13.3 Fire & Smoke Detection: Spot Detectors, Continuous Loops & Optical Sensors

13.3 Fire & Smoke Detection: Spot Detectors, Continuous Loops & Optical Sensors

FAA Airframe Subject Matter Focus: Aviation maintenance technicians must master the engineering classifications of aircraft fire zones, spot thermal switches, rate-of-rise thermocouples, continuous-loop resistance and eutectic systems (Kidde and Fenwal), pneumatic gas-charged systems (Lindberg and Systron-Donner), and cargo smoke detection technologies.

1. Designated Aircraft Fire Zones

Federal Aviation Regulations (14 CFR Part 25) mandate that aircraft engine nacelles, auxiliary power unit (APU) bays, and combustion heater compartments be classified into designated fire zones to establish detection and extinguishing requirements.

                  DESIGNATED FIRE ZONE CLASSIFICATIONS

   ┌────────────┬─────────────────────────────────┬───────────────────────────┐
   │ Zone Class │ Airflow & Aerodynamic Geometry  │ Typical Aircraft Location │
   ├────────────┼─────────────────────────────────┼───────────────────────────┤
   │ Class A    │ Heavy airflow over bluff /      │ Reciprocating engine      │
   │            │ irregular rough surfaces        │ power section             │
   │ Class B    │ Heavy airflow over aerodynami-  │ Turbine engine compressor │
   │            │ cally clean, smooth surfaces    │ and accessory sections    │
   │ Class C    │ Low or restricted airflow       │ APU compartments, tail-   │
   │            │                                 │ cone accessory bays       │
   │ Class D    │ Extremely low / unventilated    │ Wing cavities, sealed     │
   │            │ zero airflow compartments       │ wheel wells               │
   │ Class X    │ High airflow, severe liquid     │ Turbine combustor, nozzle │
   │            │ fuel leakage & ignition hazard  │ and turbine wheel casing  │
   └────────────┴─────────────────────────────────┴───────────────────────────┘

Fire Detection System Performance Mandates

An FAA-approved fire detection system must:

1. Provide rapid detection and cockpit warning within under 5 seconds of fire initiation.
2. Accurately indicate when a fire has been extinguished (automatic reset).
3. Resist false alarms caused by vibration, oil, fuel, wash water, or extreme ambient temperature swings.
4. Incorporate continuous built-in integrity monitoring to detect broken wires, loop ground faults, or lost pneumatic charge.

2. Spot Fire & Overheat Detectors

Spot detectors monitor specific localized hazard locations (such as near engine exhaust manifolds or combustor casings).

                     SPOT DETECTOR MECHANISMS

   1. FENWAL THERMAL SWITCH (Bimetallic Expansion / Fixed Temperature)
             Inconel Outer Tube (High Thermal Expansion)
      ┌─────────────────────────────────────────────────────────┐
      │  ───[ Contact ] ───                      ─── [ Contact ]│
      └─────────────────────────────────────────────────────────┘
             Internal Low-Expansion Struts Pull Contacts Closed

   2. EDISON THERMOCOUPLE (Rate-of-Temperature-Rise / Millivolts)
         [ Active Hot Junction ] ─── Exposed to Fire Flash (Rapid Heat)
                   │
                   ▼ Net EMF (mV) Energizes Alarm Relay
                   ▲
         [ Cold Ref Junction ]   ─── Shielded in Insulated Cage (Slow Heat)

1. Thermal Switch Systems (Fenwal Spot Detectors)

• Construction: A bimetallic switch consisting of a curved outer Inconel casing with a high coefficient of thermal expansion, enclosing two low-expansion alloy internal struts carrying gold-plated electrical contacts.
• Fixed Temperature Operation: When ambient temperature rises to the calibrated setpoint (e.g., $325^\circ\text{F} ext{ to }600^\circ\text{F}$), the outer Inconel tube expands longitudinally. This expansion relieves tension on the internal struts, causing the electrical contacts to snap closed.
• Circuit Wiring: Thermal switches are wired in parallel with each other and in series with the cockpit warning lamp. Closing any individual switch completes the circuit to ground, illuminating the fire warning light.

2. Thermocouple Systems (Edison Spot Detectors)

• Principle of Operation: Senses the rate of temperature rise rather than reaching a fixed temperature threshold. It operates on the thermoelectric (Seebeck) effect.
• Hot vs. Cold Junctions:
  • Active Hot Junction: Exposed directly to engine compartment airflow; heats up almost instantaneously during a fire flash.
  • Reference Cold Junction: Encased inside a heavy, thermally insulated protective cage; heats up very slowly.
• Dynamic Response:
  • Normal Engine Warmup: When an engine warms up normally (e.g., slow climb), both hot and cold junctions heat up at nearly the same rate, generating equal and opposing voltages that cancel out (net EMF $\approx 0\text{ mV}$).
  • Rapid Fire Event: A sudden fire flash heats the exposed hot junction immediately while the cold junction remains cool. The resulting temperature differential produces an active electromotive force (typically $10\text{ to }30\text{ mV}$) that energizes a sensitive galvanometer relay in the control unit, sounding the fire bell and illuminating the fire light.
  • Automatic Reset: Once the fire is extinguished, the hot junction cools down to match the cold junction, and the warning automatically clears.

3. Continuous-Loop Fire & Overheat Detection Systems

Modern turbine aircraft utilize continuous-loop systems because a long, flexible sensing tube routed throughout the engine nacelle provides complete zone coverage and can detect localized hot spots as well as average zone overheat.

