# Aeronautical Communication Associated Terms and Message Priorities

> ICAO communication terms, message priority order, and definitions including distress, urgency, and flight regularity messages.

- **Subject:** 090 — Communications
- **Section:** 090-01 — Concepts
- **Topic code:** 090-01-01
- **Last reviewed:** 2026-04-01
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/communications/concepts/associated-terms/

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## Categories of Messages
According to **ICAO Annex 10, Volume II**, the messages handled by the aeronautical mobile service are classified by priority. The order of priority for establishing communications and transmitting messages is as follows:

1. **Distress** (Signal: **MAYDAY**): Distress calls, messages, and traffic.
2. **Urgency** (Signal: **PAN PAN**): Urgency messages, including those preceded by the medical transports signal.
3. **Direction Finding**: Communications relating to direction finding.
4. **Flight Safety**: Messages concerning movement and control (ATC instructions), messages of immediate concern to an aircraft in flight, and important meteorological advice.
5. **Meteorological**: Meteorological messages.
6. **Flight Regularity**: Messages regarding the operation or maintenance of facilities, or company messages (e.g., schedule changes, fuel requests).

## Communication Methods
There are distinct ways to classify communication channels:

*   **Simplex**: A method in which telecommunication between two stations takes place in **one direction at a time**.
    *   Examples include standard ATC communications where you push-to-talk.
*   **Duplex**: A method in which telecommunication between two stations can take place in **both directions simultaneously**.
    *   Example: Telephone conversation.

It is also important to distinguish between air-ground relationships:
*   **Air-ground communication**: **Two-way** communication between aircraft and stations or locations on the surface of the Earth.
*   **Ground-to-air communication**: **One-way** communication from stations or locations on the surface of the Earth to aircraft (e.g., **ATIS**, **VOLMET**).

## Q-Codes (Navigation)
A specific set of Q-codes is used to describe bearings and headings.

| Code | Definition | Description |
| :--- | :--- | :--- |
| **QDM** | Magnetic heading **TO** the station | Magnetic bearing to the station (zero wind). |
| **QDR** | Magnetic bearing **FROM** the station | The magnetic radial you are currently on. |
| **QUJ** | True heading **TO** the station | True bearing to the station. |
| **QTE** | True bearing **FROM** the station | The true bearing from the station to the aircraft. |
| **QNH** | Altimeter setting | Atmospheric pressure adusted to Mean Sea Level (MSL). |
| **QFE** | Altimeter setting | Atmospheric pressure at aerodrome elevation (or runway threshold). |

## Service Availability Codes
These abbreviations indicate the operational hours of a service (e.g., in NOTAMs or AIP):

*   **H24**: Continuous day and night service.
*   **HJ**: Sunrise to sunset (Day schedule).
*   **HN**: Sunset to sunrise (Night schedule).
*   **HS**: Service available during hours of scheduled operations.
*   **HX**: No specific working hours.

## Key Definitions

*   **CAVOK (Ceiling And Visibility OK)**: Used when visibility is **10 km or more**, there are no clouds of operational significance (no clouds below 5000 ft or MSA, no CB/TCU), and no significant weather.
*   **AIS (Aeronautical Information Service)**: Ensures the flow of information/data necessary for the safety, regularity, and efficiency of international air navigation.
*   **AFIS (Aerodrome Flight Information Service)**: Provides information and instructions to aircraft at non-controlled aerodromes to assist in safe operations.
*   **RNAV (Area Navigation)**: A navigation method allowing flight on any desired path within the coverage of station-referenced aids or within the limits of self-contained aids, rather than flying strictly "to" or "from" ground stations.
*   **Clearance Limit**: The point to which an aircraft is granted an air traffic control clearance.
*   **Expected Approach Time (EAT)**: The time ATC expects an arriving aircraft, following a delay, will leave the holding fix to complete its approach.
*   **Visual Approach**: An approach by an IFR flight when either part or all of an instrument approach procedure is not completed and the approach is executed in visual reference to terrain.

## Instrument Approach Types
*   **Non-Precision Approach (NPA)**: A 2D instrument approach (lateral guidance only).
*   **Approach with Vertical Guidance (APV)**: A PBN instrument approach with vertical guidance (3D) that does not meet precision approach standards (e.g., LPV).
*   **Precision Approach (PA)**: A 3D instrument approach based on systems like ILS, MLS, or GLS.

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