cam switch contacts. This action cycles first the
seconds on, 51-66 seconds off. The icing condi-
tions during flight determine the switch position.
Setting the selector switch to SLOW or FAST
With the propeller deicer switch set to SLOW,
permits dc power from the essential bus to
the operation is the same as the fast cycle with
energize the deicer timer motor and turn on the
indicator light. Resistances in the timer determine
the one exception. Direct current enters the timer
through a different pin (pin H) in the plug and
the speed of the timer motor. The motor, through
flows through the two fixed resistors and the
reduction gears, causes the camshaft to rotate.
variable resistor to the motor. (Dc power to the
This rotation positions the cam switches
control cam switch is through the same pins as
alternately between the right and left contacts.
for the fast cycle.) Because of the increase in
Current flows through these contacts to cycle their
resistance, the motor operates at a slower speed.
respective propeller deicer relays. With the relays
Thus, with motor speed reduction, rotation of the
being energized alternately, current from the
camshaft, through the reduction gear, is slower,
three-phase generator ac buses, through propeller
and the timer now functions at the slower cycle.
deicer circuit breakers, flows to the propeller
brush pad bracket assemblies. Carbon brushes
Several aircraft have an anti-icing and deicing
contact the copper slip rings, transmitting ac
system that prevents the formation of ice (anti-
power through the slip rings to the blade heating
icing) on the forward portion of the propeller
elements. Placing the selector switch in the OFF
spinner. The system removes any ice formation
position stops the propeller deicing operation, and
(deicing) from the blades and cuffs, aft portion
the indicator light goes out.
The propeller deice timer is a two-speed,
of the spinner, and spinner islands. This system
operates similarly to the system described
automatically controlled timer. It regulates, in
previously, except that the anti-icing elements are
cycles, the time duration and sequence of electrical
on continuously and the deicing elements cycle.
impulses to the propeller blade heating elements.
Figure 5-11 shows the location of the heating
The unit is located in a moistureproof, airtight
elements. The system usually contains a safety
case, which isolates the unit from temperature
feature for testing the propellers on the ground.
extremes and vibration. The deice timer consists
This feature provides a low voltage to the heating
of a fractional horsepower, constant-speed dc
elements, which prevents damage to the prop from
motor, including reduction gear, camshaft with
overheating.
three cams, three cam switches, two fixed
resistors, and variable resistor. The unit also
includes a filter to minimize radio interference.
With the propeller deice switch set to FAST,
direct current flows from the left dc bus, through
REVIEW SUBSET NUMBER 5
the propeller deicer circuit breaker and switch, to
the timer. This current follows two paths in the
Q1. In supersonic flight, icing doesn't occur for
timer. One path, from pins E and F that connect
what reason?
in the timer, directs the current flow to the control
cam switch. The other path, from pin G, directs
the flow through the variable resistor and one
fixed resistor to the timer motor, the filter, and
to ground. The adjustment of the variable resistor
Q2. What are the two types of anti-icing/deicing
determines the speed of the motor. The motor,
systems used on aircraft?
through the 3,000 to 1 reduction gear, rotates the
camshaft and cams. Two single-lobed shift cams
and a single two-lobed control cam are on the
camshaft. Positioning on the camshaft is so the
Q3. At what point in the engine is the air the
two single-lobed shift cams are on either side of
coldest and most likely to ice?
the two-lobed control cam. As the control cam
rotates, it alternately makes and breaks its right
and left cam switch contacts. This permits the
flow of current to the shift cam switches. As the
Q4. What is the difference between propeller
current flows to the other cam switch, rotation
deicing and anti-icing?
of the single-lobed cam makes and breaks the shift
