(4)
In the air, all air/ground relays are in the air position. This connects 115-volt ac power to each pitot probe heater directly. The CSRs energize due to current flow in the heaters. Each pitot probe heater is connected to single phase, 115-volt ac power. A ground path for the pitot lights on the auxiliary annunciator panel opens. Therefore, in the air all pitot probes are heated by 115-volt ac power, and no pitot lights are on. If a probe heater failure occurs, the associated amber pitot probe light comes on and a pitot message is displayed on the EICAS (Ref 31-41-00).
(5)
If two or more probe heaters have failed, the EICAS displays a consolidated PROBE HEAT message. 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 pitot probe heaters are tested on the ground with engines off. The test 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 left and right PROBE HT TEST relays. (The left PROBE HT TEST relay is in P33. The right PROBE HT TEST relay is in P37.) The test relays provide a path for rectified 115-volt ac power to flow through the pitot probe heaters. If all heaters are working, no pitot probe heater lights on P5 are on. If a probe heater fails the test, its amber 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.)
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30-31-00
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A
757
MAINTENANCE MANUAL
C. Control
(1)
To place system in operation, apply electrical power
(AMM 24-22-00/201).
(2)
Check that the following circuit breakers on main power distribution panel P6 are closed:
(a)
PITOT HEAT CAPT MAIN
(b)
PITOT HEAT L AUX
(c)
PITOT HEAT F/O MAIN
(d)
PITOT HEAT R AUX
(3)
Check that the following circuit breakers on overhead circuit breaker panel P11 are closed:
(a)
PROBE HEAT IND PITOT R
(b)
PROBE HEAT IND PITOT L
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