TM 55-1520-228-10
SECTION IX. UTILITY SYSTEM
2-46. PITOT HEATING SYSTEM.
a. Description. The pitot heater is installed on
the pitot head and functions to prevent ice forming in
the pitot tube. Electric power for the pitot heater oper-
ation is supplied from the 28 VDC helicopter electrical
system. Circuit protection is provided by a 5 ampere cir-
cuit breaker switch on the overhead console. The pitot
heater is controlled by a switch located on the overhead
console (figure 2-12) labeled PITOT HTR.
b. Operation. The pitot heater switch should be
in PITOT HTR position to prevent ice forming in pitot
tube. To shut off pitot heater, position switch to OFF.
SECTION X. HEATING AND VENTILATION
2-47. BLEED AIR HEATING SYSTEM.
a. Description. The bleed air heater is installed
in the equipment compartment aft of the electrical
shelf
behind the passenger seat. A HTR switch (figure 2-12)
located on the overhead console actuates the heater.
The pilot may select air temperature by means of a
HEAT control knob (figure 2-17) located above and aft
of his head. This knob operates a control cable in the
cabin roof to adjust an air temperature se
nsor mounted
in the heater discharge duct. Adjusting the sensor mixes
the fresh and bleed air to obtain the air temperature
selected by the pilot. The bleed air hea
ting ducts are
connected to windshield defog nozzles at the bottom of
the forward windshield through blowers mounted in the
fuselage nose. The blowers are ac
tivated by DEFOG &
VENT circuit breaker switch (figure 2-12) located on the
overhead console to increase airflow velocity over the
windshield. Lower nose window
defog is continuous
and automatic when HTR switch is turned on. Outside
fresh air from an intake on the top of the fuselage
nose is directed through the
windshield defog nozzles
mounted on the lower windshield frame. A VENT PULL
control on the right side of the instrument panel (figure
2-16) opens and closes th
e fresh air intake.
b. Operation. A circuit breaker switch labeled
HTR on the overhead console (figure 2-12) actuates a
solenoid valve. The switch in the ON position permits
air from the engine compressor section to pass th
rough
the bleed air nozzle. A venturi working in conjunction
with the bleed air nozzle draws in outside air through
the outside air vent. Bleed air and outside air
is fed into
the mixing valve where a sensor determines the mixing
ratio produce the desired temperature. Bleed air forces
heated air through the duct system to re
gisters under
the seat and/or to the defroster nozzles. Temperature
is regulated by a manual control knob labeled HEAT
located above and to the rear of the pi
lot’s head, and
connected to a variable remote sensor in the heater
compartment. The sensor has a bimetallic element
which controls the mixing valve
. The defogging and
vent system is installed in the nose and consists of a
ram air intake, two blower fans, defroster nozzles and
ducts. The bleed air system is
also connected to the
defogging and vent system. Outside air flow to the
cabin and defogging nozzles is controlled by manual
push-pull type controls la
beled VENT and DEFOG
located on each side of the instrument panel (figure
2-16). The blowers direct air to the defogging nozzles
and are controlled by a 5
AMP circuit breaker switch,
labeled DEFOG & VENT, on the overhead console.
(Refer to figure 2-12).
Change 10 2-35
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