1.1 Background

The National Park Service (NPS) was created by Congress to

 "… promote and regulate the use of Federal areas known as national parks ... [so as to] conserve the scenery and the natural and historic objects and the wild life therein and to provide for the enjoyment of the same in such manner and by such means as will leave them unimpaired for the enjoyment of Future generations. "1

In doing so, the NPS's mission was seemingly defined with an inherent conflict between the goal of conservation and the goal of providing for enjoyment. Enjoyment requires that visitors have access to the parks, and conservation requires that such access not damage or diminish the resources that the park was created to protect. But the NPS Organic Act was amended by the Redwoods Act of 1978, and this act unambiguously defines resource preservation as the primary responsibility for the Park Service. Given that natural quiet is a clearly identified resource, that aircraft overflights can disturb this resource and that the Federal Aviation Administration (FAA) controls use of the airspace, meshing the disparate missions of the NPS and FAA has, until recently, appeared to be an intractable task.

For three main reasons, aircraft flying low over parks present the NPS with a very different and unusual set of problems. First, the "natural quiet" found in many national park units has long been regarded as a park resource. This perspective is reflected in law, policy and by park visitors. It is a highly valued and increasingly rare resource in some parks that can be affected by low-flying aircraft. 

Second, the effects produced in parks by aviation are perhaps less obvious and, in some ways, less permanent than the effects produced by visitors on the ground. Worn trails and facilities, crowded camping areas and automotive traffic jamming park roads are easily observed and agreed upon effects that threaten visitor enjoyment. The overflight of a single aircraft, however, creates what is often perceived as a temporary visual effect, produces non-natural sound levels that are audible for a finite period, and may, if flying low enough and fast enough, startle visitors or the horses / mules of mounted visitors resulting in some risk of injury. Because of the paucity of studies and the difficulty of quantitatively proving impacts on visitors, wildlife, or other resources, the visual and audible effects produced by aircraft tend to be judged subjectively with the acceptability of these effects being a matter of personal opinion (Dunholter, et. al. 1989)2.



1. NPS Organic Act, 16 USC 1

2. "While extensive research has been completed on the effects of aircraft overflights on urban populations in the vicinity of airports, [a literature search conducted for the National Park Service] revealed a shortage of information on the subjects of enroute aircraft sound, aircraft sound in wilderness settings, or the acoustic effects on a park visitor population."


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The third main reason this is such an unusual and difficult problem is that the authority to legally exercise control on aviation access3 does not lie with the NPS. For all visitors to parks who travel by ground, the NPS has the authority to manage their impacts. Whatever type of ground transportation visitors use, the NPS can control where, when and how park areas are accessed and used. Further, any

 "... public accommodations, facilities, and services as have to be provided within those areas should be provided only under carefully controlled safeguards against unregulated and indiscriminate use, so that the heavy visitation will not unduly impair [park] values and so that development of such facilities can best be limited to locations where the least damage to park values will be caused."4

Thus, for access on ground, the NPS can exercise controls based on park policies developed to fulfill the mission of the NPS and of the specific park.

On the other hand, Congress unambiguously vested authority for all aspects of airspace management in the Administrator of the FAA. The Federal Aviation Act of 1958 gives the Administrator the authority and the mandate to prescribe rules and regulations governing the flight of aircraft, including rules as to the safe altitude of flight, for the purposes of 1) the navigation, protection, and identification of aircraft, 2) the protection of persons and property on the ground, 3) the efficient utilization of navigable airspace, and 4) protection to the public health and welfare from aircraft noise and sonic boom.5

Thus, to the extent that use of airspace has effects on park lands, the NPS must work with the FAA to determine what controls are possible. Such a division of authority has meant in some cases that where NPS and FAA are unable to agree, resolution has had to occur through the President or Congress.

This type of conflict resolution between resource protection and airspace use is not new. Aircraft flights over and into the area now designated as the Boundary Waters Canoe Area Wilderness present an early example. As early as the 1930's, floatplanes provided sport fishing access to the Superior National Forest (now the Boundary Waters Canoe Area Wilderness) in northern Minnesota, and by 1948 Ely, Minnesota was reputed to be the largest freshwater floatplane base on the continent with approximately 70 planes making multiple round trips per day into remote lakes. Considerable publicity through writings and a short documentary film brought national attention, and on December 17, 1949, President Truman issued Executive Order 10092 establishing an airspace reservation of certain areas of the Superior National Forest. The order prohibited, with a few exceptions, flight below the altitude of 4,000 feet above Mean Sea Level (MSL) over designated areas. In 1978, Congress passed the Boundary Waters Canoe Area Wilderness Act into which E.O. 10092 was incorporated by reference. (FAA, *)



3. Access via aviation means, for this report, aircraft flying over land areas administered by the NPS for use and enjoyment of park resources.

