Because fire is one of the most dangerous threats to an aircraft, the potential fire zones of modern multiengine aircraft are protected by a fixed Fire Protection System.
Fire protection systems on aircraft usually consist of two separate operating systems. One system is for fire or overheat detection and the other is for fire suppression or extinguishing. In some cases, the systems can be interconnected so extinguishing takes place automatically after fire is detected.
Classes of Fires
- Class A – Fires involving ordinary combustible materials, such as wood, cloth, paper, rubber, and plastics.
- Class B – Fires involving flammable liquids, petroleum oils, greases, tars, oil-based paints, lacquers, solvents, alcohols, and flammable gases.
- Class C – Fires involving energised electrical equipment in which the use of an extinguishing media that is electrically nonconductive is important.
- Class D – Fires involving combustible metals, such as magnesium, titanium, zirconium, sodium, lithium, and potassium.
Requirements For Fire Protection Systems
- The fire protection system must provide an immediate warning of fire or overheat by means of a red light and an audible signal in the cockpit or flight compartment.
- The system must accurately indicate that a fire has been extinguished or indicate if a fire reignites.
- The system must be durable and resistant to damage from all the environmental factors that may exist in the location where it is installed.
- The system must include an accurate and effective method for testing to assure the integrity of the system.
- The system must be easily inspected, removed and installed.
- The system and its components must be designed so the possibility of false indications in unlikely.
- The system must require a minimum of electrical power and must operate from the aircraft electrical system without inverters or other special equipment.
Fire Extinguishing Agents And Portable Fire Extinguishers
Fire-extinguishing agents are those chemicals that are injected into the compartment or area to extinguish a fire. These agents work by either displacing the oxygen or chemically combining with the oxygen to prevent combustion. Some additional extinguishing effect can occur by the low temperature at which the agents are discharged.
The A-320 has:
- Fire detection and extinguishing systems for the Engines and APU
- Smoke detection in the avionics bay
- Smoke detection and fire extinguishing systems in the cargo compartments and lavatories
- Portable fire extinguishers for the flight compartment and the cabin
- The Engines and APU have individual fire detection systems
Each system consists of:
- Two identical detection loops (A and B) mounted in parallel
- A Fire Detection Unit (FDU)
Fire Sensing Element in Engine:
- Pylon nacelle
- Engine core
- Fan section
Fire Sensing Element in APU:
APU Compartment – Each engine has an identical fire detection system, comprising two parallel detection loops monitored by a fire detection unit (FDU). Normally, both loops must indicate a fire to produce a warning. The loops are monitored and automatically disabled if they malfunction. Loss of a single loop produces a level 1 ECAM warning, loss of both loops, or loss of an FDU, leads to a level 2 ECAM warning indicating loss of fire detection capability on that engine. If both loops break within 5 seconds, a fire warning is triggered. Each engine has a guarded fire button on the overhead fire panel, this lights up to provide fire indications. When pushed, the fire button pops out as a physical indication of activation, pushing the fire button arms the squibs and closes the following on the affected engine:
- Pneumatic bleed valves
- Pack valves
- Fuel valves
- Hydraulic valves
The APU has an identical fire detection to the engine.
Each engine has two fire bottles and the APU has a single fire bottle. These are discharged by pressing one of the AGENT buttons situated near their respective fire buttons. The APU fire extinguisher may also be discharged automatically in the case of an APU fire on the ground. Secondary engine fire indications are provided by fire lights on the pedestal ENG panel, and a secondary external APU fire indication, together with a guarded APU SHUT OFF button, is provided on the EXTERNAL POWER panel. Once a fire is extinguished, the light in the fire button will extinguish.
Test buttons for each system are situated near their respective fire button.
There are two detector loops for the Cargo compartments. Each loop has two detectors in the aft cargo compartment and one in the forward. A Smoke Detection Control Unit (SDCU) receives indications from these detectors and forwards them to the FWC which displays warnings on the CARGO SMOKE panel. If smoke is detected, the inlet and exhaust valves for the affected cargo bay are automatically closed. A single fire bottle is provided for both compartments. The aft compartment has two nozzles, and the forward compartment only one. When a DISCH button is pushed on the CARGO SMOKE panel, the bottle is completely emptied into the selected compartment. Smoke warnings will not extinguish once a fire is extinguished as the smoke will remain isolated in the cargo compartment and the smoke detectors are sensitive to the extinguishing agent.
A smoke detector is provided for the avionics in the air extraction duct. Each lavatory has a smoke detector.
These are connected to another SDCU which transmits data to the Flight Warning Computer (FWC) and Cabin Intercommunication Data System (CIDS). Each lavatory waste bin has an automatic fire extinguishing system.
For more tech talks on the Airbus 320, click here.