Aircraft Engine Fire Detection & Protection Systems
In the realm of aviation, fires are a very rare occurrence. With highly advanced systems and intense testing of the aircraft engine and components, most fires are preventable altogether. Nevertheless, it is still always a possibility for accidents or malfunctions to happen, whether caused by the spilling of oil or a leakage. Because of this, aircraft feature a robust fire protection system to detect aircraft engine fires as soon as they occur. Two of the most common means of fire protection is the spot detector and continuous loop system, each containing their own types. In this article, we will discuss the fire detector systems present in aircraft, and how each function.
Four types of spot detector systems that are present across aircraft include the thermal switch, thermocouple, optical fire detection, and pneumatic-based thermal fire detection systems. Systems such as the thermal switch and thermocouple systems are widely featured on a number of old aircraft and utilize sensors to detect aircraft engine fires. The thermal switch fire protection system consists of indicator lights that are connected to a number of thermal switches. The thermal switches sense heat and complete the circuit when a certain temperature threshold is reached, causing the indicator lights to illuminate and warn the pilot.
Unlike the thermal switch system, thermocouple systems detect fires based on the rate in which temperature increases within the system. This is achieved with the use of a “hot junction”, which is the point in which two metals are conjoined and are exposed to potential fires to serve as the fire detector. A similar junction, the reference junction, is placed between insulation within a space of dead air. As normal operation of the engine can cause great amounts of heat, the indicator light circuit is only completed if the rate of heat increase between the hot and reference junctions is different.
Both the optical and pneumatic-based thermal fire detection systems have a much simpler operation than the thermal switch and thermocouple systems. With the optical fire detector, engine fires are detected by spotting infrared (IR) or ultraviolet (UV) radiation emissions that are produced by hydrocarbon flames. Pneumatic-based thermal fire detectors, on the other hand, consist of enclosed helium that increases in pressure directly caused by rises in heat, eventually causing a switch to close and alarm the pilot when heat reaches a certain level.
For larger, commercial aircraft, continuous loop detector systems are the most widely used for fire protection and include those such as the Fenwal and Kidde systems. The fenwal system features a control unit with various attached inconel tubes containing nickel conducting wires and eutectic salt that is thermally sensitive. Fire is detected in a Fenwal system when there the eutectic salt resistance drops due to excessive heat. This results in the current within the wires to run between the wire and the outer sheath, causing the control unit and relay to alert the pilot. Within the Kidde system, two conducting wires surrounded by thermistor are sheathed. One wire connects to the fire detection control unit, while the other is grounded and attached to the tubing. During a fire or overheat condition, electrical resistance drops until the fire detection control unit alerts the pilot. Due to the control unit monitoring the rate of change, it can discern between a fire and a short.
When a fire occurs, it is important that the pilots or crew can resolve the situation as quickly and safely as possible. Many aircraft have a fire extinguishing system, and there may be multiple extinguishers in different locations of the aircraft. Fire extinguishers contain discharge valves and a discharge indicator to help fight the fire and know when the container has emptied. Fire switches are also useful to shut down the engine and starve the flame, aiding in mitigating damage immediately. With a proper fire protection system in an aircraft, maintaining safety and fighting fires becomes easy.
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