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5 Helicopters are unique aircraft in terms of how they fly and how they are used. Because they can lift off and land vertically they do not need a long runway for takeoff and landing. They also can operate at low speeds, hover, make tight turns, and reverse course in mid-air. This operational flexibility is the primary utility of the helicopter and has allowed for their use by a wide range of operators. In the vicinity of civilian airports the helicopters people see and hear on a daily basis may be flown by or for police, medical facilities, news organizations, tour operators, and construction or maintenance, training, or personnel transport. Helicopters are used for many of the same purposes as fixed-wing aircraft, such as surveillance by law enforcement, air tours, aerial photography, search-and-rescue operations, and crop-dusting; however, in many situations their unique characteristics may make them better suited for these mis- sions. Other missions, such as heavy lifting in construction or utility line projects, cannot be per- formed by fixed-wing aircraft. Helicopters can also provide an advantage over ground transportation, particularly in congested urban areas or remote wilderness. For example, helicopters have proven effective for medical evacuations and search-and-rescue operations for which other means of transpor- tation cannot compete in terms of minimizing travel time to emergency care and operating without the need of a runway. For all the benefits that helicopters provide, they also have undesirable characteristics. Helicop- ters may be seen as a potential invasion of privacy because of their ability to fly and hover at low altitudes. Because helicopters operate at significantly lower speeds than most fixed-wing aircraft, in congested airspace ATC may keep them at a lower altitude to avoid conflicts between the two, and this is readily noticed by the community. This separation of altitudes between helicopters and fixed-wing aircraft is a key safety factor often not well understood by the public. Although the light civilian helicopter is significantly quieter than the heavy military helicopter, people recognize heli- copter noise as distinct from fixed-wing aircraft noise and may react more negatively to it. Much of this report is dedicated to explaining the unique noise characteristics of the helicopter, including low-frequency noise from the main and tail rotor, and impulsive noise (âhigh speed impulsiveâ noise and blade slap) (see Appendix A1 for a more detailed discussion). UNDERSTANDING HELICOPTER NOISE IN COMPARISON WITH FIXED-WING AIRCRAFT NOISE For many reasons community reaction to helicopter noise has not been as thoroughly studied and is less well understood than community reaction to fixed-wing aircraft noise. Most conspicuously, there are far fewer helicopters than fixed-wing aircraft. For example, of 232,567 active aircraft in the domestic U.S. fleet, including commercial and general aviation aircraft, only 11,245 are helicopters (FAA 2011). Despite the smaller numbers of helicopters, people affected by exposure to helicopter noise nonetheless may find it to be distinctive and annoying. The noise emissions of helicopters are more complex, variable, and more unpredictable than those of fixed-wing aircraft. Appendix A1 provides a brief tutorial on the sources and characteristics of helicopter noise in various flight regimes. Helicopter noise emissions vary considerably between approach, departure, hover, and overflight. Rotor High Speed Impulsive noise is concentrated in the plane of the rotor disk and in the direction of forward flight. Blade Slap Impulsive noise radiates more out of plane and is most intense in the direction of forward flight. Tail rotor harmonic noise chapter two UNIQUE ROLE OF HELICOPTERS, THEIR COMPLEX NOISE CHARACTERISTICS, AND THE ROLE OF STAKEHOLDERS
6 has the same physical origins as the main rotor and can be a notable noise source with its radiation pattern rotated by 90 degrees. Broadband emissions of rotary-wing aircraft are typically greater on the side of the aircraft. Furthermore, the low-frequency noise emissions of helicopters can cause more indoor rattle and vibration in residences than fixed-wing aircraft. This vibration is caused by the helicopter noise interacting with the structure of the home and is a result primarily of the lower frequency content of helicopter noise. Fixed-wing aircraft noise increases as an aircraft flies toward an observer, reaches a peak at about the time that the aircraft is directly overhead, and then decreases as it flies away from the observer. In contrast, helicopters typically operate at lower altitudes and at slower speeds than fixed-wing aircraft and may hover or fly in tight circles over a given area. Banking, turning, and changing flight speeds can substantially change noise radiation. These flight characteristics can render individual helicopter operations more variable and more audible for longer periods of time than fixed-wing aircraft overflights. In areas within a few miles of runway ends, high-speed, fixed-wing aircraft usually follow predict- able paths and distribute their noise emissions symmetrically with respect to the flight path. In contrast, helicopters