Summary of Aircraft Engine Fire Systems
Aircraft Engine Fire Systems: Detection, Extinguishment & Operation
Introduction
Aircraft engine fire detection and suppression systems protect the aircraft, crew, and passengers by detecting fires early and extinguishing them quickly. This material explains common detection system types, how they operate, how extinguishing systems are used, and cockpit actions when a fire is indicated. It breaks concepts into digestible parts and shows real-world applications.
Fire Detection Systems — Overview
Aircraft engine fire detection systems are designed to detect either a localized flame/overheat or a rapid rise in temperature. Four common types are used on turbine-powered aircraft:
Types of engine fire detection systems
- Thermoswitch (spot-type) system
- Thermocouple (rate-of-rise) system
- Thermistor-type continuous-loop system
- Pneumatic-type continuous-loop system
Definition: A spot-type detector uses discrete sensors (spots) placed at locations of suspected fire risk; a continuous-loop detector uses a single length of sensing element routed through the fire zone.
Thermoswitch (spot-type) system
- Detects a high temperature at a specific location (a spot).
- It is not capable of indicating a general overheat condition across a larger area.
Definition: Thermoswitch — a temperature-activated switch that closes or opens when the sensing element reaches a preset temperature.
Practical example: A thermoswitch mounted near an engine accessory gearbox will actuate only if that specific spot reaches its trigger temperature, indicating a localized fire or hot spot.
Thermocouple (rate-of-temperature-rise) system
- Measures the rate at which temperature increases rather than absolute temperature.
- Designed to detect rapid temperature rises associated with developing fires.
- Cannot indicate a general overheat condition (slow, uniform temperature increase across an area).
Definition: Thermocouple system — uses junctions of dissimilar metals that produce a voltage proportional to temperature change; used to sense rapid temperature rise.
Practical example: If an engine compartment temperature climbs quickly due to ignition, the thermocouple output increases rapidly, triggering a fire warning.
Thermistor-type continuous-loop system
- A continuous resistance-type loop whose resistance changes with temperature along its length.
- Provides detection over the entire routed path rather than at discrete spots.
- Faults such as breaks are indicated by cockpit test, but a break does not necessarily prevent fire detection where the intact portion is exposed to fire.
Definition: Thermistor continuous-loop — a sensing loop composed of thermistor material whose electrical resistance varies with temperature.
Pneumatic-type continuous-loop system
- A gas-filled continuous tube routed through the protected area. When heated, the gas inside is released and actuates a pressure-sensitive switch.
- Can detect both small high-temperature spots and lower elevated temperatures over an extended length, thus can indicate general overheat as well as fire.
Definition: Pneumatic continuous-loop — a sensing tube that releases gas when heated; the released gas closes a diaphragm or pressure switch to signal a fire condition.
Real-world note: Pneumatic continuous-loop detectors are valued in some engine installations because they provide both local and distributed sensitivity.
How to check a thermocouple-type system
- The thermocouple-type system is checked by verifying it responds to a rapid temperature change (rate-of-rise) during maintenance test procedures. Specific test steps depend on the aircraft maintenance manual and the test equipment used.
Note: Detailed maintenance circuitry such as the thermal test unit circuitry is covered in other material and is intentionally excluded here.
Continuous-loop system fault behavior
- A break in a continuous-loop detector will cause a fault indication when tested. However, that break does not necessarily prevent fire
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Engine Fire Systems
Klíčové pojmy: Four types: thermoswitch, thermocouple, thermistor continuous-loop, pneumatic continuous-loop, Thermoswitch (spot) detects local high temperature only, not general overheat, Thermocouple senses rate-of-temperature-rise, cannot indicate general overheat, Thermocouple operation is checked by verifying rate-of-rise response per maintenance procedures, A break in continuous-loop shows a fault but may not prevent fire detection on intact sections, Pneumatic continuous-loop detects both small high-temp spots and lower widespread overheat, Most turbojet engine HRD extinguishers use Halon 1301, Pulling the T-handle arms bottle discharge, trips generator field relay, shuts off fuel, hydraulics, and bleed air, Thermistor continuous-loop varies resistance with temperature and monitors its entire routed path, Always follow aircraft maintenance manual for testing and preflight checks