(3)
The localizer deviation from both VHF NAV receivers is applied to a difference circuit and a sum circuit. The difference circuit compares the two signals and applies the difference to the level detector. The sum of the two signals adjust the threshold level of the localizer level detector. As the sum is decreased, the threshold is lower. When the threshold is exceeded for 2 seconds, the LOC annunciator lights are turned on. The threshold difference is large when approaching the LOC center line and decreased when on the center line (Fig. 2).
(4)
The GS deviation difference monitor is very similar to the LOC difference monitor, except activation requirements. The GS deviation monitor requires that both ILS frequencies be selected and that both GS super flag voltages are present. The threshold difference is large when approaching the GS and decreased when on the center line (Fig. 2).
Radio Altimeter Difference Monitor
(1)
Radio altitude and a 500-foot trip signal from each radio altimeter is provided to the comparator.
(2)
The radio altimeter difference monitor is activated by the approach trigger (GS Engage). The difference monitor turns on the ALT lights if the threshold is exceeded for 2 seconds. The summation of both radio altimeter signal is used to adjust the threshold level. The threshold is increased as altitude increases. A step threshold increase is implemented by the radio altitude trip discrete.
(3)
The radio altimeters are compared provided one of the FCCs is in the GS capture mode. The GS capture signal is also used to lower the threshold for heading, roll attitude and pitch attitude comparison. Refer to the comparator threshold chart (Fig. 2) for the values of the threshold.
Flight Instrument Accessory Unit
(1)
The flight instrument accessory unit monitors GS capture status from the FCCs and relays that status to the comparator unit. An approach trigger is provided from logic in the flight instrument accessory unit. The logic into the accessory unit is GS capture from the AFCS. This is a parallel ground circuit to the AFCS mode annunciator at GS capture. The power to the logic gates is the VOR 1 dc circuit breaker.
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737-300/400/500MAINTENANCE MANUAL
(2) AIRPLANES POST-SB 34-1197; a relay installed in the flight instrument accessory unit isolates the instrument comparator test circuit from other annunciators in the master dim and test circuits. This prevents unwarranted illumination of the instrument comparator annunciator when no significant difference exists between the outputs of the instruments being compared.
H. Self-Test
(1) Testing of the comparator is accomplished by pressing either the test switch on the comparator or on the forward instrument panel (P1). This injects a test signal, which is just above the threshold and enables the monitors. Each annunciator light comes on after the respective time delay.
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INSTRUMENT COMPARATOR SYSTEM - TROUBLE SHOOTING
_______________________________________________
1. General_______
A. These trouble shooting procedures are assembled on the performance of the system operational test and are shown in a tree-type format.
2. Prepare Instrument Comparator and Associated Systems for Trouble Shooting_________________________________________________________________________
A. Do the steps that follow before you trouble shoot the instrument
comparator system.
(1)
Supply electrical power (Ref 24-22-00/201).
(2)
Make sure all the ADI, HSI, and flight instrument switching circuit breakers are closed.
(3)
Make sure all the VOR/ILS systems are on (Ref 34-31-00/501).
(4)
Make sure all the low range radio altimeter systems are on (Ref 34-48-00/501).
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