Powerful Facts About Aircraft Fire Protection Systems

Table of Contents

All You Need to Know About Aircraft Fire Protection Systems

Needless to say, the outbreak of fires on aircraft can be catastrophic on varying levels. This is why all aircraft must have suitable fire protection systems to detect and protect the aircraft and its occupants from fire outbreaks.

Fire protection systems comprise fire detection and fire extinguishing systems. Before a fire can be extinguished, it must first be detected.

Therefore, these systems work hand in hand. For larger transport categories or military aircraft, these systems are elaborate. However, the chance of fire outbreaks in light aircraft is minimal, so only the engines of such aircraft typically have fire protection systems.

Aircraft Fire Protection Systems

In this article, I will talk about:

The types of fires that may break out on a plane.
Requirements for fire protection systems.

Types of Fire Outbreaks and Extinguishing Agents on Planes

There are four classes of fires that are likely to occur in aircraft. They are:

Class A- involves combustible light solids such as wood, paper, cloth, rubber, and plastics.
Class B- involves flammable liquids such as petroleum oils, tars, greases, oil-based paints, alcohols, and so on.
Class C- fires involving electrical equipment. Non-conducting extinguishing agents should be used for these.
Class D- involves combustible metals such as titanium, magnesium, sodium, lithium, zirconium, and so on.

These classes of fire have different contents for their fire extinguishers.

For the Auxiliary Power Unit (APU) and engines, CO₂ extinguishers are commonly used. It is used outside of the aircraft because it can be slightly intoxicating and suffocating when one is exposed to it for a few minutes. They are also used in the cargo/freight compartment since that is away from the passengers. CO₂extinguishers can handle both class B and C fires.

Methyl bromide (MB) fire extinguishers may also be used in the engine bay compartment. They are much more effective than CO₂, but they are also more toxic and so cannot be used in the cabin or cockpit.

Dry powder extinguishers are usually kept in handheld extinguishers in plane galleys. It is effective for Class A, B, and C fires.

However, a generally accepted extinguisher is used in some aircraft. This is the halon extinguisher. Although over the years, halons have been known to be harmful to the environment, their benefits as pertaining to aviation are more greatly considered.

Moreover, they have been modified recently to halocarbon clean agents which are less harmful. Halon is a choice chemical because it leaves no residue, does not conduct electricity, is not very toxic, can handle 3 classes of fires (A, B & C), and a relatively small amount is required to be effective.

Requirements of Fire Protection Systems

Aviation regulatory bodies expect that fire protection systems meet standard requirements such as:

No tendency to give false warnings.
Quick detection and an indication of a fire and its accurate location.
An accurate indication that a fire has been put out, as well as an indication if the fire is re-ignited.
Sustained indication for as long as the fire is still present.
A means of self-testing the detector system from the cockpit.
Damage resistance from exposure to oil, water, vibration, extreme temperatures, and poor handling.
Light-weight and easily mountable.
Direct electrical circuity that does not involve connection to the static inverters in aircraft electrical power systems.
Separate detector systems for each engine.
An audible alarm signal and functional cockpit light signal.
A minimum number of onboard handheld fire extinguishers, depending on the size of the aircraft, and some other factors.
Proper distribution of handheld fire extinguishers.
The minimum amount of extinguishing agent per extinguisher.
Minimal use of toxic gases.

The Components of Fire Protection Systems and Their Workings.

The fire protection systems are situated in areas of the aircraft that are likely to have fire breakouts due to excessive temperatures. The fire detection systems do not only detect fire but also overheating and smoke. There are fixed and portable components of the systems. The systems are set up thus:

Fire and overheat detection and extinguishing systems for the APU and engines.
Smoke detection and extinguishing systems for the cargo compartments and lavatories.
Smoke detection for the avionics bay.
Portable fire extinguishers for the flight compartment and passenger cabin.
Fire protection systems in bleed air ducts.

The systems will be explored according to the different sections where they are set up. The Airbus A320 aircraft will be used as a reference.

APU and Engines.

The combination of the extremely high temperatures at which these components operate, and the flammable liquids found in them makes the risk of fire very high. They have their own individual detection systems which function the same way. The components within these detection systems are:

Gas detection loops: there are two of them in a parallel configuration. The loops each comprise three sensing elements for each engine’s pylon nacelle, core and fan section, and one sensing element in the APU. The sensing elements send signals to Fire Detection Unit when there is heat.

