Which of the following is not a listed type of air filter?

• A. Dry Air Filter
• B. Viscous Air Filter
• C. Oil Bath Filter
• D. Water Filter

Answer: (D)

Air filters for engine air intake use different trapping methods to remove dust before it reaches the engine. The common types you’ll see are a dry filter, which relies on a fiber medium to block particles; a viscous filter, which uses a sticky film to capture particles as air passes through; and an oil bath filter, where air flows through or over oil so dust sticks to it and can be separated from the airstream. Using water as a filtration medium isn’t employed in these systems because introducing water into the intake brings moisture, icing risk, potential for corrosion and contamination, plus added weight and maintenance. So, water filtration isn’t a recognized type for aircraft air filters.