# Aircraft Ice Warning Systems and Detection Technologies

> Vibrating probe, differential-pressure, and visual ice detection methods, plus advisory versus primary warning system logic.

- **Subject:** 021 — Airframe, Systems, Electrics, Powerplant
- **Section:** 021-07 — Anti-icing and De-icing Systems
- **Topic code:** 021-07-02
- **Last reviewed:** 2026-03-27
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/airframe-systems-electrics-powerplant/anti-icing-and-de-icing-systems/ice-warning-systems/

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Ice warning systems provide flight crews with timely alerts regarding ice accretion on the aircraft. These can range from simple visual cues to sophisticated automatic detection systems.

## Types of Detectors

### Vibrating Probe (e.g., Rosemount)
- **Principle:** A short cylindrical probe vibrates axially at a referenced **resonant frequency**.
- **Detection:** As ice accumulates on the probe, its mass increases, causing the **frequency of vibration to decrease**. This change is detected and triggers a warning.
- **Operation:** The probe is equipped with a heating element to melt the ice after detection, allowing the system to reset and continue monitoring the accretion rate.
- **Technology:** Often uses **piezoelectric crystals** to sense the change in frequency.

### Differential Pressure (e.g., Smiths)
- **Principle:** A hollow tube with holes on its leading and trailing edges.
- **Detection:** Under normal conditions, air pressure builds up in the probe. In icing conditions, the holes on the leading edge become blocked by ice.
- **Mechanism:** The blockage causes a **decrease in dynamic pressure** inside the probe. The resulting change in pressure differential between the holes triggers the warning.
- **Heating:** The probe is heated to clear ice. The **smaller holes tend to clear first**.

### Visual/Manual Detectors (e.g., Hot Rod)
- **Design:** A simple probe/rod mounted outside the cockpit window in the pilot's field of view.
- **Function:** Pilots visually observe ice buildup.
- **Feature:** Includes a heater (often manually operated) to clear ice and allow assessment of the accumulation rate.
- **Eye Reference:** Design requirements ensure that visual ice indications (like buildup on wipers) are visible from the pilot's **correct eye reference position**.

## System Classifications

- **Advisory System:** The crew primarily uses flight manual criteria (Total Air Temperature, visible moisture) to activate anti-icing. The detector serves only as a backup.
- **Primary System:**
    - **Automatic:** The detector **automatically activates** the ice protection systems at the optimal moment.
    - **Manual:** The crew activates the systems based on the **primary** indication from the detector.

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