may approach and depart a landing pad at low speeds and to and from more than one direc- tion. In addition, the spatial distribution of helicopter noise varies based on source directivity, depen- dence of emissions on operation (takeoff, approach, hover, overflight), and the operational flexibility of rotary-wing flight. The location, timing, and duration of helicopter noise are less predictable than that of fixed-wing aircraft. For example, at an air-carrier airport airlines tend to have busy periods in the morning and late afternoon/evening, with busy Fridays and Mondays and slow Saturdays. Helicopters are used for a number of different activities, many of which do not follow a predictable pattern. For these reasons, researchers have theorized that helicopter noise may be more annoying on a per-event basis than fixed-wing aircraft noise of comparable sound level. It is also theorized that the repetitive impulsive nature of helicopter noise is its most annoying characteristic. Neither of these interpretations has been conclusively proved by research. In particular, it remains unclear whether the supposed âexcessâ annoyance of helicopter noise (vis-Ã -vis that of fixed-wing aircraft noise) is acoustic or nonacoustic in origin or is a combination of both. A compounding factor is the age of the helicopter fleet. The United States has phased out older jet aircraft; however, there is no mandatory retirement of older helicopters. A recent study estimates that 60% of the helicopters built 40 years ago (1975) and 95% of helicopters built 20 years ago are still in the fleet (see http://www.ascendworldwide.com/2014/03/what-factors-influence-helicopter- values.html). This compares with the U.S. commercial jet aircraft fleet, where the average aircraft is retired at 27 years (http://www.airfinancejournal.com/Article/3341243/Aircraft-Retirement-Trends- and-Outlook.html). ROLE OF STAKEHOLDERS FAA plays several different roles in helicopter noise management. It sets noise certification standards through Federal Air Regulation Part 36. These are noise levels set at defined measurement locations and tested under specified conditions, as part of FAA certification of the helicopter type prior to its sale in the United States. Each noise certification standard is designated as a different âstage,â with Stage 1 being the loudest. Helicopters are certified as Stage 1, Stage 2, or Stage 3. FAA manages the airspace in which helicopters operate and is responsible for the safe operations of aircraft. The Report on the Los Angeles Helicopter Noise Initiative (FAA 2013) describes airspace management and safety considerations in greater detail. It is described in the annotated bibliography (Appendix B) and available on the FAA website: http://www.faa.gov/regulations_policies/policy_guidance/envir_policy/media/la_helicopter_ noise%20report_final_053113.pdf.
7 FAA also licenses pilots; there are a relatively small number of licensed helicopter pilots com- pared with fixed-wing pilots (in 2011, 37,000 helicopter pilots out of a total of 619,000 pilots, including commercial aircraft pilots: http://www.aopa.org/About-AOPA/General-Aviation- Statistics/FAA-Certificated-Pilots). This is a benefit because voluntary noise abatement measures are easier to communicate to the smaller helicopter pilot population. States play varying roles in managing helicopter noise. They generally have statewide aviation system plans that in some cases include helicopter forecasts and facility needs. An important role that states play in managing helicopter noise is through enabling local land use legislation. This can vary widely from state to state, but includes the ability for local jurisdictions to allow for zoning or land use restrictions on private heliports or helipads. State restrictions on helicopter operations may be preempted at public use airports or heliports. Local government has control over private use heliports and helipads through zoning and land use restrictions, but may have more limited control over helicopter operations at public use facilities. The restrictions may include limits on the number of operations, time of operations, size of helicopters, and noise levels. These kinds of restrictions are often found as part of conditional use permits or zoning restrictions for hospitals or other private heliports or helipads. Local government is also responsible for compatible land use planning. Only local government can adopt policies that ensure compatible land uses around airports and heliports pending the exis- tence of enabling state land use legislation. Currently, there are no unique noise/land use compat- ibility guidelines for helicopter noise and further research is needed to determine if such policies are needed or warranted. The airport operator is generally a part of local government and is responsible for the operation of the airport. Members of the community often confuse the role of the airport operator and FAA. Although the airport operator is responsible for maintaining and operating the airport facilities and can develop and propose voluntary noise abatement procedures, FAA has exclusive control over aircraft in flight. Although an airport operator can make requests to FAA, only that agency may chart flight routes and altitudes.