Chapter 16 Fire Detection Book (Section A)

A fire must have what to ignite or continue to burn?

A fire must have oxygen, fuel, and enough heat to cause the fuel or oxygen to ignite. If any element is missing, the fire will not ignite or continue to burn.

How many classes of fires are defined by the National Fire Protection Agency and what are they?

Four classes of fires are defined:

• Class A

• Class B

• Class C

• Class D.

These classifications are based on the type of fuel involved in the fire.

What do Class A fires involve?

Class A fires involve ordinary combustible materials such as wood.

These are typically found in cabin or structural materials.

What do Class B fires involve?

Class B fires involve flammable liquids.

These fires commonly occur with fuels such as oil or other liquid combustibles.

What do Class C fires involve?

Class C fires involve energized electrical equipment.

These fires present a risk of electrical shock if not handled properly.

What do Class D fires involve?

Class D fires involve flammable metals such as magnesium.

These fires burn with high intensity and require special extinguishing methods.

How are aircraft compartments classified into fire zones?

Aircraft compartments are classified into fire zones A, B, C, D, and Z based on the amount and characteristics of airflow through the particular compartment.

What is a Class A fire zone?

• Has large amounts of airflow past similar obstructions

  • Example: power section of a reciprocating engine

• Airflow can carry extinguishing agent away from the fire

• Requires a system that can still be effective despite high airflow conditions

What is a Class B fire zone?

• Has large airflow over smooth, aerodynamically clean surfaces

  • Examples: heat exchangers, exhaust shrouds, turbine engine areas

• Surfaces are designed to be smooth, clean, and well drained

• Prevents flammable liquids from pooling

• Typically includes fire detection and extinguishing systems

What is a Class C fire zone?

• Has relatively low airflow

  • Example: APU compartment

• Fire is less likely to be blown around compared to A or B zones

• May include fire detection and extinguishing systems

• May also include features to isolate flammable materials like fuel or oil

What is a Class D fire zone?

• Has very little or no airflow

  • Examples: wing compartments and wheel wells

• Fire may self-extinguish due to lack of oxygen

• Usually no extinguishing system required

• Fire detection is often used to alert the crew of possible damage

• Important because damage (like tires or structure) can still occur even if fire goes out

What is a Class X fire zone?

• Has large airflow but irregular/unusual construction

• Contains deep recesses, pockets, and structural spaces

• Makes fire detection difficult

• Makes even distribution of extinguishing agent difficult

• Requires about twice the amount of extinguishing agent compared to a Class A zone

• Designed systems must overcome both airflow and complex geometry

What do modern overheat and fire detection systems consist of and what do they do?

Modern overheat and fire detection systems consist of electrical or electronic sensors in remote locations.

These systems warn the pilot of impending hazards by sounding an audible alarm and illuminating a warning light that identifies the location of the hazard.

What are the two primary types of engine fire-detection systems?

The two primary types are spot-detection systems and continuous-loop systems.

These systems differ in how they sense and monitor fire conditions.

What three types of systems does Kidde-Fenwal produce for fire detection?

Kidde-Fenwal produces thermoswitch, thermocouple, and continuous-loop systems for fire detection. Each system uses a different method to detect heat or fire conditions.

What is a Fenwal thermoswitch detection system?

A Fenwal thermoswitch detection system is a spot-detection system that uses bimetallic switches arranged in a single loop or double loop configuration to detect heat at specific locations.

What is an Edison thermocouple system?

An Edison thermocouple system is a spot-detection system that uses thermocouples to indicate a rapid rise in the surrounding air temperature, rather than a fixed temperature point.

What is a Fenwal continuous-loop detection system?

A Fenwal continuous-loop system uses a single fire or overheat sensing element made of small diameter INCONEL tubing. Inside is a center conductor and ceramic beads wetted with a eutectic salt that changes resistance when heated.

What is a Kidde system?

A Kidde system is a continuous-loop type system that uses a sealed INCONEL tube containing two conductors embedded in a thermistor material. The resistance changes as temperature increases to detect fire.

What is the Lindberg fire detection system?

The Lindberg system is a pneumatic continuous-loop system that consists of a stainless steel tube filled with an inert gas and a gas absorbing material that changes pressure with temperature.

What is the Systron-Donner system?

