(3)  
If one engine is operating on the ground, and engine speed N2 is greater than 38.8% the engine out relay associated with the "on" engine energizes. This provides a path from the 115v ac power source, through the engine probe heater, to ground. The CSR associated with the engine probe of the "on" engine energizes. This opens the ground paths for left and right Engine Indication And Crew Alerting System (EICAS) computers.

 (4)  
Therefore, with one engine operating on the ground, the engine probe associated with the "on" engine is heated with 115v ac power. If the engine probe heater of the operating engine fails, a status and maintenance EICAS message L (R) ENG PROBE HEAT appears (Ref 31-41-00).

 (5)  
If both engines are operating on the ground, the engine out relays of both engines energize. This provides heating to the engine probes of both engines. All the CSRs energize, opening the corresponding ground paths for left and right EICAS computers.

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MAINTENANCE MANUAL

ENGINE SPEED
PROBE CSR
CARD-L/R
Engine Probe Heat Schematic
Figure 2

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MAINTENANCE MANUAL

 (6)  
Therefore, on the ground with both engines running the engine probes are heated. If either engine probe heater fails, its corresponding EICAS status and maintenance message L (R) ENG PROBE HEAT appears. If both engine probe heaters fail, both L and R ENG PROBE HEAT messages appear.

 (7)  
In the air, all air/ground relays de-energize to allow heating of the engine probe of each engine. Engine probe failure annunciation is identical as with two engine operation on the ground.

 B. Control
 (1)  
To operate the system, supply electrical power to main ac buses (Ref 24-22-00).

 (2)  
Make sure these circuit breakers on the overhead circuit breaker panel, P11 are closed:

 (a)  
ENG PROBE HTR LEFT

 (b)  
ENG PROBE HTR RIGHT

 (c)  
PROBE HEAT IND ENG LEFT

 (d)  
PROBE HEAT IND ENG RIGHT

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