Once both loops (A and B) detect a specific temperature, they activate the fire warning system. An electrical-related fault in any loop has no effect on the warning system as the loop is disconnected by the flight crew and the operative loop alone is made to trigger the warning signal.

If the loops detect an APU fire while the aircraft is on the ground, the APU is turned off automatically and the extinguishing agent is released.

The sensing systems function as thermistors, meaning their resistances decrease as the temperatures increase and increase as the temperatures decrease. They have the same relationship between their resistance and lengths.

A short length of the element is heated above the alarm temperature so that the temperature of the element then drops to the alarm temperature. 28 V DC is applied to the detector wire so that the resistance networks within the sensing elements can be monitored.

The fire detection unit (FDU): It uses logic to control the detection system. The input from both loops causes the FDU to generate a warning signal which is then transmitted to the information and indication systems in the cockpit. This setup can be seen in the figure below.

The AND gate changes to an OR gate when one loop is faulty. The FDU operates by means of an automatic ‘self-interrogation’ circuit in the loop. If one loop signals fire, the self-interrogation circuit checks if the other loop is operative.

If it is operative, the circuit will suppress the fire signal since the other loop should have signaled failure too. If the other loop is inoperative, it would not have signaled a fire, so the signal can then be accepted.

This process occurs in very little time so that there is virtually no delay. Another option is for a ‘flame effect’ whereby breaks occur in both loops less than 5 seconds after the other. Overheat warning signals are first sent so that appropriate actions can be taken to prevent fire breakout. When a fire is detected, another signal is sent.

As for the fire extinguishing systems, the engines each have two extinguisher bottles that have electrically controlled squibs. These squibs are responsible for discharging the bottles’ contents. Each squib has a dual electric supply.

The squibs’ discharge is controlled by the crew from the ENG FIRE panel. The APU’s system is similar, except that it has only one bottle and it is controlled from the APU FIRE panel. The bottle automatically discharges if there is an APU fire on the ground.

The extinguishers function with high discharge rates so that the extinguishing agents are delivered in seconds and the fire is eliminated quickly. The bottles contain a pressurized gas (typically nitrogen) and a halon agent.

A temperature-sensitive relief diaphragm prevents the pressure in the bottle from exceeding the bottle test pressure when the temperature gets extremely high. There is a discharge valve, which when open, lets the agent be released through the squib.

There is a red thermal disc on the body of the fuselage that lets the flight and maintenance crews know if the APU extinguisher bottle needs to be replaced while the aircraft is on the ground. The disc removes in the event of excess heat as the bottle’s contents are dumped through the relief valve fitted to the bottle.

Avionics Bay and its Normal Operation

Here, there is no actual fire that is detected or extinguished. Ventilation action is performed by the avionics ventilation system instead when smoke is detected. A smoke detector in the air extracting duct of the ventilation system detects the smoke. It then sends a signal to the ECAM (cockpit display) so that a warning is shown. After 5 seconds of smoke detection have passed:

A single chime sounds.
The amber MASTER CAUTION light comes on.
A warning memo is shown on the EWD.
The amber SMOKE light on the EMER ELEC PWR panel comes on.
The BLOWER and the amber EXTRACT fault light come on.

After 5 minutes of smoke detection, the caution can be cleared from the ECAM. However, the caution is still present and can be recalled later. On the ground, resetting both Flight Warning Computers (FWCs) will unlatch the caution.

The smoke detecting system is a chamber in which air flows between an LED source and a scatter detector photodiode. Typically, only a small amount of light gets to the scatter photodiode from the LED. If the air that reaches the detector photodiode has smoke in it, the smoke particles present will reflect more light on the detector photodiode. This triggers an alarm signal. An intensity monitor photodiode ensures the LED source stays on and regulates constant light output from it. That way, the contamination of the LED and detector photodiode can also be detected.

The smoke detector has numerous sampling ports from which the fans draw air through a water separator and heater unit.

