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From
your friends at ATCMonitor.com. © 2006 - Reproduction without
permission is prohibited.
In the early 1960s when turbojet aircraft became part of the growing population of personal travel, it brought the inconvenience of noise along with it.
If you have lived near a metropolitan city or major airport, then you are familiar with this occurrence.
In the early 1970s, Congress gave the Federal Aviation Administration (FAA) the power to regulate aircraft design and equipment for noise reduction.
Today at airports across the country, this is known as Noise Abatement policies.
 Aircraft noise is generated primarily by two major sources in the aircraft engines: the external turbulent jet exhaust and the internal compressors and combustion process.
The main portion of the engine that causes noise is the exhaust. This is due to exhaust exiting the engine at a rate faster than the air that it is hitting behind the aircraft, which causes a thunderous roar.
Consequently, people will usually notice more aircraft noise during the warmer months of the year, when their windows are open, or they are outside.
Also, during the summer season people travel more, and thus there is more air traffic.
Aircraft also climb slower in hot, humid air, because of reduced lift in warmer air.
Additionally, during the warmer months, construction projects on the airport increase and result in runway closings, which significantly reduce the flexibility to rotate runways, and therefore, increase the chance that aircraft will be flying over residential communities with greater frequency.
Wind direction, however, is the primary determining factor in runway selection, because aircraft operate best when flying into the wind.
Because aircraft noise affects a large portion of the population in the United States, the federal government is required to reduce the impact of aircraft noise that affects people who live in areas surrounding airports.
Even aircraft manufacturers, air carriers, airport authorities and residents living in these areas surrounding airports are involved in the noise abatement efforts.
However, since most people react differently to noise, and every airport location is different, it's difficult to derive one formula for reducing aircraft noise that will please the total population.
Since there is no specific ruling for noise reduction and abatement procedures for airports across the country, the FAA studies each airport and its surrounding area individually.
However, in most cases the solution is some combination of getting aircraft onto or off of the runway with the least amount of noise production.
With this in mind the FAA currently recommends, a procedure that incorporates a reduction in engine power from takeoff thrust to normal climb power at an altitude of 1500 feet above ground level after takeoff with subsequent acceleration and climb after passing through 3000 feet by changing the deck angle and retracting the flaps.
This procedure is generally used by scheduled air carriers. However, it is the responsibility of Air Traffic Control (ATC) and other airport authorities to enforce these procedures if needed to obey noise restrictions.
For some airports, normal approach paths cover substantial residential populations, such as aircraft arriving and departing Atlanta
Jackson International Hartsfield airport. Therefore, approach and departure paths are typically designed to avoid residential areas where possible.
In order to prevent increases in noise from aircraft engines, some airports have special noise abatement procedures.
For example, at some airports, steep climbs are used on takeoff and during departure so that aircraft will be higher than they otherwise would have been when they reach heavily populated residential areas.
In other cases when aircraft must climb directly above residential areas they often do so with reduced power in order to minimize excessive noise from greater engine thrust.
In addition to the FAA's recommended noise abatement procedures, there are also noise standards for regulating aircraft noise from civil subsonic turbojets.
These standards are generally based on an aircrafts weight and number of engines. Essentially, the heavier the aircraft and the greater the number of engines, the more thrust that's required to takeoff and therefore the more noise the aircraft is allowed to generate and still comply with the required noise limits.
For example, if an aircraft weighed 200,000 pounds and had four engines, it would probably take 100 percent power to produce enough thrust to get that aircraft off the ground, so it would be allowed to use a higher percentage of power to safely takeoff.
Oh the other hand, an aircraft that weighs 50,000 pounds and has four engines producing the same amount of thrust as the heavier aircraft could use less power to perform the same operation, and therefore produce less noise during takeoff, which would accommodate the noise abatement.
That doesn't mean that the heavier aircraft can just barrel down the runway and roar over a residential area with no consequences.
But different aircraft have different procedures for approach and departure in order to accommodate the noise abatement policies.
For example, let's say that a neighborhood and residential area is positioned 3 miles from the departure end of the runway.
The smaller jet mentioned earlier could probably climb to 3000 feet prior to ever reaching that neighborhood and the noise wouldn't be an issue for residents. But the heavier aircraft may only be at 1200 feet in that distance, because it is much heavier and takes longer to climb.
In this case, ATC may require the pilots to make an immediate turn in one direction or another upon takeoff in order to facilitate their climb in a direction that will not affect the neighborhood.
These requirements issued by ATC are generally published for airmen so that they know ahead of time what factors will effect their arrival and departure.
Aircraft noise affects many people. Real estate agents, home buyers and owners, air carriers, aircraft manufacturers and so on.
Because of this reason, the FAA makes every effort to ensure that the general public is considered in their efforts to reduce aircraft noise and that the air carriers and manufacturers abide by noise abatement policies in the most efficient means available.
However, even with all of these factors working together, there are still issues with noise. One of the main ways that the FAA is pursuing reduction in aircraft noise is by participating in the development of future technology in new engines.
Pratt & Whitney and
General Electric show potential for further reductions in engine noise levels through improved designs of the internal compressors and exhaust mechanisms.
This improved design along with more efficient wing design, and more effective control surfaces such as flaps and spoilers will require less engine thrust for safe flight, therefore providing further noise reduction.
However, in the past there has been a lag between newly developed noise reduction technologies and the implementation of those technologies into commercial aircraft.
For example, the L1011 and B747 have implemented a noise reduction technology that was developed in the 1970's.
Even though this gap between laboratory research and implementation seems rather large, the aircraft manufacturers and the FAA are working harder to close this gap.
However, due to certification and financial responsibilities from air carriers, it is unlikely that engine quieting technology being developed today will reach commercial service for at least five to six years from now.
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