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747-400 MAINTENANCE MANUAL
AIR DATA STANDBY INSTRUMENTS - DESCRIPTION AND OPERATION
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1. General
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A. The air data standby instruments provide the pilots with standby indications of airplane altitude and indicated airspeed (IAS). The air data standby instruments are pneumatic instruments and receive their inputs from the pitot-static system.
B. The air data standby instruments are the standby altimeter and the standby airspeed indicator.
(1)
The standby altimeter is connected to the alternate static system. The standby airspeed indicator is connected to the auxiliary No. 1 pitot system and the alternate static system. Both indicators are connected to the instrument lighting system.
(2)
The internal vibrator in the standby altimeter receives 28 volts dc from the main battery bus.
2. Standby Altimeter
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A. The standby altimeter serves as a backup altimeter for the pilots. The standby altimeter pointer and altitude counter display altitude from -2000 to 50,000 feet.
B. The indicator can be adjusted for local barometric pressure by the BARO set knob. Two counters indicate the barometric setting. One reads in inches of mercury from 22.00 to 31.99 (IN. HG) and the other reads in millibars from 745 to 1083 (MB/HPA).
C. The standby altimeter is located on the center instrument panel.
3. Standby Airspeed Indicator
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A. The standby airspeed indicator serves as a backup airspeed indicator for the pilots. Indicated airspeed (IAS) from 60 to 450 knots is displayed by a pointer read against a non-linear graduated scale.
B. The standby airspeed indicator is located on the center instrument panel.
4. Operation
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A. Standby Altimeter
(1)
Two aneroid diaphragms in the standby altimeter case drive a gear train in response to static pressure changes. The gear train positions the rotating pointer and the altitude counter. Indicated altitude is corrected for barometric pressure by rotating the BARO knob on the front of the instrument, which differentially corrects the altitude gear train value.
(2)
A vibrator in the standby altimeter is used to reduce friction errors in the mechanical linkage and to improve indicator response. The vibrator receives 28v dc power from the 28v dc main battery bus. Power is controlled by the STBY INSTR ALTM VIB circuit breaker on the P7 overhead circuit breaker panel.
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747-400
MAINTENANCE MANUAL
STANDBY AIRSPEED INDICATOR
Air Data Standby Instruments Component Location Figure 1
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747-400
MAINTENANCE MANUAL
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747-400
MAINTENANCE MANUAL
B. Standby Airspeed Indicator
(1) The airspeed mechanism is contained in an airtight case. A diaphragm within the case is actuated by differential pressure from the pitot-static system. The diaphragm expands or contracts in proportion to the difference between the pressure around it and the pressure in it. This information is transferred from the diaphragm to the airspeed pointer through mechanical linkage. The pointer shows indicated airspeed (IAS) on a non-linear dial graduated in knots. The indicator is a pneumatically controlled mechanical instrument with no failure monitors or automatic failure mode. It requires no electrical connection other than scale illumination and is always on.
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