Fire Protection Systems

Where are fire protection systems built-in to aircraft?

In designated fire zones and potential fire zones

What are the requirements for a fire detection system?

- Warns immediately of fire/over heat

- Red light and audible signal

- Indicates extinguished fire

- Indicates re-ignition of fire

- Shows location

- Durable and resistant to damage

- Auto re-sets when fire is extinguished

When would smoke and toxic gas detection be used?

When type of fire anticipated is expected to produce more smoke before sufficient temperature change

What will a smoke and toxic gas detection system be?

Generally in cargo/baggage hold and lavatory

- Light refraction type (photoelectric cell)

- Ionisation type

- Solid-state type

What is the light refraction type of smoke detector?

Smoke interferes with light beams and reflects it into a lens and photocell which sets off alarm

What is the ionisation type smoke detector?

Ions align creating a circuit with electricity from battery. As smoke particles enter alarm, ion path is broken up and weakens circuit, setting off alarm

Where would detection systems typically be located?

- Powerplant

- Cargo hold

- APU

- Lavatory

What are the three systems for fire detection?

Thermo-switch

Thermocouple

Tubular type

- Fenwal

- Kidde

- Pneumatic type (Systron-donner)

- Lindberg system

What is a thermo-switch system?

Aka a spot detector, a circuit with one or more thermal switches connecting in parallel. Outer casing expands with heat (bimetallic design) and elongates, the two contacts inside then meet and close the circuit activating the warning system. Will reset itself

What is a thermocouple system?

Two dissimilar metals with one end fused together, placed in an open frame, as airflows over the probe, the exposed wires at one end create a hot junction and the other end is under insulating material which creates a cold junction. The temperature difference creates a current which sets off a sensitive relay. If temperature rise is slow, cold junction heats up along with the hot junction

What is a fenwal system?

Continuous loop, small, lightweight, flexible inconel tube, pure nickel wire centre conductor, space between conductor and tubing wall has ceramic insulating material and eutectic salt packing which has a low melting point. When heated, current flows between the wire and the tube wall (salt melts and drops resistance) which signals control unit. Will correct itself once the fire is out

What is a kidde system?

Continuous loop system, inconel tube with thermistor material embedded with two electrical conductors. When heated, thermistor drops resistance, sensed by electronic control circuit monitoring system (wire conductors). Second wire is the grounding wire, grounded to outer shell

What is the pneumatic system (systron-donner)?

Very similar to Lindberg pneumatic fire detection system

Continuous-length system, stainless steel tube with two separate gases and a gas absorption material (wire). Tube will normally have helium gas under pressure. Titanium centre wire has hydrogen gas, wrapped in helical shape. Gaps between turns release hydrogen gas under certain temperatures and once corrected, titanium reabsorbs hydrogen. Constant volume means pressure increases with temperature (helium gas in tube exerts more pressure and closes pneumatic switch)

What is the role of fire extinguishing agents?

- Displaces oxygen (CO2/dry powder)

- Chemically combines with oxygen to prevent combustion

Why aren't dry chemical extinguishers used?

They cause damage by toxic and corrosive chemicals

What are the two types of airframe fire extinguishing systems?

- Conventional (older)

- High Rate of Discharge System (HRD System)

How does a conventional fire extinguishing system work?

Perforated ring and distributor nozzle, when activated, agent bottles are opened and gas flows through lines to selected engine, flowing out of the perforated ring to smother fire

What is an HRD system?

Spherical bottles under high pressure, actuated by explosive charges (squibs) to flood compartment and displace oxygen. Propels extinguishing agent at high speeds

How does an HRD system work?

Fire switch closed by crew member. Explosive charge (squib) at neck of bottle is detonated and a cutter driven through sealing disk in neck, instantly releasing extinguishing agent to area selected

How do you determine thermal discharge on an airframe extinguishing system?

Gauge on bottle visible through window on the outside of the aircraft, red blow-out disk or yellow disk being displaced indicates bottle cartridge has fired (red due to over heat or over pressure). When thermal discharge occurs, pressure in line blows out disk