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737-300/400/500MAINTENANCE MANUAL
 STANDBY HORIZON SYSTEM - DESCRIPTION AND OPERATION
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General (Fig. 1)
_______
 A.  The standby Horizon indicator is a backup system for the Inertial Reference System. It functions as a completely independent system which provides its own visual indication of airplane attitude in pitch and roll at all times during flight.
 B.  The system consists of the standby horizon indicator located on the center instrument panel (P2).
 C.  Power for the system is provided from the STANDBY HORIZON circuit
 breaker, situated on the load control center (P18).Standby Horizon Indicator (Fig. 1)
_________________________
 A.  The indicator is of the conventional mechanical type. Attitude is provided by a 3-phase vertical gyro suspended in a two degree-of-freedom gimbal system. This system is connected to a drum-type roll and pitch attitude display. The system displays approximately 90 degrees of freedom in climb, 80 degrees of freedom in dive and 360 degrees of freedom in roll.
 B.  The gyro system utilizes a spinning pendulum and counterweight device to automatically erect the gyro at turn-on. The quick caging knob on the front face is used to mechanically accelerate the erection process. The fast erection process stabilizes the display drum to the reference vertical position determined by indicator orientation.
 C.  An integral static inverter converts the 28v dc input to three-phase 400Hz ac voltage for the gyro motor. It also monitors the power supplies and the speed of the gyro.
 D.  The gyro failure flag appears in view in the event of a power failure or
 if gyro rotor speed is low.Operation (Fig. 2)
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 A.  Within 3 minutes of power application the gyro reaches operational speed and the gyro failure flag retracts from view. The normal rate of gyro erection is 3 degrees per minutes, but a fast erection device is fitted to bring about erection within a few seconds. The mechanism for fast erection is actuated by pulling gently on the quick caging knob, located in the lower right-hand corner of the indicatior front face. The quick caging knob may be used to adjust the horizon line to zero pitch by turning the knob clockwise and counterclockwise. After power has been removed, the gyro takes approximately 9 minutes to come to rest.
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A 737-300/400/500MAINTENANCE MANUAL

KNOB

STANDBY HORIZON INDICATOR

Standby Attitude Reference System Component Location
Figure 1

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A 737-300/400/500MAINTENANCE MANUAL

Standby Attitude Reference System Schematic
Figure 2

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A
737-300/400/500MAINTENANCE MANUAL
 STANDBY HORIZON SYSTEM - TROUBLE SHOOTING
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 1.  General_______
 A.  These trouble shooting procedures follow the system operational test. They are made in a special sequence to isolate the failure quickly. When all steps are done, the system is serviceable.
 2.  Prepare for Trouble Shooting____________________________
 A.  Supply the electrical power (Ref 24-22-00/201, Maintenance Practices (Apply Power)).
 B.  Close the STANDBY HORIZON circuit breaker on the P18-2 panel.
　
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