(c)
The DH control knob is mechanically coupled to the DH index and to two contacts of the DH switch. Adjustment of the DH control knob positions the DH index to the selected altitude and positions the contacts of the DH switch. The DH switch contacts are connected to an ON coil and an OFF coil in the DH relay. The opposite sides of both coils are connected to the power supply positive flag supply. When the DH index is set to an altitude greater than the airplane altitude, the power supply negative flag supply is applied through the DH switch to the ON coil closing the relay contacts to turn on the DH light(s). When the DH index is set to an altitude less than the airplane altitude, the DH switch closes in the opposite direction and the power supply negative flag supply is applied to the OFF coil. The DH relay is a latching relay and remains in one position until the opposite coil is energized.
(3)
Altitude Indication (Vertical Scale Height Indicators)
(a)
In vertical scale height indicators, the dc altitude voltage from the receiver-transmitter and a dc feedback voltage from the wiper of the feedback potentiometer are applied to the differential chopper. The dc feedback voltage represents the position of the altitude tape, and the dc altitude voltage represents airplane altitude.
(b)
When the two voltages applied to the differential chopper are not equal in value, a difference voltage is produced and applied to the servo-amplifier. The servo-amplifier drives the motor which repositions the feedback potentiometer (and altitude tape) until the voltages are equal. When the two voltages are equal, the altitude tape indicates the correct airplane altitude.
(4)
Decision Height Set/Operation
(a) The decision height is set by rotating the SET/TEST knob until the desired DH is displayed by the DH counter. The wiper of the DH potentiometer is also adjusted by the SET/TEST knob. When the altitude signal and the voltage from the DH wiper are equal, the output of the DH Light driver turns on the DH light. The DH light remains illuminated until extinguished when altitude of the airplane is approximately 75 feet lower than the DH index altitude setting, the DH index is reset to an altitude lower than the airplane altitude, or the airplane ascends to an altitude higher than the DH setting.
(5)
ADI Rising Runway
EFFECTIVITY
LH ALL; IB 405 CONFIG 4 01 Page 14 Apr 25/8834-33-00
(a) A dc altitude signal from receiver-transmitter 1 and 2 is applied to the captain's and first officer's ADI, respectively. The altitude signal controls the vertical movement of the rising runway symbol. When the airplane descends to 200 feet absolute altitude, the 200-foot altitude trip signal allows vertical movement of the symbol provided the required valid signals are supplied to the ADI logic circuits to retract the runway flag from view. The LRRA system flag alarm signal is provided as one of the required valid signals. Additional signals are required from the VOR/ILS system (Ref 34-31-00).
(6)
Self-Test Operation (Fig. 3)
(a)
Self-test of LRRA system 1 and 2 is initiated at the captain's and first officer's height indicator respectively. There are no provisions in the control cabin for self-testing LRRA system 3 when a third LRRA system is provided. When the test button is depressed, a ground signal is applied through a relay in the AFC accessory box to the test circuits in the receiver-transmitter. The relays, one for LRRA system 1 and one for LRRA system 2, inhibit the self-test command after glide slope capture by the autopilot system.
(b)
Actuating the test switch causes a bias signal to be applied to the circulators in the receiver-transmitter. The resultant field set up in the circulators switches the circulators from the antenna system to the delay line. The delayed signal is processed through the receiver circuits in the same manner as a normal altitude signal. During self-test, the indicator should display the test altitude of 100 | 4 feet.
(7)
Warning, Flag, and Integrity Monitor Operation (Fig. 3)
(a)
For LRRA systems 1 and 2, a warning flag will come into view on the associated height indicator to indicate unsatisfactory system performance. For LRRA system 3 (if installed), a failure annunciator on the pilots' center instrument panel P2 will announce unsatisfactory system performance by changing from a black to a red display. The height indicator flag control circuit functions as a monitor of the servo loop, the ac voltage for the power supply, the flag alarm voltage, and the +30-volt dc reference voltage. When the signals applied to the null detector differ by more than the predetermined value, or if any of the other inputs to the flag control circuit are missing, the flag solenoid is de-energized and the warning flag comes into view. The maintenance monitor circuit provides a flag monitor voltage to the receiver-transmitter indicator fault circuits.
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本文链接地址：747飞机维护手册AMM CHAPTER 34 - NAVIGATION 第34章导航2(71)
