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 1.  General
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 A.  The low range radio altimeter (LRRA) system supplies vertical position information for use by the pilots during approach and landing phases of flight operation. The LRRA system provides an accurate measurement of absolute altitude (height above terrain) from 2500 feet to touchdown. Flag alarm, dc altitude, and vertical terrain clearance (altitude trip) outputs are available for use by other systems.
 B.  Two complete systems are installed in the airplane. Each system consists of a receiver-transmitter, height indicator, decision height (DH) light and rising runway display on an attitude director indicator (ADI), transmit antenna, and receive antenna. Location of components is shown on Fig. 1.
 C.  Power distribution to the LRRA equipment and the interface between the LRRA systems and other airplane systems is shown on Fig. 2.
 2.  Receiver - Transmitters (Bendix ALA/51A)
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 A.  The receiver-transmitters are located side by side in center equipment center E9. A test connector with protective cover, six monitor lamps, and a PRESS TO MONITOR switch are located on the front panel. The monitor lamps identify a failure in the receiver-transmitter, height indicator, or antenna and coaxial cables. Two locking lugs secure the unit to its mount. A handle for carrying the unit is provided on the front panel.
 3.  Antennas
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 A.  The antennas are flush-mounted, probe-fed conical horns. Two identical antennas are used in each system, one for transmitting and one for receiving. The No. 1 and 2 system transmitting antennas are located to the left and right, respectively, of the lower fuselage centerline at body station 913. The No. 1 and 2 system receiving antennas are located to the left and right, respectively, of the lower fuselage centerline at body station 933. Coaxial cables connect the antennas to the receiver-transmitters. Each systems transmit and receive antennas are interconnected by a coaxial cable which provides a controlled leakage input signal to the receiver-transmitter integrity monitor circuits.
 4.  Height Indicators (Bendix INA-51A)
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 A.  A height indicator is located on each pilot's flight instrument panel. The height indicator provides a visual display of airplane terrain clearance. The operating controls and indicators consist of a DH control knob, DH index, test pushbutton switch, dial, altitude pointer, pointer mask, and warning flag. The circular dial is graduated from -20 to 2500 feet. The graduation is linear from -20 to 500 feet and logarithmic from 500 to 2500 feet.
 B.  The warning flag appears and covers the dial in the vicinity of the 100-foot indication when a malfunction occurs in the system. The altitude pointer is driven underneath the pointer mask at altitudes above 2500 feet and when a malfunction occurs in the system. The DH control knob adjusts the position of the DH index on the altitude scale thereby selecting the altitude at which the DH light(s) operates.
 EFFECTIVITY

 SN 301-350  CONFIG 3  03 Page 1  Apr 25/8834-33-00

DH (MDA) INDEX
DH INDEX
CONTROL ATTITUDE DIRECTOR INDICATOR HEIGHT INDICATOR
B
Low Range Radio Altimeter System Component Location Figure 1
EFFECTIVITYSN 301-350

CONFIG 3799 Page 2
34-33-00
Apr 25/88

LRRA System Block Diagram
Figure 2

EFFECTIVITYCONFIG 3 SN 301-350

799 Page 3
34-33-00
Apr 25/88
 5.  ADI Displays
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 A.  A DH light is located on each pilot's ADI. The operation of the DH light is controlled by the height indicator located on the same instrument panel. The DH light has a press-to-test for light burnout feature.
 B.  The rising runway symbol displays both radio altitude and localizer deviation. Radio altitude is presented by the distance between the top edge of the runway symbol and the base of the fixed airplane symbol. Localizer deviation is presented by the runway symbol moving left or right across a horizontal scale (Ref 34-31-00).
 6.  Operation
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 A.  Functional Description (Bendix LRRA System)
 (1)  Altitude measurement operation (Fig. 3)
 (a)  
The LRRA system provides altitude data when 115-volt ac power is supplied to the receiver-transmitter. Low voltage power is then supplied to all circuits in the receiver-transmitter and to the height indicator power supply. The system is turned on by closing its circuit breaker(s).

 (b)  
The modulator generates a 155-Hz timing signal which is wobbulated by a signal from the wobbulation generator and then applied to a triangular wave generator. The triangular wave frequency modulates an oscillator signal in the transmitter. The resultant signal is amplified and frequency multiplied to produce the nominal 4.3 GHz transmitter output signal.
　
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