4. 79 Stat. 969

5. S. Rep No 1811, 85th Cong 2d Sess. 14 (1958)


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Grand Canyon National Park (GCNP) has had ever increasing overflights by aircraft. But whereas aircraft provided transportation to lakes in the Superior National Forest, aircraft use over Grand Canyon National Park is primarily for sightseeing purposes. Scenic tour flights began over the park in 1926 when an airstrip was developed on the south rim near Red Butte. The completion of the GCNP Airport in 1965, two miles south of the park boundary in Tusayan, contributed in a major way to the expansion of an air tour industry. By 1987, about 40 companies provided over 50,000 air tours over the Canyon.

The increasing number of flights over the Grand Canyon, combined with increases in air traffic over parks in Hawaii, the Colorado Plateau, and elsewhere raised both NPS and visitor concerns that overflights were having a significant impact on park values and on the visitor experience. For the Grand Canyon, this concern grew to such an extent that in January 1975, when Public Law 93-620, the Grand Canyon National Park Enlargement Act was passed, its Section 8 recognized "natural quiet" as a value or resource in its own right to be protected from significant adverse effect. In addition it specifically addressed the potential for aircraft or helicopter operations to cause a significant adverse effect on the natural quiet and experience of the park.

Public Law 93-620 led to early research to determine if adverse effects were being caused by aircraft overflights. Acoustic research to develop a baseline on levels of aircraft sounds and sociological surveys to determine visitor reactions to the sound of aircraft were undertaken. Acoustic research found sound levels from aircraft to be quite high in various locations due to extensive numbers of flights. Surveys established that a range of visitors (from 20% of rim visitors, to 70% of backcountry users) were dissatisfied with aircraft overflights or related sound levels. Also in response to Section 8 of the law, a public process was begun in October 1984 to review research and to discuss associated issues. By March of 1986, this process convinced the NPS that aircraft activity occurring over or within the park was causing a significant adverse effect on the natural quiet and experience of the park, and was likely to cause an injury to the health, welfare, or safety of visitors to the park. In June of 1986, two tour aircraft collided over the park, killing 25 people. This tragedy focused national attention on the aircraft overflight issue at the Grand Canyon, and led in part to passage by Congress, in August 1987, of Public Law 100-91, the National Parks Overflights Act. This report responds to the requirements of that law.


1.2 Public Law 100-91 and this Report

This law directed the NPS and the U.S. Forest Service to study the effects of aircraft overflights and report to Congress on the results. The Forest Service reported on work conducted earlier (USDA, 1992); this report presents the results of further studies conducted by the NPS. The law required that specific questions be answered. Table 1.1 below lists these questions, the section of P.L. 100-91 in which they appear and the chapter or chapters that address each of the question areas.


1.3 Organization of Report

The organization of the report is based on the questions posed by Public Law 100-91. The nine major research areas of the report are described in this section:


1.3.1 The Nature and Scope of Overflight Problems

Chapter 2 presents information provided by park managers about the nature and scope of the problem. It examines which and how many parks are affected, what type of adverse effects or impacts are perceived, what types of aircraft and aircraft operations are responsible for overflights, how many


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Table 1.1 Questions Posed by P.L. 100-91


Question to be Answered

Section of P.L. 100-91 where Question is Posed

Chapters of Report that Address the Question

1. What is the nature and scope of the overflight problem in the National Park System?



2. What are other injurious effects of overflights on the natural, historical, and cultural resources for which such units were established?


3, 4, 5

3.a. What is the proper minimum altitude which should be maintained by aircraft when flying over units of the National Park System?

§1.(a )


3b. What have been the effects of the minimum altitude established over Yosemite and Haleakala National Parks?



3c. Has the plan for management of airspace above the Grand Canyon succeeded in substantially restoring the natural quiet in the park?



3d. What revisions in the airspace management plan for the Grand Canyon may be of interest?



