Aircraft Smoke Detection Systems

General Principles

• Application: Smoke detectors are mandatory in areas that cannot be kept under constant physical surveillance by the crew.
  • Locations: Cargo holds, Lavatories, Crew rest areas, Electronic/Avionics bays.
  • Note: Engines and APUs typically use heat/fire loops, not smoke detectors.
• Indication: Flight deck warnings usually include a Master Warning (Red) and specific zone lights to draw immediate attention.

Optical Smoke Detectors

These rely on the interaction of light with smoke particles.

1. Light Refraction / Scattering (Tyndall Effect)

• Mechanism: A light source (LED) and a photocell are positioned at an angle within a dark chamber (labyrinth).
• Normal State: Light passes into a trap/catcher; the sensor sees darkness.
• Alarm State: Smoke particles enter the chamber and scatter/reflect the light onto the photocell.
• Usage: Common in "point" detectors (e.g., Lavatories).

2. Light Attenuation / Obscuration

• Mechanism: A transmitter and receiver are placed in a direct line of sight.
• Normal State: The receiver sees full light intensity.
• Alarm State: Smoke passes between them, blocking or attenuating the light beam.
• Usage: Suitable for larger open areas (e.g., Cargo compartments).

Ionization Smoke Detectors

• Mechanism: Uses a small amount of radioactive material to ionize the air between two electrically charged plates, creating a steady current flow.
• Alarm State: Smoke particles attach to the ions, making them heavy and slow. This disrupts/reduces the current flow between the plates, triggering the alarm.
• Sensitivity: Very sensitive to small smoke particles.