Ice Protection

Propeller and engine de-ice & anti-ice protection systems

Pneumatic Ice Protection Systems

Airfoil deicing is accomplished via inflatable boots on the leading edges of the wings and stabilizers. Precooled and regulated engine bleed air is used to provide the pressure boot operation. The bleed air is controlled by normally open shutoff valves that are operated with the AIR supply switch located on the overhead panel. From the shutoff valve the air is ported to a pressure regulator which reduces the air supply to 8 psi. The pressure regulator also provides pressure relief at 25 psi in the event that the pressure-regulator valve fails in the open position. A check valve is incorporated to prevent reverse flow during single engine operation. From the pressure regulator the air is ported to a common supply manifold which supplies the three distribution valves located in each nacelle and the vertical fin. These valves are solenoid-actuated and arc normally deenergized to the closed position. Incorporated in each distribution valve is an air ejector which allows a small portion of regulated air to be ported overboard when the valves are closed. This provides suction that prevents the boots from automatic inflation due to aerodynamic forces.

Boot De Ice System Components

Shutoff Valve

A shutoff valve is located in each engine compartment downstream of the precooler. They are solenoid-operated valves deenergized to the open position.

Pressure Regulator

A pressure regulator is located downstream of each shutoff valve. It reduces pressure to 18 psi for deicer boot operation and provides pressure relief at 25 psi if a valve should fail open. Each valve also incorporates a check valve to prevent reverse flow during single-engine operation.

Temperature Sensor

An overheat temperature sensor is located just below the firewall in the supply line to the distribution valve. The switch is set for 150° C. When activated, it provides a signal to illuminate the DE-ICE OV TEMP light on the boot control panel. The ICE PROT and MASTER CAUTION lights also illuminate, accompanied by a single-stroke chime.

Distribution Valves

There are three distribution valves, one in each engine nacelle and one in the tail section for the stabilizer boots. These solenoid-operated valves are normally closed. When energized open, they deliver the air supply to the boots. The valves also incorporate ejectors that pass a small amount of air to provide the suction that prevents autoinflation. The pressure sensors that activate the timer light are located in the valves.

Pneumatic Boots

All boots are made of synthetic fabric and neoprene and are cemented to the leading edges. All tubes in a given boot inflate simultaneously.

Timer Control Unit

The Timer controls the boot inflation sequence via single cycle and continuous modes. The timer warning light illuminates if boot inflation pressure is absent at the outlet of a distribution valve less than four seconds after the boots have been turned on. The light also illumimates if the activated timer fails to issue an inflation signal within a three-minute period. The manual override switches operate a parallel circuit that bypasses the timer unit. This unit is located under the floor in the forward cabin. With Mod 1784 installed, to monitor the boot deice system a TIMER light comes on should any of the following faults be detected:

• No pressure is sensed downstream of the valve sequenced for opening within four seconds.

• The activated timer gives no inflation signal.

• The boots are not cycling.

• Pressure remains on (above 10 psi) in the stabilizer boot zones after more than eight seconds (AUTO CYCLING switch is in CONT).

• Pressure remains on (above 10 psi) in the stabilizer boot zones when next cycle is activated (AUTO CYCLING switch is in ONE CYCLE).

• Pressure remains on (above 10 psi) in left and right inboard wing or left and right outboard wing boot zones after more than 8 seconds.

• Control power to timer is lost.

In CONT or one cycle or by manually over riding the timer, the inflation sequences can be monitored by following the illumination of the green indication lights, which will come on whenever the respective boot zone is pressurized if the BOOT IND switch is in the ON position. In the OFF position no illumination will occur.


An engine anti-ice system prevents ice formation on certain areas of the engine where ice buildup otherwise could be expected. These areas are the inlet lip, intake ducts, including the inlet protection device (IPD) with exhaust nozzle, which are electrically heated, and the split lip and inlet guide vanes, which are heated with bleed air.Both the electrical and the bleed air parts of the system are controlled by the same L and R ENGINE anti-ice switches.


