The effect of aircraft overflights on public lands has been a concern to land management agencies for many years. Beginning in the 1940's with the floatplane access to the Boundary Waters Canoe Area Wilderness and sightseeing tours over the Grand Canyon in the late 1960's, an established air tour industry has developed that provides tours over 30 to 40 national parks including such diverse areas as the Grand Canyon, the Hawaiian parks, the Badlands of South Dakota, Cape Cod National Seashore, and Great Smoky Mountains National Park. In addition, the need for military training space and the preference that this be located over unpopulated areas, as well as increasing long distance commercial air travel mean that the airspace over public lands is under increasing demand. Public Law 100-91 recognized the increased concern about aircraft, and in directing the study of park overflights, gave specific requirements that the nature and scope of the problems be determined. Section l(b) of the law states:

 The study shall identify any problems associated with overflight by aircraft of units of the National Park System and shall provide information regarding the types of overflight which may be impacting on park unit resources. The study shall distinguish between the impacts caused by sightseeing aircraft, military aircraft, commercial aviation, general aviation, and other forms of aircraft which affect such units.

 In response to this section, the NPS took three major actions:

 First, in 1992 a list of parks affected by aircraft overflights was developed and a questionnaire mailed to each park manager. Detailed data were collected about the nature and extent of the problems. (McDonald et al. 1994) This questionnaire asked about both factual matters and matters of opinion. Questions of fact addressed the use of aircraft by the park, the numbers and types of other aircraft that fly over the park, visitor complaints and safety issues. Questions of opinion, specifically to be answered by the park manager, asked about the general types of problems occurring in the park, the level of concern about various problems, the existence and significance of any problems created by aircraft overflights, and opinions about how any aircraft-related problems should be resolved.

 Second, a study to estimate the aircraft-produced sound exposure for all park units as well as USFS wilderness areas was completed. (Tabachnick et al. 1992) Since data on exposure were prohibitively expensive to collect for all parks, this effort to characterize the sound exposure for all parks used secondary information such as maps of the parks, maps of military and commercial


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aviation routes, and estimates of overflight operations. This report provides estimates of noise exposure for each NPS unit and for each Forest Service Wilderness1, and permits a rank ordering of parks or wildernesses by "exposure".

Third, four sound measurement studies were completed. (Dunholter et al. 1989; Fidell et al. 1994; Horonjeff et al. 1993; Miller et al. 1994) Each of these studies served a different purpose, and all but the first provide comparable quantitative data on aircraft and non-aircraft sound levels in various parks. First, the Mestre Greve report addressed the techniques for measuring aircraft sounds within park and wilderness settings and examined the acoustic parameters that are important in describing aircraft sound within such settings. Second, though the BBN study had several objectives, a primary one was to determine the extent to which natural quiet had been restored in the Grand Canyon; as part of this investigation, sound measurements were made in the Grand Canyon during the fall of 1989 and the spring of 1990. Third, the purpose of the first HMMH study was to measure and present detailed "acoustic profiles" for 23 locations in Grand Canyon National Park, four locations in Hawaii Volcanoes National Park, and four locations in Haleakala National Park. Fourth, the second HMMH study developed a simplified method for collecting sound level data, and park personnel were trained in the method and used it to collect data in five units of the National Park system: Cumberland Island National Monument, Glacier National Park, Mount Rushmore National Memorial, Petroglyph National Monument, and Yosemite National Park.

This chapter reports on the information provided as a result of these three specific actions. The information is combined and summarized to provide an overall picture of the numbers of park units affected, the aircraft overflight types and numbers, the degree of concern about the impacts of overflights, and the sound levels that result from these overflights.


2.1 Survey of Park Managers

The survey of park managers collected information about the numbers of park units experiencing aircraft overflight problems, the types of aircraft operations affecting each park, the types of impacts produced by the overflights, and additional detail about the park managers' perception of aircraft overflights and associated problems.


