Cabin Altitude Warning

Cabin Altitude Warning Explained

The cabin altitude warning system is a critical safety feature in pressurised aircraft. Its main function is to alert the crew if the cabin altitude rises above a predetermined limit—most commonly 10,000 feet (3,048 m). This threshold is set to ensure that, should pressurisation fail, the crew is warned before oxygen levels become dangerously low for passengers and crew.

How the Warning Works

• The warning is typically an audible horn or chime, often accompanied by a visual alert on the flight deck.
• Activation occurs automatically if the cabin altitude exceeds the set limit, regardless of the aircraft's actual altitude.
• The warning prompts the crew to immediately don oxygen masks and initiate the cabin altitude checklist, which includes actions to restore pressurisation or begin an emergency descent if necessary.

Cabin Altitude Procedures

• On hearing the cabin altitude horn, pilots must cross-check the cabin altitude indication and verify the pressurisation system status.
• The cabin altitude checklist guides the crew through steps to manage depressurisation, such as switching to manual pressurisation control or descending to a safe altitude.
• Crew communication and coordination are essential, as rapid action is required to prevent hypoxia.

Regulatory Requirements

• Large transport aircraft are required to have a cabin altitude warning system that activates at or before 10,000 feet cabin altitude.
• Operators must ensure that flight crews are trained in both the recognition of the warning and the correct emergency procedures.

Common Terms

• "Cabin altitude horn," "depressurisation warning," and "cabin pressure warning" all refer to this alert system.
• The cabin altitude warning is distinct from altitude alerting systems, which warn of deviations from assigned flight levels.

Understanding and responding to the cabin altitude warning is a vital part of flight crew operational procedures and is regularly tested in ATPL exams.