airfoil. Elimination of fairing strips, screws, and rivnuts used  on older types of deicing boots reduces the weight of the deice  system. The deicer boot air cells are connected to system  pressure and vacuum lines by non-kinking flexible hose. When gluing the deice boots to the leading edge of wings  and stabilizers, the manufacturer’s instruction must be strictly  followed. The glue is typically a contact cement normally  spread on both the airfoil and the boot and allowed to become  tacky before mating the surfaces. Clean, paint-free surfaces  are required for the glue to adhere properly. Removal of old  boots is performed by re-softening the cement with solvent. Maintenance on pneumatic deicing systems varies with  each aircraft model. The instructions of the airframe or  system components manufacturer should be followed in  all cases. Depending on the aircraft, maintenance usually  consists of operational checks, adjustments, troubleshooting,  and inspection. An operational check of the system can be made by operating  the aircraft engines or by using an external source of air.  Most systems are designed with a test plug to permit ground  checking the system without operating the engines. When  using an external air source, make certain that the air pressure  does not exceed the test pressure established for the system.  Before turning the deicing system on, observe the vacuum- operated instruments. If any of the gauges begin to operate,  it is an indication that one or more check valves have failed  to close and that reverse flow through the instruments is  occurring. Correct the difficulty before continuing the test.  If no movement of the instrument pointers occurs, turn on  Examples of adjustments that may be required include  adjusting the deicing system control cable linkages, adjusting  system pressure relief valves, and deicing system vacuum  (suction) relief valves. A pressure relief valve acts as a safety  device to relieve excess pressure in the event of regulator  valve failure. To adjust this valve, operate the aircraft  engines and adjust a screw on the valve until the deicing  pressure gauge indicates the specified pressure at which the  valve should relieve. Vacuum relief valves are installed in a  system that uses a vacuum pump to maintain constant suction  during varying vacuum pump speeds. To adjust a vacuum  relief valve, operate the engines. While watching the vacuum  (suction) gauge, an assistant should adjust the suction relief  valve adjusting screw to obtain the correct suction specified  for the system. the deicing system. With the deicer system controls in their  proper positions, check the suction and pressure gauges for  proper indications. The pressure gauge fluctuates as the  deicer tubes inflate and deflate. A relatively steady reading  should be maintained on the vacuum gauge. It should be noted  that not all systems use a vacuum gauge. If the operating  pressure and vacuum are satisfactory, observe the deicers  for actuation. With an observer stationed outside the aircraft,  check the inflation sequence to be certain that it agrees with  the sequence indicated in the aircraft maintenance manual.  Check the timing of the system through several complete  cycles. If the cycle time varies more than is allowable,  determine the difficulty and correct it. Inflation of the deicers  must be rapid to provide efficient deicing. Deflation of the  boot being observed should be completed before the next  inflation cycle.  [Figure 15-32]  Adjustments Inspection, Maintenance, and Troubleshooting of  Rubber Deicer Boot Systems Operational Checks 15-21 Figure 15-32.  Test equipment used to test a wing deice system (left), and test equipment installed in the aircraft for testing (right).