# Aircraft Smoke Detection Systems

> Optical and ionization smoke detection principles, installation zones, and cockpit warning logic.

- **Subject:** 021 — Airframe, Systems, Electrics, Powerplant
- **Section:** 021-12 — Protection and Detection Systems
- **Topic code:** 021-12-01
- **Last reviewed:** 2026-03-24
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/airframe-systems-electrics-powerplant/protection-and-detection-systems/smoke-detection/

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## 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.

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Free study notes from pilotsyllabus.com. Not a substitute for training at an approved ATO.