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747-400 MAINTENANCE MANUAL
DME SYSTEM - DESCRIPTION AND OPERATION
______________________________________
1. General (Fig 1)_______
A. The distance measuring equipment (DME) system measures the airplane-to-station slant-range (line-of-sight) distance. This distance is sent to the EFIS/EICAS interface units (EIUs) where it is made available to be displayed as nautical miles to go on flight deck navigation displays (NDs) and primary flight displays (PFDs).
B. Two DME systems are installed in the airplane and each contains an interrogator and an antenna. The DME system is controlled by the FMC when placed in the autotune mode. When both FMCs are disabled, the systems are placed in the manual tune mode and the left and right DME systems are controlled by the left and right control display unit (CDUs) respectively. In both modes, controlling information is sent to the interrogator in an encoded a 32-bit control word via an ARINC 429 data bus.
C. The DME interrogator tunes to a particular VOR/ILS/MLS -DME paired ground station frequency and transmits pulsed rf signals and receives replies from that ground station. Distance is computed by the DME interrogator by determining the time it takes to receive a reply from the station.
D. Maximum range of the DME interrogator is 300 nautical miles with velocity of 0 to 1000 knot.
E. The interrogator also receives from the ground station an audio signal. This signal is decoded and sent to the audio management unit for audio presentation in the flight deck. The audio signal is also decoded and encoded into two digital words which make up the four characters which represent the ground station identification (ident). These characters are sent to the EIUs for presentation on the flight deck NDs. The DME station ident is displayed only when a VOR station idents are not available.
F. The DME system operates in the L-band frequency range. It has a transmit frequency of 1025 to 1150 MHz and a receive frequency of 962 to 1213 MHz. The DME and ATC systems operate in the same L-band frequency range. To prevent interference between systems only one system is permitted to transmit at any one time.
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747-400
MAINTENANCE MANUAL
LIGHSHIELD PILOTS' SEE E PILOTS' CENTER INSTRUMENT PANEL SEE D LIGHSHIELD PILOTS' RIGHT DME (STA 690) ANTENNA LEFT DME
SEE D ANTENNA
(STA 830)
CAPTAIN'S
INSTRUMENT FIRST OFFICER'S
PANEL INSTRUMENT PANEL
SEE B C SEE B C
AFT ELECTRONIC
FORWARD ELECTRONIC
CONTROL MODULE
CONTROL MODULE
SEE F
SEE F
FLIGHT COMPARTMENT
RIGHT DME
INTERROGATOR,
B82 (E2-2)
SEE A
LEFT DME
INTERROGATOR,
B81 (E2-3)
SEE A
FWD
117AL
MAIN EQUIPMENT CENTER
DME System - Component Location Figure 1 (Sheet 1)
F55791
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BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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747-400
MAINTENANCE MANUAL
TEST PUSHBUTTONS
TEST
DME System - Component Location
Figure 1 (Sheet 2)
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L33614
A
747-400
MAINTENANCE MANUAL
135 SPD LOC LNAV G/S VNAV 00012 TASGS 338 351 336`/15 140 GRH 0385.4 Z
80 157`/10.3` 32.5 NM
13.5DME 700
10 400
100 OM 6
2 T/O
120 10 10 10 200 1
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