Hush Kits
Engineer to airplane: Stifle
- By Roger A. Mola
- Air & Space magazine, January 2005
IN 1969, IN RESPONSE TO COMPLAINTS ABOUT AIRPORT NOISE, the Federal Aviation Administration began restricting aircraft noise levels near runways.
The new Federal Aviation Regulation Part 36 measured noise at various locations: beneath the approach path; at the takeoff or go-around point; and next to the runway centerline, at the point where engines are typically at full power. For older and heavier aircraft, the rule set noise levels based on their age and maximum takeoff weight. And the signatory nations of the International Civil Aviation Organization agreed that as engine technology improved, they would impose tighter standards.
For civil air transports, standards are now at Stage 3, which says airlines may buy new aircraft that meet Stage 3 requirements or replace the Stage 2 engines with quieter ones. Until a recent action by the European Union, airlines could also modify the old engine with a device called a hush kit. Stage 4 takes effect after January 2006, mandating aircraft that are quieter by another 10 EPNLdB—effective perceived noise level in decibels, a unit based on a complex formula.
In the United States, the FAA has defined aircraft noise as “significant” if its average hourly level, day and night, tops 65 dB or, more precisely, 65 dB DNL (day/night noise level, also sometimes stated as LDN). At each airport, microphones sample the noise and record it continually. Airports have DNL contour maps (see below) that indicate the areas subjected to a day-long hourly average of 65 dB or above, usually measured over a year’s time. A 10-dB penalty is added to noise measured between 10 p.m. and 7 a.m.
Controversy surrounds not only noise standards but also noise measurement technologies. Tests are hard to duplicate with precision, and placing a microphone on concrete versus grass, or at varying distances, yields different contours. Those who count decibels, or sound quantity, are at odds with those who find sounds of certain qualities—a shrill whine, say—irritants even at a lower intensity.
Few older aircraft can meet Stage 3 standards, but for airlines in developing countries and for freight operators, old aircraft are more affordable, and to meet the noise issue, the answer is often hush kits.
Most hush kits address the process by which high-velocity hot jet exhaust clashes with cooler ambient air, generating the thunderous roar associated with jets. Slowing that exhaust, or spreading out the area in which the rumble takes place, is the goal. Sound-absorbing materials similar in function to acoustic ceiling tile enclose not only the exhaust but also the engine fan and intake cowl to reduce the noise projected forward.
Some kits replace the round exhaust nozzle with a fluted shape like that of a bundt cake pan. The increased surface area diffuses and calms the stream of exhaust. Adding exhaust pipes can lower the speed of each stream; lengthening the exhaust duct reduces the velocity out the back end. Some kits tackle the exhaust farther forward in the engine, injecting ambient air . Each of these methods slows the airflow, reduces effective thrust, adds weight, and increases fuel consumption.
A new design, the chevron, consists of cutouts around the nozzle that create vortices in the exhaust flow. Developed by General Electric and refined at NASA’s Glenn Research Center in Cleveland, Ohio, chevrons resemble shark’s teeth, set randomly and capped like mushrooms, to hasten mixing of streams of ambient air, fan flow, and the engine’s core.
Pilots can also reduce noise by applying noise abatement procedures: raising or re-directing their approach paths, climbing rapidly, or reducing power near airports. Though engine noise predominates, airflow around the wings, slats, flaps, and landing gear contributes its share. In 1997, James Raisbeck, a former Boeing engineer, offered what some called “the non-hush-kit hush kit” for the Boeing 727-200. It reduces the angle of deflection of the wings’ leading edge slats, increases lift, and enables takeoff with reduced power and less noise.
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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