A

757
MAINTENANCE MANUAL

ANNUNCIATOR PANEL
RLY
Total Air Temperature Probe Heat Schematic
Figure 2

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 30-33-00
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A

757 MAINTENANCE MANUAL
 (4)  
If one engine is on, the engine speed card energizes the corresponding ENG OUT relay. This opens the ground path for the TAT probe heater light. Therefore, with at least one engine operating on the ground, both the TAT probe heater and TAT light are off.

 (5)  
The air/ground relays de-energize in the air. This allows current to flow through the TAT probe heater and its CSR. The TAT probe is heated by 115-volt ac power. The current flow energizes the CSR, which opens the ground for the TAT light. The TAT light on P5 will be off. If there is no current flow sensed by the CSR, the TAT will light. There is also a message on the EICAS display, if the TAT probe heater fails (Ref 31-41-00).

 (6)  
If two or more probe heaters have failed, the EICAS display shows the advisory message PROBE HEAT. The individual probe heat failure messages are not displayed. (This applies to any two probe heater failures in the entire system. Included are pitot, angle of attack, and total air temperature probes.)

 B. Ground Test (Fig. 2)
 (1)  
The TAT probe heater is tested on the ground with the engines off. This is done by the WING ANTI-ICE WINDOW/PROBE HEAT switch, on the miscellaneous test panel. The switch is a momentary action, center off type. Holding the switch in the WINDOW/PROBE HEAT position energizes the right PROBE HT TEST relay. (This relay is in P37.) This provides a path for 115-volt ac power to flow through the TAT probe heater.

 (2)  
The current flow to the TAT probe heater energizes the CSR, which removes the ground path for the TAT light on the auxiliary annunciator panel. If the heater operates correctly, the TAT light on P5 goes off. If the probe heater fails the test, the amber TAT probe heater light comes on. When the switch is released after testing, the system returns to normal ground operation. (The WINDOW/PROBE HEAT switch position tests all other probe heaters also.)

 C. Control
 (1)  
To operate the system, supply electrical power (Ref 24-22-00).

 (2)  
Make sure the TAT PROBE HEAT circuit breaker on the main power distribution panel, P6, is closed.

 (3)  
Make sure the PROBE HEAT IND TAT circuit breaker on the overhead circuit breaker panel, P11, is closed.

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