(3) Altitude Indication (Collins 339H-2 HI)
(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.
EFFECTIVITY AZ ALL; IB 406-499
CONFIG 06 Page 12 Aug 25/88
34-33-00
(c) 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
airplane descends to approximately 75 feet lower than the DH
index altitude setting or until the DH index is reset to an
altitude lower than the airplane altitude.
(d) 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. The 200-foot altitude
trip signal will initiate the 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).
(4) 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. (See following Warning, Flag, and Integrity Monitoring
paragraph). 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 test altitude of 100 |10
XA feet on AZ and IB airplanes, 40 |5 feet on LH airplanes.
(5) Warning, Flag, and Integrity Monitor Operation (Fig. 3)
EFFECTIVITY AZ ALL; IB 406-499
CONFIG 02 Page 13 Aug 25/88
34-33-00
(a)
A warning flag will come into view on a height indicator to indicate unsatisfactory system performance. Also for LRRA system 3, 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 circuit.
(b)
The signal presence monitor monitors the return signal strength. When the level decreases below a predetermined level, the circuit provides a warning output to the flag logic gate.
(c)
The VSWR monitor monitors the transmitter output power, receiver gain, and condition of the antennas and antenna cables. If any of the monitored parameters fall out of tolerance, a signal is sent to the flag logic gate.
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