The Systron-Donner system is a pneumatic continuous-loop system that uses a stainless steel tube with a central titanium wire sensing element surrounded by helium to detect temperature changes.

What should you be alert for when inspecting continuous-loop fire detection systems?

You should be alert for sharp bends, kinks, crushed sections, and loose support clamps, as these can affect system performance and reliability.

What are flame detectors?

Flame detectors are photoelectric sensors that measure visible light or infrared radiation.

They send an alert warning when the intensity of light striking the sensor increases, indicating a flame.

How are smoke detectors classified?

Smoke detectors are classified by their method of detection, which determines how they sense the presence of smoke particles in the air.

What are typical smoke detector methods?

Typical methods include light refraction, ionization, and solid-state detection. Each method detects smoke using a different physical principle.

Where are carbon monoxide detectors typically installed and what do they detect?

Carbon monoxide detectors are typically installed in aircraft cockpits and cabins.

They are used to sense the presence of deadly carbon monoxide gas in the air.

What should always be used when inspecting and testing fire systems?

The manufacturer’s maintenance manual directives should always be used when inspecting and testing fire systems to ensure proper procedures are followed.

What are the most common problems associated with fire detection systems?

Intermittent or false alarms are the most common problems associated with fire detection systems, often caused by system faults or environmental factors.

Define active thermocouple

The active component of an Edison fire detection system that uses thermocouples to detect the presence of an overheat condition or a fire. Reference and test thermocouples are also used in this system to determine system functionality and integrity.

Define averaging function

A pneumatic, continuous-loop fire-detection system, provides two modes of operation, an averaging mode and a discrete mode. The averaging mode or function works according to the law of gases to provide a warning if the average temperature of the area where the loop is placed increases. As the area temperature rises, gas pressure in the loop will also increase. This pressure is enough to close the diaphragm switch in the responder unit and activate the warning system alarm.

Define bimetallic THERMOSWITCH®

A rugged, autonomous thermal sensor using two bonded, dissimilar metal strips that bend at specific temperatures due to differential expansion.

Define continuous-loop system

The fire detection function of a pneumatic, continuous-loop fire detection system. When the loop is exposed to a localized high temperature such as a fire or bleed air leak, the titanium wire inside the tube releases hydrogen gas. The released gas increases the total gas pressure, which closes the diaphragm switch and activates the alarm.

Define double-loop system

An aircraft fire detection system in which two continuous loops are installed side by side on the same support tube for system redundancy. Commonly referred to as Loop A and Loop B, each loop has it own control box and testing capabilities.

Define Edison

A fire detection system used in early air carrier aircraft. Edison fire detection systems are now obsolete, having been replaced by more conventional pneumatic responder systems.

Define electronic control unit (ECU)

1. A device in the engine management system that monitors, controls, and displays engine operating parameters.

2. A device in a Kidde continuous-loop fire detection system that constantly measures total system resistance and system redundancy.

Define eutectic salt

A chemical salt used in continuous-loop fire detection systems. The salt insulates the positively charged center wire from the sides of the negatively charged tube. When heated, the eutectic salt melts into a liquid, which lowers the resistance and allows the center wire to electrically contact the side of the tube—completing the circuit to ground and powering the alarm circuit.

Define integrity switch

A switch inside a Systron-Donner fire detection responder unit that is normally closed by the pressure of the helium in the sensor tube. The system test circuit is connected through this switch. If the helium escapes due to damage to the sensor tube, the system pressure is diminished, which opens the integrity switch. When this occurs, the system test feature will not work.

Define JETCAL® analyzer

A trade name for an electronic test apparatus for checking the calibration of the EGT system, the RPM system, and the accuracy of their associated instruments.

Define overheat function

1. A behavior of a hot bond machine that turns off or reduces heat to the vacuumed part during the curing operation.

2. A function in the fire warning system that causes the warning circuit to close so that the alarm will sound when abnormally high temperatures occur in an area of the aircraft.

Define overheat-sensing element

The sensing wire portion of a continuous-loop fire detection system.

Define pneumatic continuous-loop system

A type of fire detection system that uses compressed gas (such as helium) inside a small tube. When heated, the gas expands to trigger a pressure-sensitive switch that activates an alarm

Define reference thermocouple

In a thermocouple fire warning system, the thermocouple that is isolated from the active fire zones but still exposed to the same air temperature as the active thermocouples. When a fire occurs, the temperature differential between the reference thermocouple and the active thermocouples causes the current to flow and activate the warning light and horn.