2.1.1 Number of Parks Affected

The survey first determined the number of park units affected by aircraft overflights. A screening telephone call to Regions and parks identified 98 out of the then-total of 34l non-Alaska parks as having some type or level of concern about overflights. Figure 2.1 depicts the relationship between parks with identified overflight problems and those without. It shows this relationship in terms of numbers of parks,



1. NPOA Report 92-1 provides in its Figure 5-3 the specific equations used to compute noise exposure.


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park acreage, and visits2. Figure 2.2 displays the 98 identified park locations, and Appendix B lists them. Thus, the following conclusion is drawn:

Conclusion 2.1.

NPS managers believe that approximately 30% of all National Park System units have aircraft overflight problems. These affected parks account for about three-fourths of the total NPS administered acreage, and about half the total park visits. Low-level overflights constitute a management problem for the NPS, one that needs to be addressed in a systematic manner.


2.1.2 Types of Aircraft Overflights

The survey asked park managers to identify the types of aircraft flying over their parks. Figure 2.3 shows the number of parks with overflight problems that mentioned each of four general types of aircraft overflights. Parks identifying each of these types of overflights included: general aviation (81), military (78), high-altitude commercial (55), and sightseeing (42).

Conclusion 2.2:

Managers report that overflights of parks result from all types of aircraft; general aviation and military aircraft overflights are the types most often mentioned. This reporting of overflights of many aircraft types reinforces the notion that a systematic approach is needed to address these issues and to identify the most serious overflight problems.


2.1.3 Types of Impacts

The survey also asked managers to identify the types of impacts they believed the aircraft overflights produced at their parks. In general, the types of impacts may be characterized as safety related, sound related and visual related.

One question on the survey asked "Do you feel visitors are concerned for their safety as a result of aircraft overflights over your park?" Eighteen NPS managers perceived overflights to be a serious or very serious safety issue for their park.

A second question asked managers to identify, from a list of sources of sounds, which types of sound they considered a problem, and also to identify the degree of the problem. Eighty-eight of 91 responding parks identified the sounds from "airplanes, jets, helicopters or any other aircraft" as a problem to some degree.

A third question asked managers to identify the extent to which aircraft activity adversely impacts visitors, using a five scale choice from "no impact" to "very large impact". Sixty-four of 89 responding parks said that aircraft overflights had some degree of impact on the "ability of visitors to appreciate park scenery." These responses are graphed in Figure 2.4 as a "visual " impact.



2. Visits are defined as the number of people entering a park over the course of the year for recreational purposes.


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Figure 2.1 Extent of Aircraft Overflight Problems in the National Park System


NPC Editor's Note: The original figure is not easily readable.  Each pie graph is described below:

Number of Parks: 341 NPS units

  • 29% of NPS Units Reporting Overflight Concern
  • 71% of NPS Units Not Reporting Overflight Concerns

Acreage: 25,470,955.01 acres

  • 76% of NPS Units Reporting Overflight Concern
  • 24% of NPS Units Not Reporting Overflight Concerns

Visitation: 273,465,349 recreation visits

  • 53% of NPS Units Reporting Overflight Concern
  • 47% of NPS Units Not Reporting Overflight Concerns


Figure 2.2 Locations of the 98 Park Units with Identified Aircraft Overflight Problems


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Figure 2.3 Types of Aircraft Overflying National Parks as Identified by Managers



Figure 2.4 Park Manager Judgments of Types of Impacts Produced by Aircraft Overflights



Figure 2.5 Reported Number of Overflights per Week by Aircraft Type for All Parks



Figure 2.6 Reported Number of Overflights per Week Reduced by Four Parks


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Conclusion 2.3:

NPS managers most often identified the sound of aircraft overflights as producing negative impacts on visitors. Safety was judged as least frequently impacted. Reported visual impacts, though perhaps highly dependent on the scenic resource, upon the way in which the question was asked (or on the person answering the survey), were nevertheless not as commonly judged as negative.