The variable geometry system of the CT7 engine includes: the stage 1 and stage 2 variable vanes of the compressor casing, inlet guide vanes (IGV's) in the IGV casing, lever arms attached to the Individual vanes, and three actuating rings (one for each stage). The actuating rings, levers, and vanes are moved and synchronized by the crankshaft assembly which 1s positioned by an actuator piston within the hydromechanlcal unit (HMU). This piston is in turn positioned by a fuel pressure operated servo system with feedback which responds to compressor or gas generator speed (Ng), compressor inlet temperature (T2) and physical position of the variable geometry actuator.

At maximum power, the variable stators are positioned to their farthest open condition to admit the greatest airflow to the engine. At this time, the overboard sleeve in the anti-ice start bleed valve (AI/SBV) is fully closed so that all the compressor discharge air is delivered to the combustion and turbine sections. When less than maximum power is required and the compressor speed (Ng) is less than 100%, the pumping characteristics of the individual compressor stages are changed. Air pumping capacity is higher in the forward stages of the axial compressor than in the aft stages. To remedy this condition, the CT7 variable geometry system acts by closing down the variable stators in the forward portion of the compressor. Similarly, changes in T2 affect the compressor by closing the variable stator vanes with increasing T2, and opening the vanes with decreasing T2. At compressor speeds below 90%, the HMU actuating system also positions the overboard and anti-ice sleeves to the open position.


The anti-icing and starting bleed functions are accomplished by a single, combined valve assembly which is mechanically operated by the variable geometry mechanism to provide a starting bleed function and a modulated anti-icing function. This modulated function is actuated by a cockpit switch. The valve is a mechanically operated modulating valve with provisions for a solenoid controlled secondary metered flow path, and external anti-ice position indicator. It consists of an actuator piston that controls secondary metered flow, and an overboard/anti-ice sleeve that is connected to a mechanical input clevis. This mechanical connection is attached to the crankshaft assembly and controls bleed and anti-ice flow. Starting bleed modulation is thus controlled as a function of Ng and T2. The bleed flow exits the valve through the overboard port and enters the inlet particle separator (IPS) duct. Anti-icing air exits the valve through the anti-ice port and enters the anti-icing duct.

Anti-Icing is accomplished by hot stage 5 discharge air ducted to the inlet frame splitter lip and the IGV's controlled by the anti-icing start and bleed valve. Additionally, heat transfer from the hot scavenge oil flowing through the Inlet frame struts and main frame scroll vanes provides constant anti-icng for the Inlet section. Anti-icing air is controlled by a solenoid valve located in the AI/SBV, operated by a switch in the cockpit. When the engine ant-ice switch is off, electrical power is on to the solenoid valve. With engine anti-ice on, the solenoid valve closes allowing chamber "A" to pressurize, keeping the actuator piston in the closed position and stopping secondary metered anti-icing airflow. A downstream facing probe provides clean air to the solenoid and chamber "A". When the cockpit switch is on, electrical power to the solenoid is off. With electrical power off, the solenoid valve opens to vent and allows chamber "A" pressure to bleed to ambient. The secondary metered pressure in chamber "B", acting on the actuator piston annular area, strokes the actuator open allowing anti-icing air to flow. With the actuator piston in the open position, anti-ice flow plus secondary metered flow, which is established by the mechanical input position, is routed to the anti-ice port, providing additional solenoid controlled anti-ice mode flow. An electrical position switch provides an indication of piston position to the airframe fault logic system.

During engine starting and low power operation, bleed air is controlled by the position of the overboard sleeve in the AI/SBV. The variable geometry crankshaft, controlled by the HMU actuator, positions the mechanical input shaft in the down position to physically hold the overboard sleeve valve open. However, as engine power is increased, the valve will close as the mechanical input shaft is pulled out of the valve and bleed air through the overboard port will reduce. Bleed air is closed at approximately 90% Ng. Anti-icing air is discharged through the anti-ice port in the AI/SBV. Air is ducted via external piping and internal passages to heat the inlet frame splitter lip. The air discharges from the splitter lip to rejoin engine airflow. Air is also piped to a manifold, formed by the inlet guide vane casing, where it enters each IGV through holes provided in the vane spindle. The air flows through the hollow IGV's and discharges into the engine airflow via trailing edge vane slots.

