In the modern and general aviation the fire suppression systems is divided into two subsystems – the Fire Detection System and the Fire Extinguish System.
Fire detection system
The aircraft systems that have fire detection are:
- Engines (Turbines)
- APU (Auxiliary Power Unit) – this is a small engine installed generally in the aircraft tail. It is used to start the big engines (Turbines) and provides electrical power and air conditioning mainly on the ground
- Cargo Compartment
- Avionic Compartment
- IFE (In-flight Entertainment)
- Lavatories
The fire detection installed on engines and APU are sensitive loops like protected wirings. This operates when it detects the temperature increasing and suffers variation in its material composition and sends an electrical signal to the smoke detector computer which informs the pilots.
The Cargo compartment has electronic detector boxes installed in the cargo ceiling. When there is signal of smoke, the detector feels the smoke density and then inform to smoke detector computer that transmit this information to the cockpit.
The Avionic and IFE compartments have similar smoke detector, when smoke is detected an electrical signal is also sent to the cockpit.
Fire extinguing system
Presentely the main fire-extinguishing agent used in all aviation systems is Halon (Bromotrifluoromethane).
Halon systems are the best for avation, but not for the environment or the ozone layer. Because of that, Halon has not been produced since the Montreal Protocol on ‘Substances that deplete the Ozone Layer’ was signed in September 1987. This is an international agreement banning the production of Halon.
There some alternative agents being studied in world to replace halon totally. At the moment there is only an alternative agent called FE36, but this one is a little heavier than halon gas and has been used in lavatory trash can only.
Generally, depending on the manufacturer, the fire extinguishing bottles installed in engines, APU and cargo compartments must be removed at established intervals for weight and hydrostatic testing. These scheduled maintenance intervals can be controlled in calendar days or in flight hours depending on the aircraft utilization.
Engine fire extinguishing operation
Considering an aircraft dual-turbine, generally there are two fire extinguisher bottles to support both engines. When the pilots are informed by the fire detection system that there is an engine fire, the pilot operates the fire extinguisher handle, which closes all sources such as electrical power, pneumatic and fuel, and further the pilot shoots the first bottle and waits to see if the fire has been extinguished. If the engine fire continues, then the pilot shoots the second bottle. In the meantime, the co-pilot has declared an emergency and prepares to land the aircraft.
Apu fire extinguishing operation
The APU is generally used on the ground, but can be used in flight as well. The operation to extinguish a fire in the APU is similar to the engines (Turbines), but generally there is only a small fire extinguishing bottle exclusively for the APU.
AVIONIC fire extinguishing operation
There is no fire-extinguishing bottle for this. The pilot controls the air ventilation / cooling in the avionics bay in order to suppress the oxygen.
IFE fire extinguishing operation
IFE is located in the passenger cabin to which the flight attendants have access. The fire extinguishing is done by portable fire extinguishers installed in the main points of the aircraft. The amount of portable fire extinguishers is proportional to the amount of passenger seats. This is regulated by the aviation authority.
Lavatories fire extinguishing operation
Although aviation authorities forbid smoke in the lavatories, the lavatory auto-discharge fire-extinguisher system comprises a fire extinguisher bottle installed in the waste disposal of each lavatory. It is charged with extinguishing agent and it is mounted with the discharge pipings extended into the lavatory waste compartment, above the waste container with the pressure gauge visible when the waste disposal door is open. When the temperature inside the waste compartment reaches predetermined temperature (about 77 degrees Celsius), the extinguishing agent is discharged to put out the fire. The discharge piping controls the direction of the flow of the extinguishing agent. See photo below.
These auto-discharge bottles have to be removed or weight checked generally every year and they also have a limited life span after which they will be discarded.
Cargo Compartment fire extinguishing operation
Generally there are two fire extinguishing bottles for aircraft cargo compartment (for example: E190, A320, B767 etc).
Some old aircraft designs, for example FOKKER 100, did not have the suppression system installed in the cargo compartment, this means, neither has fire detection nor fire extinguishing, but to comply with new requirements from FAA/EASA/ANAC and others, the operators of Fokker 100 had to embody the suppression system. It happened around 2005. Nowadays the new projects must born with fire suppression in the cargo compartment, for example, the new design like A350, E190E2, B787 etc.
