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 ú ú 08 Page 2 ú Nov 15/90
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A
737-300/400/500MAINTENANCE MANUAL
 (3)  
Test Switches

 (a)  Two switches, three positions, spring-loaded to the center position, are provided to self-test the VHF NAV and DME systems. Activation of a test switch sends a test ground to the appropriate system initiating the self-test. Activation of a VOR, UP/LT, or DN/RT test interrupts the flight management system interlock circuit, which disengages auto tuning of the DME system if auto tuning is in operation.

 (4)  
Frequency Displays

 (a)  The MANUAL frequency display shows the manual frequency selected. When the system is in the AUTO mode, a dayglo fire orange FLAG (bar) is raised over the manual display window, and the frequency selected by the flight management system appears in the AUTO display. With the system in the manual mode, the flag is retracted from the manual display and the automatic display is blanked. The frequency displays and mode annunciator are tested through the airplane master test circuit. When the master test switch is actuated, both displays read 188.88, flashing 2 seconds on and 1 second off (Fig. 4). Both the AUTO and MAN annunciations are illuminated steadily.
 C.  DME system No. 1 is controlled from the captain's control panel and DME system No. 2 from the first officer's control panel.
 3.  Antennas________
 A.  The DME antennas are blade-type units which are connected to the interrogator units by coaxial cable. The No. 1 system antenna is installed on the bottom of the fuselage at station 468. The No. 2 system antenna is installed at station 580.
 B.  The DME antenna provides the means of radiating the RF energy from the DME interrogator's transmitter, and receiving RF reply signals transmitted by the ground station.
 C.  The DME antenna is designed for L-band use. It has a nominal RF impedance of 50 ohms and a maximum VSWR of 2/1 over the frequency range.
 4.  Interrogator Units__________________
 A.  The interrogator units are standard 1/2 ATR short assemblies which mount on the E2-2 shelf in the electrical/electronics equipment center (Fig. 1).
 B.  DME-1 interrogator receives 115v ac, 400 Hz, power thru DME-1 circuit breaker located on the P18 circuit breaker panel. DME-2 interrogator receives its power thru DME-2 circuit breaker located on the P6 circuit breaker panel.
 C.  Interrogator units provide the capability to transmit interrogation pulse pairs to a ground station, receive and process the returned reply pulses, which provide the slant range distance to the DME display and other airplane systems.
 D.  The slant range capability of the interrogator is from 0 to 389.99 nautical miles.
 EFFECTIVITYùùùùùùùùùùùùùùùùùùùùùùùùùùùùùù. AIRPLANES WITHOUT EFIS ú

 34-55-00
 ú ú 04 Page 3 ú Mar 15/88

BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
A 737-300/400/500MAINTENANCE MANUAL
 5.  Indicators
__________
 A.  Dual DME indicators are installed in each RDDMI as shown in Fig. 1. DME No. 1 distance is displayed both above the captain's and F/O's DME-1 indications and DME No. 2 distance is displayed above both DME-2 indications.
 B.  The DME indicator is a magnetic wheel type counter which receives a BCD pulse signal from the related DME system. Logic in the indicator decodes this signal and converts it into a digital display showing miles-to-go to the interrogated ground station.
 C.  Warning flags (shutters) cover the distance display when the system is inoperative.
 6.  Operation
_________
 A.  DME operation consists of an operational description and a functional description. The operational description briefly covers the DME system. The functional description provides in-depth coverage of system components and their interface.
 B.  Operational Description
 (1)  
The DME system automatically measures the distance (in nautical miles) between the airplane and a selected ground station. The DME frequency is selected when the corresponding VHF NAV frequency is selected (Fig. 3). Manually, this is accomplished using the VHF NAV control panel. When AUTO is selected on the control panel, tuning of the VHF NAV and DME systems is accomplished by the flight management computer. NAV must be selected on the NAV - VOR/ILS transfer switch to arm AUTO tuning.
　
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