Switches and annunciator lights on the overhead ice panel:

• AIR SUPPLY switch—Deenergized and guarded to the ON position. Controls the air supply valves.

• BOOT DE ICE switch—Selects deicer boots for one cycle or continuous operation.

• BOOT IND switch—Normally in the ON position. Allows the respective boot monitor light to illuminate with boot inflation. The OFF position deactivates the lights.

• Manual Override switches—Spring loaded off. When depressed, the associated section of boots is inflated. Use of these switches bypasses the timer.

• TIMER light (amber)—Illuminates to provide a warning that no pressure is sensed at the outlet of a distribution valve four seconds after the boots have been turned on. The light also illuminates if the activated timer docs not produce an inflation signal in a three-minute period. A pressure sensing device is incorporated in each distribution valve. With Mod 1147 or Mod 1784 installed, the timer light will also illuminate if control power to the timer is lost and if the boot indicating switch is off.

• DE-ICE OV TEMP light—Provides a warning if the temperature in the supply line is in excess of 150° C.

• Three monitor lights (green)—Illuminate when the associated section of boots is inflated,

provided the indicator switch has not been positioned to OFF. The sequence of boot inflation is controlled in the automatic mode by a timer control unit located under the floor in the forward cabin. The sequence of boot inflation, depending on the modification, is as follows:

• Horizontal and vertical stabilizer

• Inboard wing

• Horizontal and vertical stabilizer

• Outboard wing

• With Mod 1147 installed, the inflation sequence is changed to: stabilizer, outboard wing, stabilizer, inboard wing. With Mod 1148, the boot sequence is changed to: stabilizer, outboard wing, inboard wing, stabilizer.

Each section inflates in sequence for six seconds (24 seconds total). If the control switch has been positioned to the continuous mode, there is a 156-second dwell period before the cycle is repeated. With the switch in the single-cycle position, the boots sequence just once. When the manual pushbutton switches are used, the selected boot section remains inflated as long as the switch is depressed.

Windshield Heating

The front and side windshields are heated with power from their respective 115v AC generator buses and is regulated by two controllers. Each controller regulates the heating of one front window and one side window on the opposite side. Over heating and fault protection is provided via two heat sensors (normal and overheat) on each pane. The window heat is applied gradually to minimize thermal stress to the panes.

A controller failure or an overheat condition results in the shutoff of heating to the related window(s). These malfunctions will cause a steady illumination of the related amber FRONT or SIDE lights on the overhead windows panel in addition to a CWP master warning and ICE PROT light. If a normal heating sensor fails, the overheat and underheat temperature sensors are used by the controller. In this scenario pane heating is window heating is applied until the overheat sensor triggers a shut off (riding the overheat control). Cycling the switches resets the controller. Windows are then heated again, which triggers the overheat sensor once again.

Side window heating is controlled by a 3 position selector (OFF, NORM, & HIGH). NORM is used for defogging and HIGH for de-icing. The NORM setting must be selected on for at least 7 minutes prior to going to HIGH to prevent thermal stress. The HIGH position is also inhibited on the ground. In late model Saabs, only ON or OFF side heat is installed.


The electrical inlet duct heating uses 115-VAC wild frequency, supplied directly from each engine's own AC generator only. Therefore, there can be no crossfeed from the other AC generator in case of malfunction. In the lower leading-edge section the duct is provided with a temperature control sensor, an undertemperature sensor, and an overtemperature sensor. These sensors are connected to an inlet duct heater controller located in the engine nacelle equipment compartment. The normal temperature control sensor has preset control levels for the inlet heat controller to "cut in" at 60° C (140° F) and to "cut out" at 80° C (175° F). A failure in the inlet

ice protection system is indicated by the L or R INTAKE light coming on. The light comes on if:

• The over temperature sensor detects a temperature exceeding 125° C (257° F), in which case the inlet heat controller will cut power to the heaters. (If the L/R ENGINE switch is left on, the light will go out when power is applied again at 40° C followed by a new overheat.)

• The under temperature sensor senses a temperature below 10° C (50° F) (inhibited for 25 seconds when switching on the system to avoid nuisance warnings)

• An open circuit is detected in one or more phases in the three-phase power supply system.