# Altitude Alert System: Acquisition/Deviation Alerts and Legal Requirements

> Understand altitude-alerting logic for capture and deviation events, regulatory applicability thresholds, and ADC/MCP signal integration.

- **Subject:** 022 — Instrumentation
- **Section:** 022-12 — Alerting Systems, Proximity Systems
- **Topic code:** 022-12-07
- **Last reviewed:** 2026-03-27
- **Author:** Pablo Asensio Martínez
- **HTML version:** https://pilotsyllabus.com/en/instrumentation/alerting-systems-proximity-systems/altitude-alert-system/

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## Overview

The **Altitude Alert System** is designed to prevent "level busts" by alerting the flight crew when approaching a selected altitude or deviating from an assigned altitude.

## Functions

The system provides two primary types of alerts:

1.  **Acquisition Alert (Approaching):**
    *   Triggers when the aircraft is approaching the pre-selected altitude during a climb or descent.
    *   Typically activates at a specific threshold before reaching the target (e.g., 900 ft or 750 ft prior).
    *   *Indication:* Visual (steady or flashing light/box) and often Aural (momentary chime).

2.  **Deviation Alert:**
    *   Triggers when the aircraft **deviates** from the selected altitude by more than a preset limit while maintaining a level flight.
    *   *Thresholds:* Typically **200 ft** (Airbus) or **300 ft** (Boeing).
    *   *Indication:* Visual (Flashing Amber light or Box on PFD) and Aural (Continuous or repeated chime).

## Legal Requirements

According to regulations, the system is mandatory for:
*   All **Turbojet** aircraft.
*   Turbine-powered aircraft with a Maximum Take-Off Mass (MTOM) **over 5,700 kg**.
*   Turbine-powered aircraft capable of carrying **more than 9 passengers**.

## System Interface
*   **Inputs:**
    *   **Barometric Altitude:** From the Air Data Computer (ADC).
    *   **Selected Altitude:** Set by the pilot on the Mode Control Panel (MCP) or Flight Control Unit (FCU).
*   **Alerts:**
    *   **Visual:** **Amber** light or Amber frame on the PFD Altitude Tape.
    *   **Aural:** C-chord, Beep, or Chime.

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