In some aircraft, in addition to fire and overheat detection, the Kidde continuous-loop system can supply nacelle temperature data to the airplane condition monitoring function of the aircraft in-flight monitoring system (AIMS).įigure 8 shows a typical aircraft fire detection system in which a control module monitors two loops of up to four pneumatic detectors each, connected in parallel. The resistance decreases more quickly with an electrical short than with a fire. The rate of change of resistance identifies an electrical short or a fire. When the fire or overheat condition is gone, the resistance of the core material increases to the reset point and the flight deck indications disappear. If the resistance decreases more to the fire set point, a fire warning occurs. Typically, a 10-second time delay is incorporated for the overheat indication. If the resistance decreases to the overheat set point, an overheat indication occurs in the flight deck. The fire detection control unit monitors this resistance. As the temperature of the core increases, electrical resistance to the ground decreases. One conductor has a ground connection to the tube, and the other conductor connects to the fire detection control unit. Two electrical conductors go through the length of the core. In the Kidde continuous-loop system, two wires are imbedded in an inconel tube filled with a thermistor core material. If multiple open circuits occur, only that section between breaks becomes inoperative. In this case, should an open circuit occur, the system still signals fire or overheat. The Fenwal system may be wired to employ a loop circuit. When the fire has been extinguished or the critical temperature lowered below the set point, the Fenwal system automatically returns to standby alert, ready to detect any subsequent fire or overheat condition. This current flow is sensed by the control unit, which produces a signal to actuate the output relay and activate the alarms. When an overheat condition occurs at any point along the element length, the resistance of the eutectic salt within the sensing element drops sharply, causing current to flow between the outer sheath and the center conductor.
The control unit, operating directly from the power source, impresses a small voltage on the sensing elements. The elements may be of equal or varying length and of the same or different temperature settings.
Lengths of these sensing elements are connected in series to a control unit. The Fenwal system uses a slender Inconel tube packed with thermally sensitive eutectic salt and a nickel wire center conductor. The slave relay then closes and completes the circuit to the warning light to give a visual fire warning. This completes a circuit from the aircraft power system to the coil of the slave relay. Any time the current is greater than 4 milliamperes (0.004 ampere), the sensitive relay closes. The ensuing voltage causes a current to flow within the detector circuit. If there is a fire, however, the hot junction heats more rapidly than the reference junction. In the engine compartment, there is a normal, gradual rise in temperature from engine operation because it is gradual, both junctions heat at the same rate and no warning signal is given. If both junctions are heated at the same rate, no voltage results.
If the temperature rises rapidly, the thermocouple produces a voltage because of the temperature difference between the reference junction and the hot junction.
A metal cage surrounds the thermocouple to give mechanical protection without hindering the free movement of air to the hot junction. There is also a reference junction enclosed in a dead air space between two insulation blocks. The point at which these metals are joined and exposed to the heat of a fire is called a hot junction. The thermocouple is constructed of two dissimilar metals, such as chromel and constantan.