When windshield temperature is above cali-
becomes too high. This action diverts the hot
brated value, the sensing element has a higher
defrosting air to the air-conditioning outlet at the
resistance value than needed to balance the bridge.
floor of the cockpit until the temperature drops.
This decreases the flow of current through the
Windshield overheating occurs only if the cabin
amplifiers, and the relays of the control unit
air temperature high limit pickup fails.
are de-energized. As the temperature of the
windshield drops, the resistance value of the
sensing element also drops. Now, the current
Windshield Anti-Icing and
through the amplifiers again reaches sufficient
Defogging System
magnitude to operate the relays in the control unit,
An electrical anti-icing and defogging system
thus energizing the windshield heaters.
When the windshield heat control switch is in
is now in the windshield panels of current naval
LOW, autotransformers provide 115 volts, 400
aircraft. The panels are constructed of two pieces
hertz ac to the left and right amplifiers in the dual
of semitempered plate glass laminated with a vinyl
windshield control unit. In this condition, the
plastic core. The core acts as a safety device to
prevent shattering in a collision with birds when
transformers provide 121-volt ac power to
windshield heating current bus bars through dual
it is in the heated condition. The resistance heating
windshield power relays. The sensing elements in
element for anti-icing and defogging consists of
the windshield operate in the same manner as
a transparent, electrically conductive film, evenly
described for high heat operation. The calibrated
distributed over the inner surface of the outer pane
of glass. The system includes the following
units maintain windshield temperature between
40C and 49C (105F to 120F).
additional components:
A windshield wire terminal box, located
Wing and Tail Anti-Icing
between the windshield panels
A
Some aircraft have a thermal anti-icing system
temperature-sensing element embedded
in each panel
that prevents formation of ice on the leading edge
of the wing panels and tail surfaces. This system
uses hot air to heat the leading edges. The hot air
Two windshield autotransformers and a
comes from a combustion heater in the wings and
heat control relay
tail section, or from the compressor section of a
jet engine. An electrically operated pump supplies
A dual windshield control unit
fuel for the anti-icer heaters. Heater demand
determines the amount of fuel the pump delivers.
A windshield heat control toggle switch
In some aircraft, thermostats automatically
located in the cockpit
determine heater demands. Various types and
The system receives power from the ac buses
combinations of solenoid-operated valves control
fuel flow and airflow in the system.
through the windshield heat control circuit
breakers. When the windshield heat control switch
is in HIGH, 115 volts, 400 hertz goes to the left
Wing and Tail Deicing
and right amplifiers in the dual windshield control
unit. The windshield heat control relay then
Some aircraft may have air-inflated, rubber
energizes, applying two phases of ac power at
deicer boots on the leading edges of wing and tail
200 volts, 400 hertz to the windshield heat
surfaces. Air pressure or vacuum is alternately
autotransformers. These transformers provide
applied to these boots and cracks off any ice that
218-volt ac power to the windshield heating
has formed. Once the ice has cracked, the force
current bus bars through the dual windshield
control unit relays. The sensing element in
of the airstream peels it back and carries it away.
Pressure for inflating the air cells in the deicer
each windshield has a POSITIVE temperature
boots is normally from engine-driven pumps. Air
coefficient of resistance and forms one leg of a
pressure or vacuum is alternately applied either
bridge circuit. (Some systems use a thermistor,
which has a negative temperature coefficient,
by the use of motor-driven rotary distribution
valves or by the combination of an electronic
as a sensing element to control windshield
timer and solenoid distributor valves.
temperature.)