4. What is the impairment of visitor enjoyment associated with flights over such units of the National Park System?



5. What are the impacts of aircraft noise on the safety of the park system users, including hikers, rock-climbers, and boaters?



6. What are the values associated with aircraft flights over such units of the National Park System in terms of visitor enjoyment, the protection of persons or property, search and rescue operations and firefighting?




overflights are estimated to occur, and how much of the time aircraft are audible at apecific locations in eight national parks.


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1.3.2 Effects of Overflights on Natural Quiet

"Natural quiet" is a resource found in many parks which, under the NPS Organic Act, as amended, is to be protected. How and why overflights effect natural quiet in national parks is examined in Chapter 3. The chapter examines the importance of natural quiet, provides both qualitative and quantitative descriptions of natural quiet in parks, and discusses why natural quiet is so difficult to preserve.


1.3.3 Effects on Cultural and Historical Resources, Sacred Sites and Ceremonies

The law requires information and evaluation of injurious effects of overflights on the historical and cultural resources of parks. Chapter 4 examines these effects from manager and visitor perspectives, and then discusses the potential for acoustic impacts and for vibration impacts.


1.3.4 Effects on Wildlife

Wildlife is one of the parks' natural resources that can be impacted by overflights, and is required to be examined by the law. Chapter 5 discusses physiological and behavioral responses of wildlife to overflights, presents a summary of observed responses for various species, and examines indirect effects of disturbance from overflights such as accidental injury, reproductive and energy losses and habitat avoidance and abandonment. It also presents factors that influence animal responses to aircraft, discusses some of the problems with detecting long-term effects of aircraft produced disturbance, and examines the limitations of current information about wildlife responses to aircraft overflights.


1.3.5 Effects of Overflights on Visitors

P.L. 100-91 directs the NPS to provide information and an evaluation of the impairment of visitor enjoyment associated with flights over units of the National Park System. Chapter 6 draws on information primarily from two different surveys of visitors to examine how aircraft overflights affect visitors and their enjoyment of the parks. Visitor opinions about overflights are presented both for the National Park System (excluding Alaska), and for 39 specific parks. Visitor opinions addressed include ratings of how overflights affect their enjoyment, how overflight sounds rate as a problem compared with other sounds, types of aircraft heard, and effects of hearing or seeing aircraft. Visitor opinions are used to rank order the 39 parks by "degree of overflight problem", and this ranking is used to better understand visitor opinions and reactions. Management rankings of park overflight problems are compared with the visitor-based ranking. Finally, study results that quantitatively relate visitor reactions (responses) to measured aircraft sound levels (doses) are presented.


1.3.6 Aircraft Overflights and Safety

Chapter 7 presents management concerns about safety, with specific examples at surveyed parks, gives visitor opinions about aircraft and safety, and discusses the special problems associated with Temporary Flight Restrictions around forest fires or other major incidents.



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1.3.7 Values Associated with Aircraft Overflights

Public Law 10391 also requires that the research provide information and an evaluation regarding "...the values associated with aircraft flights terms of visitor enjoyment, the protection of persons or property, search and rescue operations and fire fighting. " Chapter 8 examines these benefits from management and air tour passenger perspectives.


1.3.8 Restoration of Natural Quiet

P.L. 100-91 explicitly states that "Noise associated with aircraft overflights at the Grand Canyon National Park is causing a significant adverse effect on the natural quiet and experience of the park...." The Act requires the NPS to implement a plan for management of air traffic, and to report on "... whether the plan has succeeded in substantially restoring the natural quiet in the park...." The Act also sets limitations on flight over Yosemite National Park and Haleakala National Park. Chapter 9 specifically examines the effectiveness of the minimum flight altitudes set in accordance with P.L. 100-91 in Yosemite and Haleakala National Parks and addresses whether Special Federal Aviation Regulations (SFAR 50-2) stemming from this legislation have succeeded in substantially restoring natural quiet in the Grand Canyon National Park.


1.3.9 Conclusions, Issues and Recommendations

Chapter 10 presents conclusions of NPS studies, issues needing to be addressed, and NPS recommendations. It also specifically addresses the P.L. 100-91 question of whether possible revisions in the SFAR are necessary.


1.3.10 Availability of NPS Studies

The series of NPS studies upon which this report is based are listed in Appendix A and are available through the National Technical Information Service.


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