3. Test Indicator
A. Provide electrical power.
B. Check that panel light circuit breakers on panel P6 are closed.
C. Check that indicator illumination lights are on.
NOTE: For servoed or servo-pneumatic indicators, air data computer should be known to be functioning properly before this test.
D. Check that indicator failure warning flags are out of view.
NOTE: Place mode switch on servo-pneumatic indicator in CADC or ON position.
E. Remove electrical power if no longer required.
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34-12-21 Page 402 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. May 01/01
AIR DATA TEMPERATURE INDICATING SYSTEM - DESCRIPTION AND OPERATION
1. General
A. The temperature of the outside air is used to calculate air density, which is required for correct adjustment of the airplane engines.
2. Total Air Temperature Probe
A. The outside air temperature probe contains a noninductive resistive transducer sensing elements and an anti-icing heater. Temperature variations of the slipstream airflow through the probe and sensor causes the element resistance to vary, thus providing air temperature data to the total air temperature indicator (Fig. 1 and 2).
B. In flight, heating of the probe assembly is controlled by a switch on the pilots' overhead panel. Refer to Pitot Tubes and Temperature Probe Anti-Icing System, Chapter 30.
3. Total Air Temperature Indicator (GJ ALL EXCEPT B2509-B2510)
A. The total air temperature sensor resistance is electrically included as one leg of a dc bridge circuit across which the electro-mechanical movement of the total air temperature TAT indicator is connected. A change in air temperature will cause a consequent change in resistance of the sensor, and thus proportionately unbalance the bridge. This unbalance produces a corresponding current flow through the indicator movement, rotating the pointer, and displaying total air temperature on the peripheral scale between -70 and +50 degrees centigrade.
4. Total Air Temperature Indicator (GJ B2509-B2510)
A. The total air temperature TAT indicator provides a visual indication of total air temperature as sensed by a total air temperature probe. Total air temperature is read on a three-drum numerical counter from -60 to +60 degrees centigrade in 0.1-degree increments.
B. Indicator input is a resistance from the total air temperature probe that varies as a function of total air temperature.
(1) The instrument transforms a variable resistance into a visual indication of temperature. As shown in Fig 2, the rebalance/feedback system slaves the numerical counter to an input from the air temperature probe such that the counter is providing a continuously variable readout of total air temperature.
5C8
Jun 20/85 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 34-14-0 Page 1
K84830
Air Data Temperature Indicating System Schematic 5C8
34-14-0 Figure 2 (Sheet 1) Jun 20/85
Page 2
BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details.
(2)
The error signal required to position the rebalance/feedback system is developed in a dc bridge. The air temperature probe forms on leg of the bridge and the rebalance potentiometer does the balancing. DC bridge balance is monitored by a solid-state chopper and when a changing resistance in the temperature probe unbalances the bridge, the chopper converts the inbalances to an error signal to drive the feedback loop. Mechanical motion in the loop then balances the dc bridge by repositioning the rebalance potentiometer to a null condition.
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