C. The master caution and overheat detector annunciator lights alert the crew to an overheat condition at either engine. The master caution and annunciator lights are controlled by the master caution light control section of the fire protection system module (Fig. 2). The overheat detector annunciator light is located at the left side and a master caution light is located at each side of the lightshield. Either master caution light may be pressed to reset the master caution system. The annunciator light panel may be pressed to recall the warning on the lightshield, if the fault still exists. For further information, refer to Chapter 33, Master Caution and Warning Lights.
6. Sensing Elements
A. Sensing elements provide overheat and fire detection at each engine. Elements are mounted above the engine on the horizontal firewall and on the bottom of the engine, and on some airplanes a third element is installed on the lower surface of each engine tailpipe. The sensing elements are attached by quick-release clamps. Each sensing element consists of two sensors mounted in a perforated metal tube. One sensor detects overheat temperatures and the other fire temperatures (Fig. 1).
B. The resistance of a sensor varies inversely as it is heated; hence, as sensor temperature is increased, it's resistance decreases. Each sensor is composed of two wires imbedded in thermister material which is encased in a heavy wall inconel tube for high strength at elevated temperatures. The electrical connectors at each end of the sensor are ceramic insulated. The inconel tubes are shrouded in a perforated stainless steel tube and supported by teflon-impregnated asbestos bushings at intervals. The shroud protects the sensor from breakage due to vibration, abrasion against airplane structure and damage from maintenance activity.
C. The resistance of a sensor also varies inversely with its length, the increments of length being resistances in parallel. The heating of a short length of sensor out of a given length requires that the short length be heated above the temperature alarm point in order that the total resistance of the sensor decrease to the alarm point. This characteristic permits integration of all temperatures through out the length of the installation rather than sensing only the highest local temperature.
D. The two wires encased within the thermistic material of each inconel tube form a variable resistance network between themselves, between the detector wire and the inconel tube, and between each adjacent incremental length of sensor. These variable resistance networks are monitored by the application of 28 volts dc to the detector wire from the detector control unit.
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May 01/99 BOEING PROPRIETARY - Copyright . - Unpublished Work - See title page for details. 26-11-0 Page 9
7. Operation
A. Normal Operation
(1)
Normal fire detection is automatic when power is supplied to the airplane and the fire detection, master warning and control, and master caution circuit breakers are closed. The overheat detector switch should be locked in the NORM position.
(2)
If a fire or overheat condition occurs in the engine area, the two sensors in a shrouded sensing element are subjected to a temperature rise. The fire and overheat sensors are connected to the associated fire and overheat temperature detector control card. The control cards are connected to the associated fire and overheat warning indicators through the overheat detector switch. The temperature rise will cause the resistance value of both sensors to decrease. At a predetermined resistance value corresponding to alarm temperature, the low temperature detection control card will trigger an overheat warning circuits. The circuits will activate the respective engine overheat warning light, release the lock on the engine fire switch and illuminate the master caution and overheat annunciator lights. The master caution and overheat annunciator lights may be extinguished by pressing the master caution light. If the warning is overheat only, no further indication will be received. If a fire condition exists, the temperature in the engine compartment will continue to rise causing the resistance of the high temperature sensor to continue to decrease. At a predetermined resistance value, corresponding to the alarm temperature, the high temperature detector control card will trigger a fire warning circuit. The circuit will activate the associated engine fire switch lights, release the lock on the engine fire switch, illuminate the red master fire warning lights and sound the alarm bell. The alarm bell may be silenced and the master fire warning lights extinguished when either the bell cutout switch or the lens cap of either master fire warning light is momentarily pressed. The associated overheat light will remain on. If an overheat condition exists, the applicable engine should be monitored and corrective action taken. When the fire warning lights come on, a fire is assumed to exist and should be extinguished. Refer to 26-21-0, Engine Fire Extinguishing System. When the fire is extinguished, the compartment temperature and sensor resistance values should return to normal, the fire and overheat signals removed and the fire warning lights extinguished.
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