Altitude Alert System: Acquisition/Deviation Alerts and Legal Requirements

022-12-07Pablo Asensio Martínez2026-03-271 min

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.