![]() The sound power radiated from a source can therefore be calculated by integrating the acoustic intensity over a surrounding surface. The sound intensity is the sound power transmission through a surface (W/m 2) - a vector quantity with direction through a surface. When sound propagates through a medium acoustic sound power is transferred. 14 CFR §91.883 Special flight authorizations for jet airplanes weighing 75,000 pounds or less – lists special flight authorizations that may be granted for operation after December 31, 2015.Sound power is the energy rate, or energy of sound per unit of time ( J/s or W in SI-units) - emitted by a source. Any Stage 2 Business jets 75,000 pounds or below that have not been modified by installing Stage 3 noise compliant engines or have not had Stage 3 hush kits installed for non-compliant engines will not be permitted to fly in the contiguous 48 states after December 31, 2015. ![]() In 2013, the FAA modified 14 CFR part 91 rules to prohibit the operation of jets weighing 75,000 pounds or less that are not Stage 3 noise compliant after December 31, 2015. EU Regulation 925/99 was passed over US threats to ban Concorde but was superseded (and effectively repealed) by EU Directive No. This regulation threatened to reduce the value of the mostly-American used airplanes that employed hush kits and hurt the profits of American hush kit manufacturers. By December 31, 1999, all Stage 2 noise jetliners 75,000 pounds or more operating in the US must have Stage 3 hush kits in order to continue to fly in the US after January 1, 2000. In 1999, this concern led to a regulatory dispute between the United States and the European Union, where the EU proposed a new noise ordinance which effectively prevented the use of hush kits in Europe. While hush kits effectively reduce noise emissions from older aircraft, noise cannot always be reduced to the level of modern planes at a reasonable cost. On a larger aircraft, like FedEx's Boeing 727s, the hush kits add 410 kg (900 lb) of extra weight (total airplane weight up to 86,000 kg (190,000 lb)) and this results in an overall 0.5% increase in fuel burn for short trips (but no measurable increase for long flights). Meanwhile, the hush kit for the Gulfstream G-III jets weighs about 170 kg (370 lb) and cause a calculated 2% drop in range, although the reduction in range has not been noticed during actual flights, which are rarely made to the very limit of the aircraft's range. For example, the hush kit fitted to the Gulfstream II adds 106 kilograms (234 lb) to the total airplane weight of 65,500 lb (29,700 kg), causing around a 1.6% reduction in aircraft range. It also reduces engine performance and aerodynamic efficiency. ![]() Hush kits can adversely affect the range and performance of the aircraft they are fitted to because of the extra weight. Several older aircraft that are still in service (typically in a cargo capacity) have hush kits retrofitted so that they are able to conform with noise regulations needed to operate in many commercial airports. Modern aircraft equipped with mid and high-bypass turbofan engines are designed to comply with contemporary aviation noise abatement laws and ICAO regulations. A hush kit installed on a Swedish Air Force Sud Aviation Caravelle
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