Internal doors

This regulation addresses internal doors in aircraft operated under Part 121 (commercial airlines) that have built-in ventilation features like louvres or vents. When such doors exist, the aircraft must provide the crew with a convenient way to shut off airflow through these openings when needed.

The practical purpose is safety and operational flexibility. In certain situations—such as smoke or fumes in one compartment, fire containment, or pressurization issues—the crew may need to isolate different areas of the aircraft by stopping air movement between them. Without a means to close off these ventilation paths, internal doors couldn't effectively separate compartments even when closed.

The requirement is straightforward: if a door has vents, there must be a mechanism (like a sliding cover or closable louver) that crew members can easily operate to block airflow when circumstances demand compartment isolation.

*This is a plain-English summary for study only. The official 14 CFR text on this page is controlling — always read the current regulation and consult a CFI.*