The aircraft ice and rain protection system has three pneumatic components being:
- The wing and empenage surface deice system,
- The windshield anti ice system, and
- The engine inlet anti ice system.
The wing and empenage surface de icing system is used to break up ice that has built up during flight in icing conditions on the wings, vertical stabiliser and horizontal stabiliser.Bleed air from both engines is routed to rubber boots mounted on the leading edges of these surfaces which expand when activated to increase the surface area and break off any accreted ice.
Surface De ice System
The pneumatic de ice boots are activated by a SURFACE DE ICE switch located on the left hand switch panel next to the pilot. The system utilises both vacuum and air pressure provided by bleed air. Vacuum pressure is normally applied to the boots to aid in holding them against the wing and stabiliser surfaces during flight.
Surface De ice switch
When the SURFACE DEICE cycle is activated bleed air pressure is directed to the vertical and horizontal stabiliser surfaces first and inflated to 11 PSI which will then activate a SURFACE DEICE light on the annunciator panel. This process for the inflation of the empenage boots takes approximately six seconds.
SURFACE DEICE light illuminated
Bleed air from the engines is then routed to the wing deice boots for inflation. Once again the SURFACE DEICE light will illuminate on the annunciator panel with the wing boot inflation process taking approximately six seconds. The SURFACE DEICE light will illuminate twice in a normal deice boot cycle.
The surface deice boots should be used when ice accumulates between ¼ - ½ inch thick. A normal surface deice cycle will take just over 90 seconds to complete from switch activation and will be accompanied by the SURFACE DEICE light illuminating twice during a normal cycle.
The SURFACE DEICE switch is a three position switch which is spring loaded to the centre 'OFF' position. The other two positions are RESET and SURFACE BOOTS with the latter being the position for activating the system. RESET is used to terminate a cycle should it become necessary.
The RESET position of the SURFACE DEICE switch should be used if the SURFACE DEICE light on the annunciator has illuminated for more than 30 seconds during a deice cycle. Should this scenario ever occur recycle the deice boots ONCE and then leave icing conditions as soon as possible if the problem persists.
If less than 11 PSI is available to the surface deice system the SURFACE DEICE light on the right hand annunciator panel will not illuminate. If this scenario is experienced icing conditions should be exited as soon as possible. Correct function of the surface deice boots should be checked during the Pre Takeoff checklist.
The surface deice system should not be operated in OAT temperatures less than minus 40 degrees Celcius as damage to the rubber boots may occur. The surface deice boots have also have a special electrically conductive coating to allow static electricity to bleed off in order to reduce radio interference.
Activation of the surface deice boots will also increase stall speeds slightly. Therefore if activation of the surface deice system is required during approach to land, extra airspeed should be used, to allow for the increase in stall speeds.
The windshield anti ice system uses engine bleed air from both engines to prevent ice accumulation on the pilots windscreen and, as an option, on the co-pilots windscreen. The windshield anti ice system is designed to be turned on prior to entering icing conditions in flight.
The windshield anti ice system is controlled by the WINDSHIELD switch located on the left hand switch panel besides the pilots leg. When the switch is turned on, bleed air from both engines is directed to a heat exchanger in the nose where the temperature is reduced before being directed to outlet nozzles mounted at the bottom of the perspex windscreen.
The WINDSHIELD anti ice switch
Windscreen anti ice bleed air from the left and right hand engines
Temperature of the bleed air from the heat exchanger is regulated at 135°C by a motor driven valve located in the ram air duct in the nose of the aircraft. If the temperature of the bleed air exceeds 157°C a solenoid valve automatically shuts off the flow of bleed air to the windscreen and illuminates a W/S AIR O’HEAT light on the left hand annunciator panel.
The WINDSHIELD O'HEAT light
The system may sense an overheat condition and cycle off and on once after initial activation. If the system cycles off and on twice, the WINDSHIELD switch should be turned off. Failure to do so will result in continuous cycling of the windshield anti ice system.
If the temperature regulating valve and the solenoid valve both fail, a third temperature valve opens at 165°C which will divert the bleed air allowing a control valve to close blocking the flow of bleed air to the windscreen. Note: Trust this system as far as you can kick it as these valves are not used that often and have failed in the past.
Windshield Anti ice system
An alternative procedure to reduce the flow of bleed air to the windscreen may also be used. This procedure was mentioned in section 8, the pneumatics section of this course and is reiterated here. If the windscreen anti ice safety valves fail with the windscreen heat ON, hot bleed air will flow over the perspex windscreen eventually leading to the windscreen melting. To aid in the control of the flow of bleed air to the windscreen the 'Right hand' fuel fire wall shutoff circuit breaker can be pulled followed by pressing the Right hand F/W SHUTOFF PUSH button. This will not terminate the flow of bleed air completely but may reduce the volume of air flow to the point that failure of the windscreen will be delayed.