Pneumatic Deicing System Description; Installation - Piper Navajo Chieftain PA-31-350 Service Manual

NAVAJO CHIEFTAIN SERVICE MANUAL
14-60. PNEUMATIC DEICING SYSTEM. (Refer to Chapter X Pneumatic System Installation)
14-61. INTRODUCTION. This portion of Section XIV provides service and maintenance procedures for
the pneumatic deicing system. This information is current as of the time of this issue.
14-62. DESCRIPTION AND PRINCIPLES OR OPERATION. The deicer is essentially a fabric reinforced
rubber sheet containing built-in inflation tubes. The type used in this installation have spanwise inflation
tubes. Deicers are attached by means of a cement to the leading edges of the surfaces being protected.
There are either aluminum or flexible rubber air connections on the backside of the deicer boots called "air
connection stems." Each stem projects from the underside of the boot into the leading edge, through a
round hole provided in the metal skin, for connection to the airplane's pneumatic air supply system.
On models with serial numbers 31-5001 to 31-7652123 inclusive, air from the pumps passes through the
two stage regulator and into the system. Air enters the ejector flow control valves (items 2 and 5, Figure 14-23)
and upon passing through the small ejector in each valve a vacuum is created which holds the boots close to the
leading edge of the surface and also helps in deflating the boots more rapidly. Deicer inflation is effected by the
deicer system control switch. This is a momentary ON type switch which returns to OFF position when
released. Upon actuation of the control switch the timer shuts off the low pressure side of each two stage
regulator (Figure 14-23, item 3) leaving the high side free to regulate pressure when reaching 18 psi. This higher
pressure enters the two wing ejector flow control valves (Figure 14-23, item 5) which have also been electrically
energized to the inflate position by closing off the bleed port. This phase of the cycle lasts for 6 seconds then the
single tail ejector flow control valve (item 2. Figure 14-23) is actuated for the last 6 seconds, completing a 12
second cycle. The system will then return to its normal low stage pressure and vacuum is reapplied to the boots.
On this system two pressure switches (item 8, Figure 14-23) connected in series in each engine nacelle
illuminate a blue wing indicator light located on the aircraft circuit breaker panel when sufficient pressure is
obtained to indicate phase one operation. A pressure switch (item 17, Figure 14-23) located in the tail
illuminates a blue tail indicator light to indicate phase 2 operation.
On models with serial numbers 31-7652124 to 31-7952250 inclusive, air from both pumps is directed
through the normally open dump valve (item 9, Figure 14-24) after passing through the ejector (Figure 14-24.
item 8). Vacuum is created by this action and is applied to all boots under normal operation. The actuation of
the momentary ON type switch causes the timer to open the normally closed control valve and to close the
normally open dump valve (refer to Figure 14-24. items 7 and 9) for six seconds. These valves when energized
along with the two stage regulator (Figure 14-24, item 3) allow higher pressured air to inflate the boots. When
sufficient pressure is reached a single pressure switch (item 14, Figure 14-24) illuminates a blue light on the
circuit breaker panel indicating proper operation.
On models with serial numbers 31-8052001 and up. a constant suction is applied to all of the surface
deicer boots by an ejector during normal operation with the deicing system off. The ejector assembly utilizes
air pressure supplied by the pneumatic system to obtain a vacuum to hold the boots in the flat. deflated
condition. The deicing system is controlled by a "momentary on" type control switch. When this WING
DEICE switch is actuated. the boots perform one complete inflation cycle. The switch must be actuated for
each additional inflation cycle. After each inflation cycle, the timer automatically resets to allow the inflation
cycle to begin when the switch is actuated. Actuation of the momentary switch triggers a system cycle timer.
which in turn shifts the two stage regulators to high pressure( 18 psi). opens the 'A'system solenoid vale to
send air to the wing boots. and cuts off air to the copilot's gyros (when installed). After six seconds. the 'A'
system solenoid is closed and the 'B' system solenoid is opened to send air to the tail boots for six seconds. At
the completion of the tail c cle. the 'B' system solenoid closes. the two stage regulators return to low pressure
(gyro pres.) and the copilot's air supply resumes. When the inflation cycle is complete. the deicer solenoid
valves permit overboard exhaustion of the pressurized boots. Suction is then reapplied to the deicer boots to
hold them close to the airfoil surface. Two blue indicator lights with press-to-test and dimming features.
illuminate when each surface deicer boot system inflates to a predetermined pressure. Illumination of the
indicator light is controlled by a pressure sensitive switch connected to the deicer pressure lines (one in the
"A" system and one in the
"B" system).
Revised: 10/5/83
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