A pressure regulator protects the system from over-
(2) Normal operation of the surface deice
When the system is not in operation, a
system is accomplished by use of the three-position
distributor valve applies vacuum to the boots for hold-
DEICE CYCLE switch, which has a momentary SGL
down. When a solenoid in the distributor valve is
(single) position, and a momentary MNL, (manual)
energized by the pneumatic deicer timer, or when either
position. The switch returns to the center, OFF position,
the surface or antenna DEICE CYCLE switch is
when the toggle is released. When switched to the SGL
positioned to MNL (manual), a servo valve changes the
position, the deicer boots automatically inflate for seven
inlet to the boots from vacuum to pressure which allows
to eight seconds, then deflate and return to the vacuum
the boots to inflate. When the solenoid valve is de-
hold down position. In MNL position, all boots inflate
energized, the airflow through the valve is cut off. The
and stay inflated while the switch is in this mode. When
air then discharges out of the boots through an integral
the switch is released, all boots deflate. The manual
check valve until the pressure reaches approximately 1
position is for use in case of timer failure. In either
in Hg, at which time the boots are ported to vacuum and
switch position, the boots cannot be over-inflated.
the remaining air is evacuated. The boots are again
held down by vacuum.
2-54. Propeller Electrothermal Deicer System.
(1) Either engine is capable of providing
Elecrothermal deicer boots are
sufficient bleed air for all requirements of the surface
cemented to each propeller blade to prevent ice
deicer system. Check valves in the bleed air and
formation or to remove ice from the propellers. Each
vacuum lines prevent backflow through the system
thermal boot consists of one outboard and one inboard
during single-engine operation.
Bleed air passes
heating element, and receives electrical power from the
through a 16 PSI regulator and then enters the deicer
deicer timer. This timer sends current to all propeller
and vacuum systems. Vacuum pressure is created by
thermal boots and prevents the deicers from overheating
the ejector and is proportional to pneumatic pressure
by limiting the time each element is energized. Four
supplied by the deicer pressure regulator valve.
intervals of approximately 3 seconds each complete one
Regulated pressure is indicated on a gage, placarded
cycle. Current consumption is monitored by a PROP
DE ICING PRESS, located on the control pedestal
circuit breaker switch, placarded PROP, on the left
subpanel (fig. 2-7), controls the propeller electrothermal
(2) The operation of the surface deicer
system is controlled by a three-position switch located
on the left subpanel. The switch is placarded DEICE
b. Normal Operation. Operation of the propeller
SURF, SGL, MNL.
The surface deicer system is
deicing system is controlled by the propeller heat switch,
protected by two 5-ampere circuit breakers, placarded
placarded PROP, which controls two inboard and
SURF DEICE located on the fight subpanel.
outboard, heating elements in each propeller boot.
When ice formation becomes visible on the aircraft, or
b. Normal Operation.
when ice is expected, place this switch in the HEAT
position. The timer will then cycle power to the heating
(1) Deice boots are intended to remove ice
elements. The timer successively delivers current to the
after it has formed rather than prevent its formation. For
outer heaters on one propeller, the inner heaters on the
the most effective deicing operation, allow at least 1/2
same propeller, the outer heaters on the opposite
inch of ice on the boots to accumulate before attempting
propeller and the inner heaters on the same propeller.
ice removal. Very thin ice may crack and cling to the
The timer energizes each of these four phases in turn for
boots instead of shedding.
about 30 seconds and then repeats the cycle as long as
the control switch is on. When the timer shuts off, it
advances one cycle. Each cycle is 30 seconds in
duration, which makes a complete cycle lasting two
Never cycle the system rapidly, since
minutes. When the timer switches from one phase to
this may cause the ice to accumulate
the next, the ammeter will register a momentary
outside the contour of the inflated
deflection. These fluctuations inform the pilot that the
boots and prevent ice removal.
timer is switching properly.