Piper Navajo PA-31 Service Manual page 986

14-20. PROPELLER DE-ICING SYSTEM.
14-21. DESCRIPTION AND PRINCIPLES OF OPERATION. (Refer to Figure 14-2 or 14-3.) The
Propeller De-Icing System consists of an electrically-heated De-Icer (1) bonded to each propeller blade, a
slip ring assembly (2) with a brush block assembly (3) to transfer electrical power to the rotating De-Icers, a
timer (9), an ammeter (5), a control switch-circuit breaker (6), shunt (8), together with wiring harnesses (4)
to complete the circuit. Power is drawn from the aircraft electrical system (7).
To conserve electrical power, current is cycled to the De-Icer heaters at timed intervals rather than
continuously. Each De-Icer has two separate heaters; one for the outer half and one for the inner half. By
heating all outer or inner heaters on only one propeller at a time, rotational balance is held during de-icing.
Current is drawn from the airplane electrical system through the switch, ammeter and timer. The timer
successively delivers current via the slip ring and brush block arrangement to (phase 1) the outer heaters on
the right propeller, (phase 2) the inner heaters on the same propeller, (phase 3) the outer heaters of the left
propeller and (phase 4) the inner heaters on the left propeller. The timer energizes each of these four phases
in turn for about 30 seconds and then repeats the cycle as long as the control switch is on. The cycling
sequence given is vital so that outboard heaters on each propeller operate before the inboard heaters. See
cycle sequence. (Refer to Figures 14-4 thru 14-7, two-blades, or Figures 14-8 thru 14-11, three blades.) The
system may be used continuously inflight if needed.
NOTE
Heating may begin at any phase in the cycle depending on the
timer position when the switch was turned off from previous use.
a. De-Icers: The De-Icers contain special heater wires protected by fabric plies and by oil and
abrasion-resistant rubber. The side of the de-Icer cemented to the propeller has a dull finish whereas the air
side finish is "glossy". Each De-Icer has a separate lead for the inboard and outboard heater and a third lead
which is a common ground. These leads are so marked. An unmarked ground can be identified by using an
ohmmeter across the three possible pairs of leads. One pair will show twice the resistance of the other pairs.
The latter are the "hot" leads and the lead excluded from the pair that shows twice the resistance of the
other pairs is the ground lead. All De-Icers used on this airplane must be of the new design, which includes a
grey plastic patch where De-Icer and strap join.
b. Slip Rings, Brushes and Brush Blocks: To transfer electrical power to the rotating De-Icers, a
brush block assembly is mounted to the engine or similar stationary member and has brushes which are
spring loaded to press against the revolving slip rings. The slip ring assembly is provided as a slip ring gear
assembly which replaces the original starter ring gear of the engine.
c. Timer: The timer is a sealed unit. If found inoperative, it must be replaced as an assembly - no
field repairs are authorized. For timer function, refer to Paragraph 14-51.
b. Ammeter: The ammeter is designed for each particular system and it is therefore important that
the correct replacement part number be used if replacement should be required. In the event of low aircraft
battery voltage (very possible in ground checks), the ammeter readings will be lower than at full voltage.
Provided the ammeter needle reads in the shaded range on the scale, (full aircraft voltage) current flow is
considered as normal.
e. Switch: The switch-circuit breaker is mounted in the switch and circuit breaker control panel.
14-22. DE-ICER SYSTEM OPERATIONAL CHECK.
a.
Lock the brakes and operate the engine at near take-off power.
b. Turn De-Icer system switch ON and observe De-Icer ammeter for at least two minutes.
c. The ammeter needle must "flicker" approximately every 30 seconds as the step switch of the
timer operates.
Reissued: 10/12/79
ACCESSORIES AND UTILITIES
PIPER NAVAJO SERVICE MANUAL
5E13