When smoke/fire is detected in cargo, initially the pilots discharge the first bottle which completely fills the affected cargo with Halon gas, then after that they deploy the second bottle. This second bottle releases the Halon in a controlled manner in order to keep the Halon concentration around 3%. This concentration of 3% to keep the oxygen out takes around 1h or more depending on aircraft certification.
In the meantime, the pilot has requested permission to land urgently at the closest airport.
Cargo Compartment classification regulations
In order to establish appropriate requirements for fire protection, a system for classification of cargo or baggage compartments was developed and adopted for transport category airplanes. Classes A, B, and C were initially established; Classes D, E, and F were added later. Class D has since been eliminated from the regulations. The classification is based on the means by which a fire can be detected and the means available to control a fire.
A Class A compartment is located so close to the station of a crewmember that the crewmember would easily discover the presence of a fire. In addition, each part of the compartment must be easily accessible so that the crewmember could quickly extinguish a fire with a portable fire extinguisher. A Class A compartment is not required to have a liner.
A Class B compartment is accessible in flight but more remote from the crewmember’s station than a Class A compartment must, therefore, incorporate a smoke or fire detection system to give warning to the pilot or flight engineer station. Because a fire would not be detected and extinguished as quickly as in a Class A compartment, a Class B compartment must have a liner. In flight, a crewmember must have sufficient access to a Class B compartment to effectively fight a fire in any part of the compartment with a hand fire extinguisher when standing at the compartment’s one designated access point, without stepping into the compartment. Therefore, Class B cargo compartments are limited to the main deck. There must be means to ensure that, while the access provisions are being used, no hazardous quantities of smoke, flames, or extinguishing agent will enter areas occupied by the crew or passengers.
A Class C compartment differs from a Class B compartment in that it is not required to be accessible in flight and must, therefore, have a built-in fire extinguishing or suppression system that is controllable from the flight deck. A Class C compartment must have a liner and a smoke or fire detection system. There must also be means to exclude hazardous quantities of extinguishing agent and products of combustion from occupied areas.
A Class D compartment was one in which a fire would be completely contained without endangering the safety of the airplane or the occupants. A Class D compartment was not accessible to a crewmember in flight, and instead of providing smoke or fire detection and extinguishment, Class D compartments were designed to control a fire by severely restricting the supply of available oxygen. Due to several uncontrollable fires in Class D compartments, they were eliminated. Class D cargo compartment standards pre-dating Amendment 25-93 are inadequate. Therefore, the FAA does not approve new Class D cargo compartments even in airplanes with a certification basis pre-dating Amendment 25-93.
A Class E compartment is found only on an all-cargo airplane. Typically, a Class E compartment is the entire cabin of an all-cargo airplane; however, other compartments (e.g., lower-deck, inaccessible cargo compartments) of such airplanes may be classified as Class E compartments. Shutting off the ventilating airflow to or within the compartment controls a fire in a Class E compartment. A Class E compartment must have a liner and a smoke or fire detection system installed. It is not required to have a built-in fire suppression system.
A Class F compartment must be located on the main deck of the airplane. Class F compartments must have means to control or extinguish a fire without requiring a crew member to enter the compartment. Class F compartments that include a built-in fire extinguishing/suppression system or require the use of fire containment covers (FCCs) or fire resistant containers (FRCs) meeting the criteria identified for these components.
FAA introduced the Class F cargo compartment as a practicable and safe alternative to the previous practice of providing large Class B cargo compartments. Class B compartments are limited to the main deck for accessibility reasons. Likewise, Class F cargo compartments must be limited to the main deck for accessibility. All lower-deck cargo compartments in airplanes carrying passengers must comply with the Class C cargo compartment.
Portable fire extinguishing operation
The portable fire extinguishers are used by crews (pilot and flight attendants) to extinguish any fire in the cockpit and passenger cabin. Some executive aircraft whom baggage compartment is accessed from passenger cabin also use the portable fire extinguishers for fire suppression.
For more information, go to www.faa.gov/airports/resources/advisory_circulars/index.cfm/go/document.information/documentNumber/25.857-1
