34-29-00
ú ú 06 Page 7 ú Jul 12/00
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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737-300/400/500MAINTENANCE MANUAL
(2) The comparator monitor unit interfaces with both pilot's instruments. The comparator provides indication to each pilot if the basic No. 1 and No. 2 flight instruments disagree by a preset value. The comparator monitors instrument indication of the following: heading, altitude, roll attitude, pitch attitude, LOC deviation and GS deviation. Figure 2 shows the threshold values where the annunciator lights are turned on in the comparator unit. The comparator also monitors its power supply. Significant differences are displayed as lights on each pilot's panel and on the comparator. If a light is illuminated, it may be dimmed by pressing the light case. This triggers a dimming circuit in the respective output section. If another light comes on, it will be bright. Pressing the light case again dims this light. Testing of the comparator is accomplished by a test switch on the comparator or on the forward instrument panel.
B. Power Supply Monitor
(1) The system receives power when the two COMPARATOR circuit breakers at the circuit breaker panel P6 are closed (Fig. 4). One supplies the primary power and the other the power monitor. The power monitor turns on the PWR-MON lights if the primary power supply fails.
C. Heading Difference Monitor
(1)
The comparator unit supplies 26 volt ac excitation to differential resolvers in the captain's HSI. The captain's HSI sends heading to the F/O HSI. If the compass card's positions do not agree, the F/O's HSI develops an error signal and through the comparator, turns both HDG lights on.
(2)
The HDG light illuminates if a significant difference in the compass cards of the HSIs is present. This is accomplished by connecting the resolver in both HSI so as to provide two output signals. These are the sine and cosine. Normally the sine is null and the cos high. The sine increases as the difference increases. The cos is used to detect an excitation loss. The sine is applied to a level detector. The level detector output is high unless the threshold is exceeded. The level detector output and the cos signal are applied to gate 1 (Fig. 4). If either goes low for 4 seconds, the amber annunciator light is turned on. When the airplane is level, the heading difference threshold is set at 6° and decreased to 4° at GS capture. The threshold is increased as the airplane is banked (Fig. 2).
D. Pitch and Roll Difference Monitor
(1) The pitch and roll difference monitor is similar to the heading difference (Fig. 4) monitor except the pitch and roll resolvers in the ADI are compared and the time delay is reduced to 2 seconds. The pitch and roll threshold is set at 4° difference prior to GS capture and 3° after GS capture (Fig. 2).
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34-29-00
ú ú 03 Page 8 ú Nov 15/93
BOEING PROPRIETARY - Copyright (C) - Unpublished Work - See title page for details.
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F.
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737-300/400/500MAINTENANCE MANUAL
LOC and GS Deviation Difference Monitor
(1)
The VHF NAV receivers supply LOC and GS deviation for comparison. The 28 volt dc ILS signal is required from both receivers to enable the LOC deviation comparison. The GS super flag and 28 volt dc ILS signals are required from both receivers to enable the GS deviation comparison. If a VHF NAV system is transferred, LOC and GS comparisons are not made.
(2)
The LOC deviation difference monitor is activated by both ILS dc signals. Both VHF NAV systems must be turned to an ILS frequency and neither switched from normal. A ten-second timer prevents nuisance tuning.
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