Either engine is capable of providing
sufficient bleed air for all requirements of the surface
deicer system. Check valves in the bleed air and vacuum
lines prevent backflow through the system during single-
engine operation. Regulated pressure is indicated on a
gage, placarded PNEUMATIC PRESSURE, located on
the copilots subpanel.
2-53. ANTENNA DEICING SYSTEM.
Description. The antenna de-ice system removes
ice accumulation from the dipole antennas. Pressure
regulated bleed air from the engines supplies pressure to
inflate the boots. To assure operation of the system in the
event of failure of one engine, a check valve is
incorporated in the bleed air line from each engine to
prevent loss of pressure through the compressor of the
inoperative engine. Inflation and deflation phases are
controlled by a distributor valve.
Antenna De-ice System Switch. The antenna de-
ice system is controlled by a switch placarded ANT
DEICE, SINGLE OFF MANUAL located on the overhead
control panel (fig. 2-12). The switch is spring loaded to
return to the OFF position from the SINGLE or MANUAL
position. When the switch is set to the single position, the
system will -,run through one, timed 6 second inflation-
deflation cycle. When the switch is held in the MANUAL
position, the boots will inflate and remain inflated until the
switch is released.
De-ice boots are intended to remove ice
after it has formed rather than prevent its formation. -For
the most effective deicing operation, allow at least 1/8 to
1/4 inch of ice to form on the boots before attempting ice
removal. Very thin ice may crack and cling to the boots
instead of shedding.
Never cycle the system rapidly, this
may cause the ice to accumulate
outside the contour of the inflated
boots and prevent ice removal.
2-54. PROPELLER ELECTROTHERMAL ANTI-ICE
Description. Electrothermal anti-ice boots are
cemented to each propeller blade to prevent ice formation
or to remove the ice from the propellers. Each thermal
boot consists of one outboard and one inboard heating
element, and receives electrical power from the de-ice
timer. This timer sends current to all propeller de-ice
boots and prevents the boots from overheating by limiting
the time each element is energized. Four intervals of
approximately 30 seconds each complete one cycle.
Current consumption is monitored by a propeller ammeter
on the copilot's subpanel. Two 20-ampere circuit
breakers placarded PROP ANTI-ICE LEFT and RIGHT
and 5-ampere propeller control circuit breaker placarded
CONTR on the overhead circuit breaker panel (fig. 2-27),
protect the propeller electrothermal de-ice system during
manual operation. A 25 ampere circuit breaker placarded
PROP AUTO, protects the system in automatic operation.
Automatic Operation. A control switch on the
overhead control panel placarded PROP OFF AUTO is
provided to activate the automatic system. A deice
ammeter above the pedestal registers the amount of
current (14 to 18 amperes) passing through the system
being used. During AUTO operation, power to the timer
will be cut off if the current rises above 25 amperes.
Current flows from the timer to the brush assembly and
then to the slip rings installed on the spinner backing
plate. The slip rings carry the current to the deice boots
on the propeller blades. Heat from the boots reduces the
grip of the ice which is then thrown off by centrifugal
force, aided by the air blast over the propeller surfaces.
Power to the two heating elements on each blade, the
inner and outer element, is cycled by the timer in the
following sequence: right propeller outer element, right
propeller inner element, left propeller outer element, left
propeller inner element. Loss of one heating element
circuit on one side does not mean that the entire system
must be turned off. Proper operation can be checked by
noting the correct level of current usage on the ammeter.
An intermittent flicker of the needle approximately each
30 seconds indicates switching to the next group of
heating elements by the timer.
Manual Operation. The manual propeller de-ice
system is provided as a backup to the automatic system.
A control switch located on the overhead control panel,
placarded PROP INNER OUTER, controls the manual
override relays. When the switch is in the OUTER
position, the automatic timer is overridden and power is
supplied to the outer heating elements of both propellers
simultaneously. The switch is of the momentary type and
must be held in position until the ice has been dislodged
from the propeller surface. After deicing with the outer
Change 2 2-43