2.1.4 Estimated Numbers of Overflights

The survey asked managers to estimate the numbers of overflights per week that their parks experienced. They were asked to make these estimates for six types of operations: 1) military training, 2) sightseeing tours, 3) transporting commercial passengers between cities, 4) park management, research and maintenance, 5) emergency services like fire fighting or search and rescue, and 6) private aircraft flights (general aviation). The numbers used here are intended by the reporting parks to include all overflights in order to provide a sense for relative numbers of overflights, and to show the nature and potential scope of the overflight problem.

Figure 2.5 sums the number of overflights per week reported by the surveyed parks to provide an indication of the relative level of operations. The total indicates that commercial operations are apparently responsible for the greatest number of overflights. Even if the estimates for George Washington Memorial Parkway (6,300 commercial overflights), Golden Gate National Recreation Area (14,000 commercial overflights), Joshua Tree National Monument (7,000 overflights) and Manassas National Battlefield Park (3,200 overflights) are removed, the results still show (Figure 2.6) that commercial overflights are judged to generate more than twice as many overflights as either sightseeing or general aviation. Military overflights and park management and emergency operations are, respectively, the least common types of overflights.

Estimates of overflights are presented in alternative form in Figures 2.7 through 2.11. The figures group parks by numbers of reported overflights per day. (Overflights per day are computed by dividing reported overflights per week by seven.) Each figure presents the distribution of parks for a different type of aircraft operation. Figure 2.7 shows the distribution of the 91 reporting parks for all operations.

(For example, 36 parks reported that the number of overflights for all types of operations as between 1 and 10 per day. Similarly, 10 parks reported fewer than 1 overflight per day and 19 parks reported having between 10 and 50 overflights per day).

Figures 2.8 through 2.11 provide the distribution for the four types of operations: military, sightseeing, commercial, and general aviation. These data suggest several generalizations. First, though many of the parks experience military overflights (78 parks, see Figure 2.3), most of the parks report relatively few military overflights per day (fewer than 10 per day, Figure 2.8). Alternatively, though a smaller number of parks report sightseeing overflights (42 parks, Figure 2.3), those that do tend to report greater numbers (up to 50 per day or more, Figure 2.9). Commercial and general aviation distributions tend to lie between these two patterns: more parks report commercial overflights than report sightseeing, and numbers range from modest to very high (over 500 per day, Figure 2.10). More parks report experiencing general aviation overflights than report any other type (81 parks, Figure 2.3), but most report fewer than ten overflights per day (Figure 2.11).



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Figure 2.7 Distribution of Overflights per Day for All Aircraft Combined


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Figure 2.8 Distributions of Overflights per Day for Military Aircraft


Figure 2.9 Distribution of Overflights per Day for Sightseeing Aircraft


Figure 2.10 Distribution of Overflights per Day for Commercial Aircraft


Figure 2.11 Distribution of Overflights per Day for General Aviation Aircraft


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Conclusion 2.4:

Reported numbers of overflights by type of operation vary considerably from park to park. Relatively few parks receive high numbers of any one type of overflight. Commercial and sightseeing operations are more prevalent than other types of overflights. Military and park administrative overflights are least common.


 2.1.5 Superintendents' Judgments of Overflight Problems

The survey specifically asked NPS managers their opinions about several matters related to aircraft overflights. The questions attempted to determine the overall level of concern superintendents have about overflights and the types of effects they believe overflights produce. Most questions were to be answered with a five-point scale ranging from "no concern" or "not a problem" to "extremely concerned" or "very serious problem".


 Each manager was asked to judge the seriousness of ten specific types of potential problems in his/her park. Figure 2.12 shows how many superintendents reported each of the problems to be a moderate, serious or very serious problem. For example, 53 responded that mechanical noises like vehicles, aircraft and generators were either a moderate, serious or very serious problem. Only maintenance of park facilities and damage to natural resources were rated as problems by more superintendents. Additionally, managers were asked to rate six different specific sounds as potential problems, and Figure 2.13 summarizes the responses. Of the 91 reporting superintendents, 65 (over 70 percent) rated the sounds from airplanes, jets, helicopters and any other aircraft to be a moderate, serious or very serious problem. The next most often identified sound-related problem was road traffic, with 37 parks (about 40 percent) rating cars, buses, trucks or motorcycles as a moderate to very serious problem.


