THERMOSTATS.- The wing leading edge pneumatic thermostat is installed adjacent to each modulating valve. (See fig. 3-2.) The thermostat controls air pressure on the modulating valve diaphragm, and thereby controls the valve opening.

The unit is composed of a probe and a valve assembly. (See fig. 3-3.) The probe is a core made of layers of high- and low-expansion material that is locked to a sliding piston. In addition, the piston contains an override spring and ball-type metering valve.

Airflow from the leading edge flows over the core and causes the materials to expand or contract. As temperature rises, the core pulls the piston and metering ball from the seated position. This allows pressure from the modulating valve diaphragm to vent. Increasing temperature causes more air to be bled from the diaphragm chamber. Because of spring action, the modulating valve moves toward the closed position. This restricts flow through the modulator valve and drops the skin temperature.

LEADING EDGE TEMPERATURE AND OVERHEAT CIRCUIT.- To monitor the overheat warning system, six skin temperature sensors (one in the inboard section, one in the center section, and one in the outboard section of each wing) form a part of an amplifier circuit. When the wing leading edge skin temperature rises in excess of 230F at any one or more sensors, the airfoil temperature control unit amplifier completes a caution light circuit, thus illuminating the leading edge caution hot light.

Also, there are three ducting overheat thermal switches installed in each wing and three installed in the fuselage adjacent to the bleed-air duct. These switches form a part of a loop that is connected to a signal light control assembly. When any one of the thermal leak detector switches closes, its respective caution light illuminates. Also, when the test switch is placed in the TEST position, both lights illuminate through their respective loop circuit.

The ducting overheat switches are single-pole, single-throw, explosiveproof, thermally actuated electrical switches with an integral temperature

Figure 3-3.-Wing leading edge thermostat. 3-4

sensing element. The switches sense still air temperature. The outboard leading edge overheat warning switches open at approximately 205F and close at approximately 220F. The other wing and fuselage overheat warning switches open at approximately 175F and close at approximately 190F.

High temperature within the leading edge is generally caused by bleed-air leakage or malfunctioning modulator valves. You can detect the portion of the leading edge that has the overtemperature by placing the rotary selector switch, located on the ice control protection panel, to the different sensor positions: INBD, CTR, and OUTBD. (See fig. 3-4.) The temperature at the selected sensor is then read at the indicator adjacent to the rotary switch. An excessive temperature reading on the indicator denotes a malfunction within the area being tested.

Operation

Figure 3-4, the ice control protection panel, shows a basic diagram of the wing deice system. Each engine is labeled by an engine number. Directly below each engine block (in the diagram) is an OPEN light that illuminates when the bleedair valve is open 2 degrees or more. The crossship manifold from the bleed-air valves goes to each modulating valve and the fuselage shutoff valves. The fuselage bleed-air shutoff valves are normally in the CLOSE position during normal deicing operation. The bleed-air pressure gauge reads cross-ship manifold pressure when one or both switches are opened.

A ground air-conditioning switch is located directly under the bleed-air manifold pressure gauge. Located above the switch is an annunciator light, which indicates VALVE OPEN when the ground air-conditioning valve is open. Either one or both fuselage bleed-air shutoff valves must be open to direct air to the ground air-conditioning unit.

A leak test switch is mounted on the upper right-hand side of the panel. This switch is used to determine if the leakage of the system is acceptable.

Three modulating valve control switches are located on the left side of the wing and empennage ice panel. The outboard switch controls the outboard modulating valve on the left and right wing, the center switch controls the two center ^{modulating valves,} and the inboard switch controls the two inboard modulating valves.

During normal operation of the deicing system, all four engine bleed-air valves are open to supply bleed air to the cross-ship manifold, and both fuselage bleed-air shutoff valves are closed.

Figure 3-4.-Ice control protection panel. 3-5

The modulating valves maintain a controlled flow of bleed air to the leading edge distribution system, and they are controlled by pneumatic thermostats. The complete system is monitored for hot spots by heat-sensing switches.

Before flight, the deicing manifold system may be tested for leakage. This leak testis performed by pressurizing the system: OPEN the No. 4 engine bleed-air valve; the Nos. 1, 2, and 3 engine bleed-air valves remain CLOSED, and both fuselage shutoff valves are in the OPEN position. When the bleed-air pressure on the bleed-air manifold reads 70 psi, the No. 4 engine bleed-air valve is closed and the leak test switch is actuated. As the bleed-air pressure drops, the time-delay relay will illuminate the ACCEPT light after an 8-second delay if the system is tight. The light will go out when the test switch is released.

Maintenance

The involvement of the AME2 and AMEC in the maintenance of the deicing system normally consists of troubleshooting. To troubleshoot intelligently, you must be familiar with the system.

In addition, you must know the function of each component in the system and have a mental picture of the location of each component in relation to other components in the system. This can be achieved by studying the schematic diagrams of the system.

As an aid, the aircraft manufacturer furnishes troubleshooting charts, which give recommended procedures to follow during troubleshooting. Figure 3-5 shows a deicing system overheat warning troubleshooting chart. This chart lists the most probable cause first and then branches to the next most probable cause. By following the recommended charts and procedures, you can save many valuable maintenance hours.