Hush Kits

Engineer to airplane: Stifle

Air & Space Magazine

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.

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