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BLEED-AIR LEAK DETECTION The S-3 aircraft bleed-air leak detection system consists of four loops of bleed-air leak sensing elements located close to the auxiliary power unit
(APU) duct, bleed-air ducts, and an engine start port leak detector located near the engine ground start port. Sensing elements are mounted between ducts and the aircraft structure because bleed-air temperature is sufficiently high to cause structural damage. Support clamps with quick-release fasteners are used to mount the sensing elements, which are protected by bushings. The bleed-air leak detector control is located in the lower section of the left load center. This system is a fire-detection type of system that responds to heat. High temperature causes a chemical reaction in the sensing element, which provides a ground for the warning circuit and turns on an indicator light on the annunciator panel. A test circuit is activated by the bleed-air leak detect switch on the eject panel located on the eyebrow panel. The bleed-air leak detection system is powered by single-phase, 400-Hz, 115-Vac power from the essential ac bus. The test circuit uses 28 Vdc from the essential dc bus. When the bleed-air leak detect switch on the elect panel is held in the TEST position, 28 Vdc is applied to the test relays. This completes the transformer circuit through the sensing elements to ground. The transformer conducts and applies a signal to a transistor circuit, which closes control relays. A ground circuit is completed through the control relay contacts to turn on the 1 BL LEAK, 2 BL LEAK, CAB LB LEAK, and APU BL LEAK indicator lights on the annunciator panel. When the momentary bleed-air leak detect switch is released to the OFF position, the annunciator panel indicator lights go off. In normal operation, when the temperature of a section or a short segment of the sensing element exceeds 255 F (107 C), the chemical in the tube conducts electric current, which completes a transformer circuit to ground similar to the test relay. The indicator light on the annunciator panel will come on to indicate the loop that is overheating. This system will function even if there is an open (break) in the loop. During test position, the open element will prevent the indicator light from turning on. The individual loops will function separately or simultaneously if a leak is located in an area common to two loops. This will allow the pilot to take action to minimize damage due to bleed-air leakage. The bleed-air leak detection system consists of a bleed-air leak detector control, bleed-air leak sensing elements, and an engine start port leak detector.
Bleed-Air Leak Detector Control The detector control is located in the lower section of the left load center. The detector control contains two modules with four electrical circuits. Each circuit has a test function and a control function. The test function verifies all sensing element loops, and the control function turns on a warning indicator light on the annunciator panel. Bleed-Air Leak Sensing Elements The sensing elements are metal tubes with center conductors isolated from the tubes by a solid chemical. The sensing elements are mounted between the bleed-air ducts and areas to be protected from very high temperatures that would develop if a duct were punctured or ruptured. The sensing elements are mounted within 2 to 5 inches of the duct. Engine Start Port Leak Detector The leak detector is mounted inside the right main landing gear door. The leak detector is a heat-sensitive element that completes a ground circuit when subjected to temperatures in excess of 255 F (107 C). Its purpose is to detect leakage from the engine ground start port in the event of a check valve failure. The leak detectors wired in parallel with the loop 2 (2 BL LEAK) sensing elements.
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