Window/Probe Heat

Question 1

WINDOW HEAT ON lights

Answer 1

illuminated = Window Heat Control Unit (WHCU) is applying power to the associated window

• if window temp is 100°F / 38°C or more, no power is applied and light remains off
• the sun could heat the glass, which will cause the ON lights to cycle on and off
• extinguished with switch OFF, or an overheat is detected, or a system failure has occurred
• if ON light does not come on
• use PWR TEST to check the ON light(s) that are not illuminated

Question 2

WINDOW HEAT switches

Answer 2

• SIDE controls #2, and if installed, #4 and 5
• FWD controls heat to #1 window
  ON
• signals the Window Heat Controller Unit (WHCU) to apply power
  OFF
• window heat not in use. Also resets the overheat light circuit
• turn OFF if window cracks or delaminates

Question 3

WINDOW HEAT OVHT / PWR TEST switch

Answer 3

OVHT
- spring loaded to neutral
- simulates an overheat condition to test overheat circuitry
- all overheat lights illuminate
- before test, place all window heat switches ON
- after the test, reset each OFF then ON
- an overheat cannot be reset until the window cools
- ON lights may extinguish immediately, or remain illuminated for as long as 70 sec.
PWR TEST
- verifies operation of the window heat system
- the test may be accomplished when any of the window heat ON lights are extinguished and the associated WINDOW HEAT switch is ON
- provides a confidence test with the window heat switch ON
- no need to power test if all 4 green ON lights are illuminated

Question 4

PROBE HEAT switches

Answer 4

ON
- supplies heat to 5 pitot probes, 2 angle of attack sensors, and the total air temperature probe
- 5 probes: Capt and Left Elev pitot on left side of aircraft; F/O, Right Elev, and Aux pitot on right side of aircraft
- (option) Capt’s pitot powered by ac standby bus
- static ports are not heated
- may ice over while parked in freezing rain so check carefully on walk-around inspection
- failure to turn on for engine runs above 70% N1 will result in EEC entering soft alternate mode (possible N1 overspeed)
AUTO (option)
- if left in AUTO, the probe heat functions become active for each side when the respective engine is running
OFF
- power off

Question 5

TAT TEST

Answer 5

• normally, power removed from TAT on ground and amber light does not illuminate
• with probe heat switch OFF
• push TAT TEST switch
• power applied to temp probe
• temp probe light extinguishes indicating the heat supply is ok
• TAT will rise on the indicator and on any CDU page (small characters) where TAT is displayed
• with probe heat switch ON
• push TAT TEST switch
• power applied to temp probe
• temp probe light remains out if the heat supply is ok; illuminates if defective

Question 6

WINDOW OVERHEAT lights 4 (amber)

Answer 6

• illuminated = window has overheated (145°F / 63°C) or loss of electrical power
• power is removed from the window and on light goes out
• window OVERHEAT light is accompanied by the MASTER CAUTION and ANTI-ICE annunciator
• reset by positioning the window heat switch to off then back on after 2-5 minutes cooling