Pneumatic Deicing System; Description And Principles Of Operation; Operational Check - Piper Navajo PA-31 Service Manual

PIPER NAVAJO SERVICE MANUAL
14-127. PNEUMATIC DEICING SYSTEM.
14-128. 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.
14-129. DESCRIPTION AND PRINCIPLES OF OPERATION. (Refer to Figure 14-52.) 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.
Through the engine driven pneumatic pumps, the system will normally apply vacuum to the deicer
boots at all times, except when the boots are being inflated. Deicer inflation is effected by the deicer
system control switch. This is a circuit breaker type switch. Through actuation of the switch, the timer
energizes the gyro relay valve (to close off pressure to the copilot's gyros), the relief valve (to increase
system pressure to 18 psig), and deicer control valve (to remove vacuum from boots and allows boots to
inflate). Air pressure from the engine driven pumps is supplied to the inflatable tubes in the boots for six
seconds. The inflation cycle is controlled by the timer and solenoid valves. The deicer pressure, normally 18
psig, is regulated by the high pressure stage of the relief valve. Upon automatic de-energization by the timer,
the relief valve lowers system pressure; the gyro relay valve allows normal system pressure to pass to the
copilot's gyros and the deicer control valve permits boot pressure to be exhausted overboard and system
vacuum is reapplied to the boots to hold them close to the surface skin. The inflation cycle will continue
every three minutes until the switch is placed in the off position, which will automatically reset the timer.
Later installations use a momentary ON type switch which returns to OFF when released. The
inflation cycle occurs only once each time the momentary ON switch is activated.
A ply of conductive neoprene is provided on the surface to dissipate static electric charges. These
charges, if allowed to accumulate, would eventually discharge through the boot to the metal skin beneath,
causing static interference with the radio equipment and possible punctures in the rubber. Also, such static
charges would constitute a temporary fire hazard after each flight.
14-130. TROUBLESHOOTING. In the utilization of the troubleshooting charts at the end of Section X, it
must be assumed that the engine driven pneumatic pumps and the airplane electrical system are operational.
It is further assumed that the deicer system installation was made in an approved manner.
14-131. OPERATIONAL CHECK. The pneumatic deicing system should be checked at least every 100
hours. This check can be done on the ground. A visual inspection should be performed to determine the
condition of the deicer boots, and any areas in need of repair should be taken care of before continuing
with the operational check of the system.
With one engine operating, activate the deicing system switch. The pressure will fluctuate as the tubes
inflate and deflate. Check the pneumatic pressure gauge. If pressure is satisfactory, observe the operation of
the deicers carefully for evidence of malfunctioning. Look for tubes which leak or fail to inflate and deflate
properly. Repeat the procedure for the other engine.
Reissued: 10/12/79
ACCESSORIES AND UTILITIES
5120