The fog observed in the cabin during rapid decompression is caused by which phenomenon?

• A. Smoke from engine
• B. Humidity change
• C. Moisture Condensation
• D. Engine oil leakage

Answer: (C)

During rapid decompression, the cabin air expands quickly and cools. The humidity already in that air can’t stay in vapor form at the lower temperature, so water vapor condenses into tiny droplets, creating visible fog. This moisture condensation is the phenomenon at work. It isn’t smoke from the engines and it isn’t oil leaking; those would show other signs such as odor or dark plumes. The fog tends to appear when humidity is high and clears as the cabin returns to stable pressure.