The system is controlled by a switch placarded RADOME
ANTI-ICE-ON located on the overhead control panel. A
temperature sensing element installed in the discharge
duct measures the hot air temperature as it leaves the
mixing chamber. When the hot air temperature reaches
130 degrees Fahrenheit, the green RADOME HEAT light
in the mission caution/ advisory panel will illuminate. If
the air temperature exceeds 200 degrees Fahrenheit, a
heat detection switch, located adjacent to the temperature
sensing element, will close the solenoid shutoff valves,
and the yellow RADOME HOT light located on the
mission caution/advisory panel will illuminate. The
shutoff valves will automatically open after the air
Fahrenheit. The system is protected by a 7. 5 ampere
circuit breaker placarded RADOME ANTI-ICE located on
the overhead control panel.
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 deice
timer. This timer sends current to all propeller deice
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
PROP ANTI-ICE CONTR on the overhead circuit breaker
panel (fig. 2-26), protect the propeller electrothermal
deice system during manual operation. A 25 ampere
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 on the center subpanel 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 deice
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
elements, the switch is to be held in the INNER position to
perform the same function for the inner elements of both
propellers. The loadmeters will indicate approximately a
5% increase of load per meter when manual prop deice is
operating. The prop deice ammeter will not indicate any
load in the manual mode of operation.
2-55. PITOT AND STALL WARNING HEAT SYSTEM.
Pitot heat should not be used for
more than 15 minutes while the
Pitot Heat. Heating elements are installed in the
pitot masts located on the nose. Each heating element is
controlled by an individual switch placarded PITOT ON
LEFT or RIGHT, located on the overhead control panel
(fig. 2-18). It is not advisable to operate the pitot heat
system on the ground except for testing or for short
intervals of time to remove ice or snow from the mast.
Circuit protection is provided by two 7. 5 amperes circuit
breakers, placarded PITOT HEAT, on the overhead circuit
breaker panel (fig. 2-26).