b. Automatic Operation. The two-position
switch located on the pilot's subpanel, placarded
PROP AUTO / OFF, is provided to activate the
automatic system. When the switch is placed to the
AUTO position, the timer diverts power through the
brush block and slip ring to all heating elements on
one propeller. Subsequently, the timer then diverts
power to all heating elements on the other propeller for
the same length of time. This cycle will continue as
long as the switch is in the AUTO position. The
system utilizes a metal-foil type, single heating
element, energized by dc voltage. The timer switches
every 90 seconds, resulting in a complete cycle in
approximately 3 minutes.
c. Manual Operation. The manual propeller
deice system is provided as a backup to the automatic
system. The spring-loaded control switch located on
the pilot's subpanel, placarded PROP MANUAL /
OFF, controls the manual override relay. When the
switch is held in the MANUAL position, the automatic
timer is overridden, and power is supplied to the
heating elements of both propellers simultaneously.
The switch is of the momentary type and must be held
in position for approximately 90 seconds to dislodge
ice from the propeller surface. Repeat this procedure
as required to avoid significant buildup of ice, which
will result in a loss of performance, vibration, and
impingement of ice upon the fuselage. The propeller
deice ammeter will not indicate a load while the
propeller deice system is being utilized in the manual
mode. However, each aircraft loadmeter will indicate
an approximate 10% increase in load while the manual
propeller deice system is operating.
2-54. PITOT HEAT SYSTEM.
Pitot heat should not be used for more
than 15 minutes while the aircraft is on the
ground. Overheating may damage the
Heating elements are installed in both pitot
masts, located on the nose. Each heating element is
controlled by an individual 7 ½ -ampere circuit breaker
switch, placarded PITOT, LEFT and RIGHT, located
on the pilot's subpanel.
2-55. STALL WARNING HEAT SYSTEM.
Heating elements protect the stall warning
lift transducer vane and faceplate from ice.
However, a buildup of ice on the wing may
change or disrupt the airflow and prevent
the system from accurately indicating an
The lift transducer is equipped with anti-icing
capability on both the mounting plate and the vane.
Stall warning vane heat is controlled by, and the circuit
is protected by, a 15-ampere circuit breaker switch
located on the pilot's subpanel, placarded STALL
WARN. The level of heat is minimal for ground
operation, but is automatically increased for flight
operation through the landing gear safety switch.
2-56. STALL WARNING SYSTEM.
transducer, a lift computer, warning horn, and a test
switch. Angle of attack is sensed by aerodynamic
pressure on the lift transducer vane located on the left
wing leading edge. When a stall is imminent, the
output of the transducer activates a stall warning horn.
The system has preflight test capability through the
use of a switch, placarded STALL WARN TEST,
located on the copilot's subpanel. Holding this switch
in the STALL WARN TEST position actuates the
warning horn by moving the transducer vane. The
circuit is protected by a 5-ampere circuit breaker,
placarded STALL WARN, located on the right sidewall
circuit breaker panel.
2-57. BRAKE DEICE SYSTEM.
a. Description. The brake deice system may
be used in flight with gear retracted or extended, or on
the ground. When the brake deice system is
activated, hot air is diffused by means of a manifold
assembly over the brake discs on each wheel. Manual
and automatic controls are provided. There are two
primary occasions that require brake deicing. The first
is when an aircraft has been parked in a freezing
atmosphere, allowing the brake systems to become
contaminated by freezing rain, snow, or ice, and the
aircraft must be moved or taxied. The second
occasion is during flight through icing conditions, when
brake assemblies are presumed to be frozen must be
thawed prior to landing to avoid possible tire damage
and loss of directional control. Hot air for the brake
deice system comes from the compressor stage of
both engines. Hot air is obtained by means of a
solenoid valve attached to the bleed air system which