Hush kit

Aircraft engines generate a lot of noise which causes noise pollution near airports. While modern jet engines have become comparatively quiet, many old aircraft like the B-727, DC-9 and DC-8 are still in service. These aircraft do not meet the newest noise regulations, as the bypass ratio of the engine is too low. In modern engines, a fan is mounted before the actual jet engine, accelerating the airflow around the core engine and cooling the airflow, reducing the noise emissions of the engine. A Hush Kit mixes the exhaust gases of the old engine with the surrounding air, which reduces the noise emissions.

Although hush kits reduce noise emissions of older aircraft, they do not succeed in making these aircraft as quiet as modern aircraft like the A320 or the B737 300-900. Therefore the European Union bans hush kitted aircraft. This caused a dispute between the US and the EU, as it dramatically reduced the chances of selling a used plane with a hush kit.