When conditioned or emergency air is no longer being supplied, ventilation is supplied from which source?

• A. From the main air conditioning pack bypass line.
• B. From an electric cabin blower.
• C. From the ram air duct through a check valve.
• D. From external air via the cabin door seals.

Answer: (C)

When conditioned or emergency air is no longer being supplied, ventilation is provided by the ram air duct. This system brings in outside air as the aircraft moves and delivers it into the cabin through a one-way check valve, which prevents backflow and helps maintain airflow when the environmental packs aren’t generating conditioned air. This ram air source is designed specifically as a fallback to keep the cabin ventilated without relying on pack operation or electrical blowers. The other sources either depend on packs, require electrical power, or rely on door seal leakage, which isn’t a controlled or reliable ventilation source.