Define relay box

In a thermocouple fire warning system, the component that houses the sensitive relay, the slave relay, and the thermal test unit. The thermocouples control the relays and the relays control the fire warning lights. Together, these components comprise one circuit. Each potential fire zone typically has between one and eight circuits.

Define responder

A type of pneumatic pressure switch used in a pneumatic, continuous-loop fire detection system. The responder is permanently connected to one end of the loop and is an integral part of the system; it cannot be removed in the event of a failure. It provides the connection to the control unit, which generates the alarm.

Define single loop system

A type of fire detection system made by Fenwal® that employs a number of thermoswitches that are wired in parallel with each other and in series with the indicator lights. Also referred to as a single loop or a continuous-loop fire detection system.

Define spot detection system

A type of fire detection system that uses individual fire detectors, each focused on one location or one section of an assembly. The detectors must be placed in locations where a fire is likely to occur because they can respond only when a fire is present in their immediate proximity. Spot detectors are also known as thermoswitches.

Define test thermocouple

A component of the Edison fire detection system test circuitry. This special thermocouple is paired with a small electric heater and wired into the detector circuit. When the test switch is pushed, the heater turns on and heats the test thermocouple. The temperature differential between the reference and the test thermocouple produces a current flow that closes a sensitive relay and a slave relay, which illuminates a warning light.

Define thermistor material

A type of material used by Kidde® in their continuous-loop fire detection system. A small inconel tube sensing element is filled with a thermal resistor material in which two conductors are embedded. As the ambient temperature increases, the resistance of the material decreases, which creates an electrical connection between the two conductors. Also known as thermistor material.

1. For a fire to occur, there must be:

a.

b.

c.

Fuel
Oxygen
A source of ignition (heat)

2. Name the NFPA class of fire that is described by each of these conditions:

a. A fire in which there are energized electrical circuits is a class ____ fire.

b. A fire with liquid fuel is a class ____ fire.

c. A fire in which there is burning metal is a class ____ fire.

d. A fire with solid combustibles as the fuel is a class ____ fire.

a. C

b. B

c. D

d. A

3. A wheel well or wing compartment would be considered to be a Class ____ fire zone.

D

4. The two general categories of engine fire detection systems are:

a. ______ systems.

b. ______ systems.

a. Spot-detection type

b. Continuous-loop type

5. The thermal switch fire detection system is a(n) ______-type system.

spot

6. Thermal switch fire detection units respond to a ______ (pre-set temperature or a rate-of-temperature-rise).

pre-set

7. Thermal switch fire detectors are connected in ______ (series or parallel) with each other.

parallel

8. A significant advantage of a double-loop detection circuit is that it can withstand a ______ such as a ______ or ______ circuit without causing a false fire warning.

fault, short, open

9. Thermocouple fire detection systems may also be known as the ______ fire detection system.

10. Thermocouple fire detection systems respond to a ______ (pre-set temperature or a rate-of-temperature-rise).

rate of temperature rise

11. In the thermocouple system, one thermocouple called the ______ thermocouple is placed in a location that is relatively well protected from the initial flame.

reference

12. The two relays in a thermocouple fire detection system are:

a. ______

b. ______

a. sensitive relay

b. slave relay

13. More complete coverage of a fire hazard area than is provided by a spot-detector system may be obtained by using a(n) ______ fire detection system.

continuous loop

14. The ______ (Fenwall or Kidde) continuous loop system uses two wires inside an INCONEL tube.

Kidde

15. A ______ (sensor-responder or continuous loop) fire detection system may also function as an overheat indicator.

continuous loop

16. The two types of sensor-responder systems that may be found in use are the:

a. ______ system.

b. ______ system.

a. Lindberg

b. Systron-Donner

17. What two gases are toxic to humans, may indicate a fire condition and may provide the earliest warning of a dangerous situation?

a. ______

b. ______

a. Carbon Monoxide

b. Nitrous Oxides

18. ______ type smoke detectors use a small amount of radioactive material to ionize some of the oxygen and nitrogen drawn into the unit.

Ionization

19. Three types of smoke detection units which may be found on aircraft are:

a. ______

b. ______

c. ______

a. The Light Refraction type

b. The Ionization type

c. The Solid-State type