Deice boot Deice boot Deice boot Bleed air flow control unit Engine P$_{3}$ bleed air source Bleed air flow control unit Engine P$_{3}$ bleed air source Vacuum regulator Pneumatic shutoff valves Pneumatic control assembly Brake deice valve Brake deice valve Deice boot Pressure Vacuum Pressure or vacuum 15-16 Figure 15-22.  Ice protection panel on a turboprop aircraft with  deice boots. Figure 15-21.  Wing deice system for turboprop aircraft. Deicing System Components Wet-Type Engine-Driven Air Pump To provide pressure for the deice boots, older aircraft may use  a wet-type engine-driven air pump mounted on the accessory  drive gear case of the engine. Some modern aircraft may  also use a wet-type air pump because of its durability. The  pump is typically a four vane, positive displacement pump.  Several components are used to construct all deice boot  systems. The components may differ slightly in name  and location within the system depending on the aircraft.  Components may also combine functions to save space and  weight. The basic functions of filtering, pressure regulation,  distribution, and attachment to a vacuum when boots are not  in use must all be present. Check valves must also be installed  to prevent back flow in the system. Manifolds are common  on multiengine aircraft to allow sourcing of low pressure  air from both engine pumps. Note that air-pump pressure is  typically expelled overboard when not needed. Bleed air is  shut off by a valve when not needed for deice boot operation  on turbine engine aircraft. A timer, or control unit with an  automatic mode, exists on many aircraft to repeat the deice  cycle periodically. to form on the expanded boots and become un-removable. If  one engine is inoperative, the loss of its pneumatic pressure  does not affect boot operation. Electric power to the boot  system is required to inflate the boots in either single-cycle  or manual operation. When electric power is lost, the vacuum  holds the boots tightly against the leading edge.