Window Heating
Window
heat is provided to improve impact resistance (bird hit), ice buildup and
defogging and should be selected ON at least 10 minutes before take-off. Each
frontal window (L & R SIDE & FWD) have an own Window Heat Control Unit
(WHCU) which receive power from their respective 115 VAC Transfer Bus.
This
heating is accomplished by laminated glass and vinyl window layers with
in-between a conductive layer that allows electrical current to flow through it
when heating is selected. This “gradual increasing”
current flow creates heat by resistance in the layer towards a target
temperature of ±43°C. The WHCU adjusts heating current in its operating range
to prevent a thermal shock and reduces the current flow at higher ranges to
prevent an over temperature. Window heat becomes active when selected to ON and
the window temperature is < 37°C, indicated by a green ON light (or
extinguished OFF light) meaning there is current flow through the conductive
layer. When the window reaches the target temperature the WHCU interrupts
electrical power and extinguishes the ON light (or illuminates the OFF light)
which could already be the case when parked into the sun on a hot day.
To
make sure the system operates when needed, there is a POWER TEST switch that
by-passes the thermistor and sends electrical current through the windows. Be
aware this action will bypass the control unit temperature regulation so when
activated too long it could cause an overtemp condition in that window.
Another
test function is the OVHT TEST switch that simulates an overheat condition with
the window heat switches in ON, indicated by amber OVHT lights on the control
panel and extinguished ON lights (or illuminated OFF lights). The simulated
overheat must be reset by selecting the window heat switches to OFF than back
to ON.
When
a window “overtemps” at values higher than 62°C, the WHCU interrupts power to
the affected windshield and illuminates an OVERHEAT indication together with an
extinguished ON light (or Illuminated OFF light). This condition can be reset
by selecting the affected window to OFF and back to ON when allowed sufficient
cooling first (2 – 5 minutes according the QRH).
When window heat is inoperative prevent speeds above 250 Kts at
altitudes below 10.000 ft to minimize the effect when a bird strike occurs on
the window
