is on, the DME frequency and an adjacent H show below the NAV radio frequency on the MFD radio menu or the MFD NAV CONTROL menu.

The DME frequency that shows below the DME HOLD display is the frequency of the paired VOR/DME radio at the time the DME H pushbutton on the DCP was pushed.

To tune the DME separately from the VOR, the MENU control, on the DCP, is first used to move the selection box around the DME frequency. Then the TUNE fine/coarse buttons, on the DCP, are turned to set the DME frequency.

CDU Controls and Displays

The control/display functions for the DME show on the CDU pages that follow:

• TUNE page 1/2
• NAV1(2) CONTROL page.

TUNE Page 1/2

The TUN (tune) key on the CDU, when pushed, shows the radio TUNE page 1/2. The radio TUNE pages are used to tune the communication and navigation radios and to control their operation modes. They also give access to CDU pages that give more radio control/display functions.

The TUNE page 1/2 shows DME 1 and DME 2 labels with their related HOLD displays. The HOLD display usually shows in small letters. If the flight crew pushes the DME1 or DME2 line-select key to set DME hold, the related HOLD annunciation will become larger.

The DME can be tuned independently (hold (H) mode) or together with the related VHF navigation receiver. If the VHF navigation receiver is tuned while the related DME is not set to the H mode, the NAV frequency and the DME frequency change at the same time.

The flight crew tunes a NAV 1 or NAV 2 radio with the entry of a frequency, preset number, or station identification (such as CID) on the scratchpad line. Then, the flight crew pushes the NAV1 line-select key or the NAV2 line-select key to move this frequency to a data field.

If the frequency is correct, it shows in the data field, and the radio is immediately tuned again. This is the VIR (VOR/ILS receiver) active frequency that is automatically related to a DME frequency (unless held).

The digit to the right of the NAV legend (1 or 2) identifies the radio that is tuned to that frequency.

The flight crew pushes the NAV1 line-select key or the NAV2 line-select key again to see the related NAV 1 (2) CONTROL page.

NAV 1 (2) CONTROL Page

The NAV1 line-select key, on the TUNE page, when pushed, shows the NAV 1 CONTROL page. The NAV2 line-select key, on the TUNE page, when pushed, shows the NAV 2 CONTROL page. The NAV1 CONTROL page is used as an example.

The NAV1 CONTROL page shows the active frequency for the VHF NAV radio below the NAV 1 label. This active frequency for VIR is automatically related to a DME frequency (unless held). The NAV 1 source annunciation shows that the CDU controls a No. 1 (left side) radio.

The flight crew records a frequency, preset number, or station identification on the scratchpad line. Then, the flight crew pushes the NAV1 line-select key to move this frequency to the NAV 1 data field. If the frequency is correct, it shows in the data field, and the radio immediately tunes again. If the frequency is valid, but has no data, the frequency is shown in yellow (amber).

The DME HOLD line-select key, when pushed, causes the NAV frequency related to the DME channel to show on the display. HOLD shows adjacent to the hold frequency.

The NAV TUNING line-select key is pushed to set the mode in which NAV is tuned. NAV is tuned automatically (AUTO) or manually (MAN). The mode selection shows in large symbols.

When MAN mode is set, the NAV radio is tuned again only when the flight crew makes an entry on the CDU. When AUTO mode is set, the NAV radio automatically tunes again because of commands from the FMS computer. The FMS tunes the NAV radio to en route stations through the flight to keep correct position data.

The TEST line-select key, when pushed, starts or inhibits the self-test of the VHF navigation system.

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The DME system has interfaces with the TCAS, the ATC transponder, the CDUs, the reversion select panel (RSP), and the RIUs. The RIUs send DME data through the IAPS to the audio system and to the PFDs and MFDs. The DME transceivers transmit and receive data on ARINC 429 high-speed (HS) buses as follows:

• The DME transceiver No. 1 transmits ground speed, station identification, and distance data to the pilot RIU, on the L-DME-3 bus
• The DME transceiver No. 1 receives DME frequency control data:
  

  • From the pilot RIU, on the LA-RIU-7 and LB-RIU-7 busses
  • From the pilot control display unit (CDU), on the L-CDU-6 bus

• The DME transceiver No. 2 transmits ground speed, station identification, and distance data to the copilot RIU, on the R-DME-3 bus
• The DME transceiver No. 2 receives DME frequency control data:
  

  • From the copilot RIU, on the RA-RIU-7 and RB-RIU-7 busses
  • From the copilot CDU, on the R-CDU 6

The DME transceiver No. 1 and the DME transceiver No. 2 receive from the reversion select panel (RSP) the discrete data that follows:

• C-SEL ground input that gives the DME frequency control from the CDU
• A-SEL ground input that gives the DME frequency control from the RIU.

When one of the L-band transceivers (DME, Mode S transponders, or TCAS) is in the transmit mode, it sends a suppression discrete to the other transceivers.

The suppression function prevents damage to the receiver sections of the transceivers during high-power emission.

The DME transceivers get power from a 28 VDC electrical source as follows:

• The DME transceiver No. 1 gets 28 VDC power from the L MAIN BUS through circuit breaker CB1-A2, on the left circuit breaker panel
• The DME transceiver No. 2 gets 28 VDC power from the R MAIN BUS through circuit breaker CB2-A2, on the right circuit breaker panel

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Component Location Index
IDENT	DESCRIPTION	LOCATION	IPC REF
A49	DISTANCE MEASURING EQUIPMENT (DME) TRANSCEIVER NO. 1 (LH)	ZONE(S) 221	34-53-01
A50	DISTANCE MEASURING EQUIPMENT (DME) TRANSCEIVER NO. 2 (RH)	ZONE(S) 222	34-53-01
-	DISTANCE MEASURING EQUIPMENT (DME) TRANSCEIVER TRAY NO. 1 (LH)	ZONE(S) 221	34-53-05
-	DISTANCE MEASURING EQUIPMENT (DME) TRANSCEIVER TRAY NO. 2 (RH)	ZONE(S) 222	34-53-05
E19	DISTANCE MEASURING EQUIPMENT (DME) ANTENNA NO. 1	FS334.00, BL0.00 ZONE(S) 141/142	34-53-09
E20	DISTANCE MEASURING EQUIPMENT (DME) ANTENNA NO. 2	FS419.00, BL0.00 ZONE(S) 141/142	34-53-09

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