Fire And Smoke Protectionsnd Detection Flashcards
Smoke detection
Often the earliest warning
Galleys, toilets and inaccessible compartments must be fitted with smoke detection
Two principle types of smoke detection: optical and ionising
Optical smoke detection
Smoke enters the optical chamber.
Smoke causes infrared light to be scattered onto the photodiode light receptor.
Ionising smoke detector
Small quantity of radioactive material in ionising chamber.
Material emits alpha particles which ionise air in chamber.
Allows small current flow between plates.
Smoke absorbs this current so when flow stops between plates a warning is triggered.
Smoke is required to allow the current to flow.
Fire detection and protection
2 main areas:
Cargo compartments
Engine bays
Wheel bays detect only using overheat firewire loop.
Engine bay fire zones.
Fireproof bulkhead in the middle between cold air zone and hot air zone,
Only monitors cold air zone.
APU fire detection/prevention
Has its own automatic fire protection and detection system
Fire is detected in APU and APU automatically shuts down.
Loop engine fire detection
Continuous loop system know as fire wire.
Nickel core surrounded by coating of eutectic salt.
Small AC electrical current flow throughs loop.
As temperature rises, resistance of salt goes down.
Change is resistsnce combined with change in capacitence.
Produces a fire signal unit when current reaches predetermined level.
Continuous loop
As temperature changes, resistance decreases.
This causes an increased current flow between the electrode and eutectic salt and a return current to the sensing unit.
This creates a fire signal.
Uses AC current.
Detection loops
Engines equipped with pair of fire detection loops and a pair of overheat
detection loops.
Loops wound in an around components on engine casing.
Both loops must detect a fire.
Loops are continuously monitored for faults.
Fault is detected and an amber fault caption illuminates.
Selecting faulty loop off restores fire detection capability.
Systron donner system
Double gaseous loops located around fire zone.
Stainless steels tube around a gas absorbing material.
Heating causes the absorbed gas to escape and increase pressure in the tube.
A bellows is sensitive to pressure increase and closes 2 switch contacts, triggering an alarm.
Both loops must detect a fire.
Bimetallic overheat sensors
Provides spot rather than area protection.
Outer sensor tube contains two metallic strips with low coefficient of expansion.
If the outer sensor tube is heated it expand.
This stretches the inner strips and pulls then together which creates an electrical circuit which triggers a warning.
Fire suppression system - B737
Pulling the fire handle out:
Closes wing soar shut off valve isolating engine bay.
Closes bleed air pressure regulating and shut off valve.
Shuts off hydraulic pump.
Isolates the hydraulic supply.
Isolates the engine driven electric generator.
Arms extinguisher squibs ready to release the extinguisher.
Closes APU shut off valve (left fire handle only).
Engine fire extinguishing- B737
Usually 2 fire extinguisher bottles.
Each engine can have 2 shots.
Pressure switch detects decrease in bottle pressure and activates the bottle discharge light
Cargo extinguishers
Two bottles can discharge into cargo compartments.
Extinguishers
Bottles usually made from alloy steel or copper materials.
Extinguishing agent usually halon pressurised by nitrogen.
Pressure release valve located on bottle.
Toilets have automatic extinguishers, fire into waste bin and undersink if heat is detected.
