30-41-00
 ALL  ú ú 01 Page 5 ú Jan 28/02

L/R EICAS CMPTR
LEFT FORWARD WINDOW HEAT SYSTEM
RIGHT FORWARD WINDOW HEAT SYSTEM

Side (No. 3) Window Heat System Schematic
Figure 4

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757
MAINTENANCE MANUAL

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757
MAINTENANCE MANUAL

 (6)  
No. 1 Windows

 (a)  
The No. 1 windows are electrically heated to prevent icing and fogging. Each No. 1 window channel can supply from zero to 4000 watts of power to its heater, to keep window temperature at 95°F (32.8°C). After ramp warm-up, the amount of power to the heaters depends on the error between the desired window temperature of 95°F (32.8°C) and actual window temperature. The further the actual temperature is below the desired temperature, the larger the power to the window heater.

 (b)  
To prevent thermal shock to a cold window, ramp warm-up is applied to the window before full power is applied to it. After six minutes, full power to a No. 1 window is reached, ramp operation ceases and the proportional mode starts.

 (7)  
No. 2 and No. 3 Windows

 (a)  
The No. 2 and No. 3 windows are electrically heated to prevent fogging. Power to the side window (No. 2 and No. 3) heaters is not continuously modulated as for the front window (No. 1) heaters. The side window control channels operate either on or off. When a side window channel is on, 800 watts is supplied to its heater. The heater is not powered when its channel is off.

 (b)  
When the temperature sensor in a side window senses a temperature of 105°F (40.6°C), the control channel will cause its side window heater to turn off. When the side window temperature is below 100°F (37.8°c), the control channel comes on and supplies power to the heaters. The side windows are not warmed up before full power is applied.

 (8)  
System Protection

 (a)  Circuits in the window heat control unit (WHCU) continuously monitor the status of the control unit, window sensors, and window heaters. The controller power line feeding each window heater contains a fail-safe relay which opens (removing heater power) due to any of the following conditions present for more than 350 milliseconds: input power not present, shorted sensor, open sensor, overheat, demand without heat, heat without demand, asymmetrical output, power on with switch in off position. Any of these conditions cause the failed channels INOP light on the window heat contol panel to illuminate and message to appear on the EICAS display (Ref 31-41-00).
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