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HELICOPTER NOISE INFORMATION FOR AIRPORTS AND COMMUNITIES This synthesis of practice provides airport operators and their communities with a better understand- ing of helicopter noise and a description of the current state of effective practices for managing helicopter noise. In many respects, helicopters are unique aircraft. They are complex machines that can land and takeoff vertically, and are flexible in the routes they can take, which makes them useful for law enforcement, fire control, medical evacuation, media events, tour operations, personnel transport, and training. Helicopters fly at much lower speeds than fixed-wing aircraft and as a result air traf- fic control (ATC) separates them from fixed-wing aircraft, usually by altitude, with the helicopters assigned to lower altitudes. Helicopters produce a unique, easily recognizable noise. Under certain flight conditions they create a sequence of sharp, equally spaced impulses at a significantly lower frequency than fixed-wing pro- peller aircraft and nothing like the broadband (many frequencies) noise from jet aircraft. Helicopter impulsive noise is a complex phenomenon that has two primary causes: The first arises from the very high air flow near the tip of the advancing rotor blade that creates acoustic disturbances that travel outward near the plane of the rotor, called âhigh speed impulsiveâ noise and can be a problem for helicopters with high rotor tip speeds in cruise. High-speed impulsive noise can be heard when the aircraft is approaching and not when it is flying away and cannot be heard in the helicopter cabin. The second occurs when the trailing rotor blade interacts with the vortex created by the leading rotor blade and is often called âblade slapâ (see Appendix A1 for more details). A portion of this noise can be heard in the helicopter cabin and is often used by the pilot to avoid blade slap flight conditions. Blade slap during landing is generally not a concern for airports as there is usually not a community near the landing pads at airports. However, landing blade slap may be a concern near police or fire helicopter landing pads, hospitals, and commercial or private helipads. Although modern light- and medium-weight civil helicopters are much quieter than older helicop- ters and much quieter than heavy military helicopters they are still the focus of community concern. Communities recognize that helicopters fly lower than fixed-wing aircraft, often without knowing there is an air traffic safety reason for these procedures. There has been discussion and research over the past 40 years about whether helicopters are more annoying than fixed-wing aircraft. There are several schools of thought, supported by research, rang- ing from the sound characteristics (low-frequency sound, blade slap, or easy to recognize sound) to operational and psychological factors (the low altitudes, the sense of loss of privacy associated with low-flying hovering aircraft, or a sense that if they have such flexibility in flight route that they should be flying somewhere else). Some of these are considered acoustic factors (that can be mea- sured in decibels) and some are considered nonacoustic factors (not related to decibels, but some other judgment about need, control, privacy, etc.). ACRP Project S02-48 includes detailed commu- nity surveys to help distinguish acoustic from nonacoustic factors, as well developing a survey tech- nique to determine if decibel-for-decibel helicopters are considered more annoying than fixed-wing aircraft. At the time of this writing, that study was not complete. SUMMARY
2 As part of this synthesis report a literature search was completed and an annotated bibliography created. In addition to the literature search, a number of airports were contacted and surveyed about their helicopter issues and their helicopter noise management programs. The mitigation strategies that surfaced as effective practices are listed here (and described and discussed in more detail in chapter seven): â¢ Outreach â To both community and operators â Flight track monitoring maps to aide discussion with community and operators â Establish local or regional forum to address helicopter noise. â¢ Helicopter noise management program â Collect and analyze complaints â Flight track monitoring n Report helicopter compliance â Published guides or brochures. â¢ Technology â Quieter aircraft â Pilot aides; that is, Global Positioning System-based routes and use of visual landmarks. â¢ Noise abatement procedures â Noise abatement routes â Minimum altitudes â Reducing high-speed impulse and blade slap n Reducing speed effect n Minimizing tight turns. â Limiting hovering. â¢ Media pooling â¢ Fees based on quiet technology â¢ Voluntary operational limits and curfews. The ten airport survey respondents (100% response) generally agreed that community outreach was the most important part of their noise management programs. These outreach programs include updated websites, educating the public and operators in person, and notifying the public of changes in helicopter routes either for temporary purposes or permanent changes (and why). Respondents agreed that simply publishing noise mitigation procedures without making operators aware of them is not all that helpful. In the literature as well as from the airport survey helicopter altitude was the next most cited control measure. This is subject to ATC and cannot be mandated by the airport. Noise reduction with increased altitude is most effective directly under the flight track and noise reduction diminishes to the side with increasing distance. The route structures also were commonly cited in the literature as well as in the airport survey. Airports can develop and propose voluntary noise abatement procedures that affect the speed, descent angle, or other operational aspects of helicopters, subject to review by FAA to ensure that they can be accomplished safely, do not compromise aircraft performance standards, and do not affect ATC clearance and separation standards. Note that this synthesis does not address the legal issues associated with control of airspace or helicopter access restrictions at public use airports. The Airport Noise and Capacity Act of 1990 and its implementing Federal Air Regulations Part 161 place constraints on the ability of the airport pro- prietors to restrict aircraft operations at public facilities, such as heliports at hospitals, police stations, private residences, and commercial buildings that are not subject to these federal laws, and local and state governments are generally able to limit hours of operations, the number of operations, and noise levels through local land use regulations.