                 CONTINUOUS-LOOP DETECTION SYSTEMS

   1. KIDDE SYSTEM (Two Wire Conductors, NTC Thermistor Core)
      ┌─────────────────────────────────────────────────────────┐ Inconel Tube
      │   (Wire 1: Live) ───[ Thermistor Core (NTC) ]─── (Wire 2: Return)
      └─────────────────────────────────────────────────────────┘ Resistance Drops

   2. FENWAL SYSTEM (Single Wire Center Conductor, Eutectic Salt Beads)
      ┌─────────────────────────────────────────────────────────┐ Inconel Tube
      │   (Center Nickel Conductor) ── [ Eutectic Beads ] ── (Ground Sheath)
      └─────────────────────────────────────────────────────────┘ Impedance Drops

   3. SYSTRON-DONNER PNEUMATIC (Helium Gas & Titanium Hydride Wire)
      ┌─────────────────────────────────────────────────────────┐ Titanium Tube
      │  Helium Gas (Avg Heat) + Titanium Hydride Wire (Hot Spot)│ (Pressure)
      └────────────────────────────┬────────────────────────────┘
                                   ▼
                  [ Integrity Sw ] ── [ Alarm Diaphragm Sw ]

1. Kidde Continuous-Loop System

• Internal Geometry: Consists of a continuous Inconel capillary tube containing two internal wire conductors embedded in a ceramic thermistor paste core (potassium and aluminum silicates).
• Negative Temperature Coefficient (NTC): At normal operating temperatures, the thermistor core has high electrical resistance ($>100\text{ k}\Omega$). As ambient temperature increases anywhere along the loop, the electrical resistance of the core drops exponentially. When resistance falls below a calibrated limit, current flows between the two internal wires, signaling the electronic control unit to trigger the cockpit fire warning.

2. Fenwal Continuous-Loop System

• Internal Geometry: Utilizes a single center nickel conductor wire surrounded by perforated ceramic beads impregnated with a specialized eutectic salt, all enclosed within an Inconel outer tube that serves as the ground return.
• Eutectic Phase Change: At normal temperatures, the dry eutectic salt is a non-conductive electrical insulator. When any portion of the tube reaches the exact melting point of the eutectic salt, the salt melts into a highly conductive liquid electrolyte. Electrical impedance between the center wire and the outer tube drops abruptly from megaohms to a few ohms, triggering the fire alarm.

3. Pneumatic Continuous-Loop Systems (Lindberg & Systron-Donner)

• Principle: Utilizes physical gas pressure dynamics rather than electrical resistance changes.
• Systron-Donner Architecture: A sealed titanium capillary tube filled with pressurized helium gas surrounding a central titanium hydride core wire that absorbs large volumes of hydrogen gas.
  • Average Overheat Detection: When the entire engine nacelle heats up uniformly, the pressurized helium gas expands per the ideal gas law ($P \propto T$), increasing internal tube pressure.
  • Localized Hot Spot / Fire Detection: When a localized hot flame strikes even a few inches of the sensing tube, the intense heat forces the titanium hydride core wire to desorb and liberate its trapped hydrogen gas, causing an immediate, massive pressure spike.
  • Dual Pressure Switches:
    • Alarm Pressure Switch: A metal diaphragm switch calibrated to close when internal gas pressure reaches the fire threshold, completing the fire warning circuit.
    • Integrity Pressure Switch: A second pressure switch held closed by the normal baseline helium charge (approx. $25\text{ psig}$). If the sensing tube is cut, crushed, or punctured, helium escapes, opening the integrity switch. This signals a LOOP FAULT warning on the cockpit annunciator and locks out false fire alarms.

4. Cargo Compartment Smoke Detection Systems

Cargo compartments (Class C and Class E) are monitored by electronic smoke detectors capable of identifying sub-visible combustion aerosols within under 60 seconds of ignition.

                 PHOTOELECTRIC SMOKE DETECTOR (TYNDALL EFFECT)

     [ Light Emitter / IR LED ] ───────────────────> [ Light Trap ]
                                      │  (Clean Air: Photodiode in Dark)
                                ☁ ☁ ☁ │ ☁ ☁ ☁
                            (Smoke Particles)
                                      │  Light Scattered
                                      ▼
                          [ Photodiode Receiver ]
                         (Triggers Alarm at >2.5%/ft)

1. Optical Photoelectric Smoke Detectors

• Tyndall Scattering Principle: An infrared light-emitting diode (LED) transmits a pulsed collimated beam across a dark sensing chamber into a light trap. A sensitive silicon photodiode is positioned at an angle ($90^\circ\text{ or }135^\circ$) off the direct beam axis.
• Clean Air State: In the absence of smoke, the light beam travels straight into the absorber trap. The photodiode remains in complete darkness, producing zero electrical output.
• Smoke Detection State: When airborne smoke particles enter the sampling chamber through convective baffles, they reflect and refract (scatter) the infrared light into the photodiode. When the scattered light intensity exceeds a calibrated smoke obscuration threshold (typically $2.5%\text{ per foot}$), the detector amplifies the signal to activate the cargo fire warning bell and discharge isolation dampers.

2. Ionization Smoke Detectors

• Principle of Operation: Uses a tiny, harmless radioactive source (Americium-241) to ionize air molecules in a sensing chamber, creating a continuous trickle current of positive and negative ions ($10^{-11}\text{ A}$) between two charged electrode plates.
• Detection Mechanism: When combustion aerosol particles enter the chamber, they attach to the ionized air molecules, neutralizing them and slowing their velocity. This causes a measurable reduction in ionization current. When the current drops below the threshold setpoint, the detector triggers the smoke alarm.