Lavatories

The lavatory smoke detection system comprises:

Smoke detectors: there is one for each lavatory. They are powered by 28 V DC(electrical) buses. If smoke is present in the air which flows in their sensing chambers, the red LED light comes on while the timing circuit works intermittently. The smoke detection unit makes a ground for the relay which then sends a ground signal to the SDCU. The alerts come in form of master call light flashes, pop-ups on the relevant cockpit displays and a lavatory call chime that sounds. The smoke indications only go away when the smoke goes away.

Smoke Detection Control Unit(SDCU): it has two channels. Signals sent from the smoke detectors are processed and sent to the FWC, to alert the cockpit and the Cabin Intercommunication Data System (CIDS) to alert the cabin (lavatory and general cabin).

The fire extinguishing systems in the lavatory are found in the waste bins. The bottles are similar to other extinguisher bottles in the aircraft.

When the temperature in the waste bins is about 77 ̊C, the solder that seals the nozzles melts so that the extinguishing agent is released. The number of the bottle’s contents can only be identified by weighing it.

Cargo Compartments

The cargo compartments have different classes according to the ease with which fire can be discovered by the flight crew, the ease of access to it, and the effect of extinguishing it on the aircraft’s occupants.

Compartments that are not easily accessible or where discoveries cannot be easily made are fitted with their own detection systems and fixed extinguishing systems.

In the aircraft, there are forward and aft cargo compartments. The forward compartment has one cavity while the aft compartment has two cavities.

The smoke detection systems have:

Smoke detectors: there are 2 located in each cavity in the ceiling panels
SDCU receives signals and processes them based on the dual loop principle.

If there is a cargo ventilation system and smoke warning signals are sent, the isolation valve in the ventilation system closes automatically and the extraction fan ceases to spin.

There is one fire extinguishing system for both the forward and aft compartments. A single extinguishing bottle is used for 3 nozzles to supply each compartment with an extinguishing agent. There are two discharge heads, one for the forward compartment and the other for the aft.

When a member of the flight crew presses the DISCH pushbutton for any of the compartments, the corresponding squib is activated. The gas pressure in the bottle then forces the extinguishing agent out.

Following this, the activated pressure switch sends a signal to the information systems so that those in the cockpit can be alerted of the discharge. There are flow control valves that direct the flow of the extinguishing agent to the correct compartment.

Conclusion

The fire protection systems prevent fire breakouts in the likely parts of the aircraft to spread to the entire aircraft. This keeps the aircraft structure and occupants safe.

Smoke detection systems enable the ventilation systems to perform appropriate actions so that toxic fumes or smoke can be vented off the aircraft. Otherwise, this can suffocate the passengers and flight crew.

The fire protection systems have ample components in them that help to preserve systems associated with the protected components, for example, the hydraulic and fuel systems.

References

Youtube (2016). A320 CBT #37 Engine Fire Protection. Available at: https://www.youtube.com/watch?v=TBPHBievPD8&list=PLsb8vTC6R_yu3H9EgV-8Mx3zIYdF8EUZx&index=41
Youtube (2016). A320 CBT #37 Engine Fire Protection. Available at: https://www.youtube.com/watch?v=TBPHBievPD8&list=PLsb8vTC6R_yu3H9EgV-8Mx3zIYdF8EUZx&index=41
Federal Aviation Administration (FAA) (2018). Aviation Maintenance Technician Handbook-Airframe, Volume 2. Federal aviation administration.
Fire Protection. (n.d.). [ebook] Airbus training. Available at: http://www.smartcockpit.com/docs/A320-Fire_Protection.pdf
SKYbrary Aviation Safety. n.d. Aircraft Fire Extinguishing Systems. [online] Available at: <https://skybrary.aero/articles/aircraft-fire-extinguishing-systems> [Accessed 20 July 2022].
Aircraft Systems. 2017. Aircraft Fire Protection Systems. [online] Available at: <https://www.aircraftsystemstech.com/2017/04/aircraft-fire-protection-system.html > [Accessed 20 July 2022].
Experimental Aircraft Info. n.d. Fire Protection in Aircraft. [online] Available at: <https://www.experimentalaircraft.info/articles/aircraft-fire-protection.php#:~:text=Fire%20Protection%20in%20Aircraft,fuselage%20to%20protect%20the%20occupants.> [Accessed 20 July 2022].