Hush Kits- page 2 | How Things Work | Air & Space Magazine

Hush Kits

Engineer to airplane: Stifle

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(Continued from page 1)

After September 11, 2001, many older aircraft, possible candidates for hush kits, were hustled off to retirement. Yet by the end of 2003, FedEx, the largest operator of Boeing 727s, held orders from 60 airlines for 740 Stage 3 kits it designed in-house. Jet Engineering and Goodrich have since released a Stage 4 hush kit for some 300 MD-80s flying in Europe, though freight carriers with night operations, including UPS, are replacing engines or buying new aircraft. In 1999, the European Union published Regulation 925, effectively freezing the number of hushkit-equipped transports in its airspace. The rule would, at a stroke, reduce the value of unsold hush kits, which are primarily U.S.-made, to scrap. Because hush kits have added weight and limited performance, the EU aimed to “improve the situation regarding fuel burn and gaseous emissions.”

Europe also required that new foreign transports traversing European airspace have, instead of hush kits, high-bypass-ratio engines. Such engines make less noise per pound of thrust than one with a lower bypass ratio, though not necessarily less noise than one with a hush kit. U.S. representatives protested this requirement, which avoided mentioning U.S. carriers while carefully wording the rule so that U.S. aircraft equipped with hush kits were barred.

In September 1999, the U.S. House of Representatives determined that if the EU persisted, House bill H.R. 661 would ban the European Concorde from U.S. airports. Congressman Vernon J. Ehlers (R-Mich.) went even further before the Subcommittee on Aviation: “The [Airbus] A320 has a very annoying noise…. It would not be at all hard to disqualify the A320 from serving the United States….”

After two years of this, the EU decided to focus on older Russian aircraft, like the Ilyushin Il-76, a cargo carrier used in global relief efforts. In retaliation, Russia threatened to cut off access by European airlines and relief flights by the Il-76.

In the future, noise abatement procedures will be even more important than hardware, so air crew training will still be a key to success.

The FAA, in a December 2003 report on its own performance, said that between 1975 and 2000, the number of people exposed to significant noise had dropped 90 percent, adding that industry may have done all it can with hush kits and technology; future gains may require residential soundproofing or relocation.

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