(2)
When the DME is in an operating mode, a pulsed signal in the 1025-1150 MHz band is transmitted. The DME ground station receives the interrogation and transmits a reply to the interrogator. The reply frequency is either 63 MHz above or below the interrogation frequency. The interrogator receives the reply, measures the time delay between the interrogation and reply pulses and converts the delay time to the equivalent DME distance.
(3)
DME distance is provided to both RDDMIs and a DAA (D/A Adapter) for use by the FMC. Each RDDMI displays the distances measured by both DME systems. The DAA converts the 6-wire ARINC 568 DME signals to a 2-wire ARINC 429 signals and distributes these digital signals to the FMC and FCC-A. DME audio is provided to the audio integrating system where it is mixed with VHF Nav audio to provide identification of the ground station.
(4)
The DME interrogators and ATC transponders are interconnected by a suppression cable. When one system is transmitting, a suppression pulse is provided to the other three systems inhibiting their operation to prevent mutual interference.
C. Functional Description
(1) VHF NAV/DME Control Panel
(a) The control panel provides control of the VHF NAV and DME systems (Fig. 3). Switches are provided to select automatic or manual frequency control, select the frequency during manual operation, and initiate self-test of the VHF NAV and DME systems. The frequency range is 108.00 to 117.95 MHz.
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A 737-300/400/500MAINTENANCE MANUAL
ELEXDME 2
PWR 2 DME 2
DME System Block DiagramFigure 2
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A
737-300/400/500MAINTENANCE MANUAL
(b)
Manual Tuning
1) Manual tuning is selected by pressing the AUTO/ANUAL switch-indicator. Relay K1 is de-energized and tuning is performed manually. MAN is annuciated on the AUTO/MANUAL selector switch. The annunciator is illuminated by 28v dc from the appropriate VOR/LOC DC circuit breaker, the ground is provided from the Master Dim circuit through K1. K1 applies the frequency select common to the 2-out-of-5 encoder, enabling the switched output. The manual lines are connected to the VHF NAV, and to the DME. The manual frequency display is driven from the unswitched output of the encoder, and AUTO is blanked.
(c)
AUTO Tuning 1) The auto function is enabled by placing the appropriate NAV
-VOR/ILS Select Switch (P1 or P3 panels) to NAV, this provides 28v dc from the VOR/LOC circuit breaker which is applied to K1 relay control and K1 is energized. With K1 energized the following actions will occur: a) AUTO will annunciate. b) The frequency select common will be removed from the
encoder and be inputed to the auto frequency display.
c) The 28v dc display power from the VOR/LOC circuit breaker will also be applied to the AUTO frequency display and manual bar solenoid, thus enabling the auto frequency display and placing the bar over the MAN display.
d) The AUTO/MAN switch is enabled which allows the pilot to switch back and forth between manual and auto modes.
2) The Flight Management System provides tuning signals to the DME, the VHF NAV, and to the control panel AUTO frequency display through the DAA.
3) The control panel can be returned to the MAN mode by either pressing the AUTO/MAN switch/indicator or by placing the NAV-VOR/ILS switch to VOR/ILS, this will remove the input to K1 relay control.
(d) Test Functions 1) When the airplane master test switch (P2 Panel) is actuated, test relay K3 is energized. The AUTO and MANUAL annunciators are illuminated and a test discrete is applied to both displays. They indicate 188.88, alternating 2 seconds on and 1 second off.
2) VHF NAV and DME tests are enabled by grounds from the test switches. When a VOR, UP/LT, or DN/RT test is initiated, the flight management system interlock is interrupted, which will disengage auto tuning of the DME system if auto tuning is in operation.
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