FAA Powerplant Written Practice Test

Which type of engines use cold stream or combined stream thrust reversing systems?

1. Low bypass turbofan engines

2. High bypass turbofan engines

3. Turbojet engines

4. Reciprocating engines

The correct answer is: High bypass turbofan engines

High bypass turbofan engines utilize cold stream or combined stream thrust reversing systems because of their design, which incorporates a large bypass duct that directs a significant portion of the air around the engine core. When these engines need to reduce speed upon landing, the thrust reversers redirect the bypass air forward, thereby generating reverse thrust. This is particularly effective for high bypass configurations because the majority of the engine's thrust is produced by the bypass airflow rather than the exhaust jet. Cold stream thrust reversers typically perform this redirection without directly affecting the engine core's exhaust, which is beneficial in maintaining efficiency while enhancing braking capability. In contrast, turbojet engines, which have little to no bypass and rely mainly on the exhaust for thrust, do not have the same capability for cold stream thrust reversal since they do not have the bypass duct structure that high bypass turbofan engines possess. Similarly, reciprocating engines and low bypass turbofan configurations are generally not equipped with cold stream thrust reversers, as their thrust reversal methods differ significantly in mechanics and effectiveness.