Hush kit

A hush kit is a device for reducing noise from an engine. It is typically used on a low-bypass turbofan engine in older commercial aircraft.

Jet aircraft generate a lot of noise, contributing to noise pollution near airports. In modern high-bypass turbofan engines, the fan mounted afore the jet engine core is very large. The bigger the fan in comparison to the jet core, the more effective the bypass air is in enveloping the jet exhaust at the rear of the engine, reducing noise. The larger turbines needed to spin the large fan slow the jet exhaust, which also reduces noise. A hush kit produces a similar effect, using several modifications to the existing engine. Primarily, a device called a multilobe exhaust mixer on the rear of the engine mixes the exhaust gases of the jet core with the surrounding air and the small amount of bypass air available. Similar systems are also employed on many modern turbofan engines as standard equipment to further reduce noise. Most kits also make further modifications to the exhaust with acoustically treated tailpipes, revised inlet nacelles and guide vanes, all of which reduce forward propagating high-pitched noise caused by the small, high-speed fan.

Modern aircraft equipped with high-bypass turbofan engines are able to comply with contemporary aviation noise abatement laws and ICAO regulations. Hush kits are used on the many older freight and passenger aircraft still in service, such as the Boeing 727 and 737-200, Douglas DC-8 and DC-9, and Tupolev Tu-154. While hushkits are normally found on older aircraft, small business jets and other aircraft too small to be fitted with large, high bypass turbofan engines will be manufactured with hushkits installed, a more economical way to meet noise restrictions than expensive engine or design changes.

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Wikipedia