Fire detection and protection system of turbojet engine

P8. a. Describe the different Fire detection & protection systems used in Turbojet Engines. b. Write short notes on: i. Fire Zones ii. Ground fire protection iii. Fire Extinguishing system Answer. (a) Turbo jet engine fire protection system: Fire protection systems are installed on aircraft to detect and protect against an outbreak of fire. These systems near the engines and in the fuselage. These systems monitor the conditions which could lead to a fire and are comprised of smoke detectors, heat sensors near engines or hydraulic systems and visible and audible warnings in the cockpit. The chance of a fire in small general aviation or experimental type aircraft are very remote but at least the engine compartment should have some form of detection as this is the area where heat, air and a combustible fluid are together in close proximity. Fire detection: Automatic systems can detect aircraft fires or potential ignition which might not be apparent to the crew until they have developed to an extent which makes their successful control difficult, or impossible. These systems are based upon both heat and smoke sensing. Heat sensing is used for cargo holds, engines/APUs, toilet waste bins, high temperature bleed air leaks and landing gear bays. Smoke detection is used in toilet compartments, avionics bays and cargo holds. Normally, Alerts or Cautions are activated locally for toilet smoke detectors though in some types a toilet detector can trigger a FIRE warning on the flight deck. All other fire and smoke detector Alerts and Cautions are normally annunciated in the flight deck. In every case, it is important that crewmembers understand exactly what type of detection system is being used in which location in their aircraft and exactly what is being detected. Only with that understanding will they be able to know exactly what any warning system is telling them. Abnormal Heat detection in an engine compartment will require manual remote activation of locally sited extinguishers, as will such detection in an APU compartment in flight. However, such detection in an operating APU on the ground triggers automatic shutdown and extinguisher discharge. Fire extinguishing system: Activation of engine fire bottles is normally also advised as a precautionary measure whenever an engine shutdown drill is carried out because of severe damage. APU Fire extinguishers are activated by the flight crew in the same way as engine extinguishers - by manual selection upon receipt of a fire warning - when airborne, but automatically, and with accompanying automatic APU shutdown, in the case of fire detection during ground running. If indications are available that an airborne engine or APU fire has been extinguished successfully, then the fire occurrence itself does not then influence the flight crew decision on how urgent it is to land the aircraft. However, in a twin engine aircraft, the shutdown of an engine will usually result in landing. Turbo jet engine fire detection system: Detection is achieved by means of linear pneumatic sensing systems, often referred to as fire loops, which are gas filled pipes routed around potentially abnormal heat sources. If the temperature in the vicinity of the sensing element rises, the electrical resistance of the core material decreases and a warning indication can be triggered. They are duplicated to allow for continued detection if a single loop in the system becomes faulty. An open loop due to a short circuit fault will be detectable during daily preflight testi g or y a u iatio of a fault i servi e. Physi al da age to a loop su h as a pi hi g may lead to a false fire warning but would also produce an independent fault annunciation. The same principles apply to the protection of APUs. There are two basic type of unit fire detectors: thermocouple and a bi-metallic switch. They are describing into the below: Prepared by: Mushfiqur Rahman Batch: ND 4th ID No: 14060406 Thermocouple detecting system: A thermocouple consists of two dissimilar materials which will generate a small voltage when its heated. They are also used in exhaust gas temperature (EGT) or cylinder heat temperature (CHT) sensors. Bi- metallic detecting system: The bi-metallic switch detects a temperature change as each of the two dissimilar materials have a different expansion thereby deforming the metal arm and contacting a switch, thereby opening or closing it depending on the type of switch that is used. Fire can also be detected by infrared sensors connected to a circuit where a threshold is set to indicate a temperature that has gone up and action of the pilot might be required. Detection loop: A different method of fire detection uses a continuous loop device. This is a thin wire inside a capillary tube insulated with a thermistor type material. This material will become a conductor when a certain high temperature is reached. When the loop is triggered a small current will flow and this trips a detector circuit and warning device. After the temperature drops the wire will stop conducting and the system is reset and can be used again. Figure: Fire detection system. Prepared by: Mushfiqur Rahman Batch: ND 4th ID No: 14060406