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A 737-300/400/500MAINTENANCE MANUAL

VERTICAL SPEED INDICATOR

WITHOUT FLAGS WITH FLAGS
ELECTRIC ALTIMETER

STANDBY ALTIMETER/AIRSPEED INDICATOR TAT/SAT/TAS INDICATOR

AIRPLANES WITH AIR DATA VSI
Air Data System Component Location
Figure 1 (Sheet 2)

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A

737-300/400/500MAINTENANCE MANUAL
PITOT STATIC PRESSURE INPUTS

TEST SELECT SWITCH
TEST
VALID LIGHT

TEST SWITCH
DIGITAL AIR DATA COMPUTER

Air Data System Component Location
Figure 1 (Sheet 3)

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A 737-300/400/500MAINTENANCE MANUAL
Computed airspeed derived from the air data computer, which is corrected for static pressure source error, drives the airspeed pointer from 60-thru 450-knot range of airspeed. The mechanism produces linear rotation of the pointer in five-knot calibration increments between 60 and 100 knots, in two-knot calibration increments between 100 and 160 knots, and logarithmically in five-knot increments between 160 and 450 knots (Fig. 1). An airspeed valid signal from the air data computer is provided to the A/S flag.Mach number indication is presented on a two drum digital readout through the range .40 to .99 mach. Below .40 mach the readout is covered by a black mask. The indicator computes mach from inputs of airspeed and altitude signals from the air data computer. Upon power failure, or loss of 28 volts dc from the air data computer a red and white OFF flag will appear to cover the digital mach readout.The colored pointer on the indicator shows maximum operating speed of the airplane as a function of corrected altitude and airspeed from the air data computer. The speed reference marker (target bug) is electrically fed from the AFCS system. The "INOP" flag indicates the absence of an operational AFCS system and the target bug will store at 440 knots. Pulling the bug set knob out will place the target bug in manual mode and will retract "INOP" flag. When operating in the manual mode, an "M" flag will appear in view at top of indicator. White peripheral pointers may be set manually to any desired airspeeds.The indicator generates a VMO signal for controlling the maximum operating airspeed pointer mechanism in the indicator. The indicator also provides an output signal to actuate an aural warning when maximum operating speed is exceeded.The air data computers provide a valid signal to the indicators. Monitoring within the indicator of the maximum operating airspeed display output signal and the maximum operating airspeed function switch is capable of detecting the following conditions; 1) absence of the altitude valid signal from the air data computer, 2) loss of electrical power to the indicator, 3) failure of indicator power supply, 4) open circuit in interlocks. When a fault condition is detected, a VMO flag in the indicator will come into view and the maximum operating airspeed switch will be inhibited from producting an airspeed warning.The indicators have a self-test capability for the maximum operating airspeed switch. Operation of the mach airspeed warning test switch results in an overspeed aural warning unless failure monitoring within the indicator detects a fault condition. The aural warning input is switched to external ground through the indicator airspeed switch. Power to the indicators is 26 volts ac through circuits breakers located on circuit breaker panel P18 for the captain's indicator and P6 for the first officer's indicator.
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