In a turbofan engine, what is bypass air?

• A. The portion of induction air that bypasses the engine core
• B. Air that passes through the gas generator portion
• C. Air used exclusively for cabin pressurization
• D. Air that mixes with exhaust gases

Answer: (A)

In a turbofan, bypass air is the portion of the incoming air that bypasses the engine core. The big fan splits the air, sending some into the core to be compressed and burned, and routing the rest around the core through a bypass duct. This bypass air never enters the gas generator or combustion chamber; it exits the engine as a separate stream and provides most of the thrust in high-bypass designs. It also helps with cooling and reducing noise. The air that goes through the core is the one that is combusted to produce core exhaust. Bypass air isn’t used exclusively for cabin pressurization, and it doesn’t mix with exhaust gases in the basic bypass flow.