The managers were also asked to identify their overall concern about aircraft activity over the park. Figure 2.14 gives the distribution of their responses. Overall, 67 out of 91 who answered this question responded that they were either moderately, very or extremely concerned about aircraft activity. The parks whose managers are very or extremely concerned about aircraft overflights are listed in Table 2.1.

Conclusion 2.6:

Most, though not all, managers of the parks with perceived overflight problems rate aircraft as one of their most important problems. Also, managers demonstrate differing degrees of concern about overflights. Hence any systematic method for assessing aircraft overflight problems should be designed to incorporate local management objectives in the identification of the problem and in developing solutions.

Conclusion 2.7:

The starting point for resolving overflight issues over national parks needs to begin with an examination of those parks whose managers are very to extremely concerned about overflights.


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Figure 2.12 Managers’ Rating of Ten Potential Problems in Their Parks

NPC Editor's Note: The categories and numbers are provided below in table form for readability.

Potential Problems

Number Reporting a Problem

Maintenence of Facilities


Damage to Plants, etc


Mechanical Noises


Development Outside Park


Damage to Artifacts


Quality of Services


Air, Water Pollution


Availabilty of Facilities


Number of Visitors


Development Inside Park



Figure 2.13 Managers’ Rating of Sound-Related Problems in Their Parks

NPC Editor's Note: The categories and numbers are provided below in table form for readability.

Type of Sound Problem

Number Reporting a Problem



Road Vehicle Traffic


Power Generators


Audio Equipment


Domestic Animals


People Talking



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Figure 2.14 Managers’ Reported Degree of Overall concern about Overflights


Figure 2.15 Managers’ Rating of Most bothersome Aspects of Overflights

NPC Editor's Note: The categories and numbers are provided below in table form for readability.

Type of Effect

Number Reporting Effects

Loudness of Some Flights


Areas Overflown


Number of Planes


Duration of Sound


When Aircraft Fly


Safety of Visitors or Staff



Figure 2.16 Managers’ Rating of Overflight Impacts on Visitors’ Enjoyment

NPC Editor's Note: The categories and numbers are provided below in table form for readability.

Type of Impact

Number Reporting Impact

Natural Quiet


Historical Significance


Interpretive Programs


Park Scenery


Normal Conversation



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Table 2.1 National Parks whose Managers are Very to Extremely Concerned about Aircraft



Bandelier National Monument

Big Cypress National Preserve

Cape Lookout National Seashore

Bryce Canyon National Park

City of Rocks National Reserve

Channel Islands National Park

Fort Vancouver National Hist. Site

Crater Lake National Park

Glacier National Park

Guadalupe Mountains National Park

Great Smoky Mountains N.P.

Joshua Tree National Monument

Haleakala National Park

Kalaupapa National Historical Park

Hawaii Volcanoes National Park

Lassen Volcanic National Park

Isle Royale National Park

Manassas National Battlefield Park

Kings Canyon & Sequoia N.P.

Mesa Verde National Park

Minute Man National Historic Park

Mount Rainier National Park

Organ Pipe Cactus Nat. Monument

Navajo National Monument

Shenandoah National Park

Perry's Victory & Int. Peace Memorial

Southern Utah Group3

Statue of Liberty National Monument

Prince William Forest Park

Pu'uhonua o Honaunau National H.P.

Puukohola Heiau National H.S.

Saguaro National Monument

San Antonio Missions NHP

White Sands National Monument


3. The Southern Utah Group (Canyonlands National Park, Arches National Park, and Natural Bridges National Monument) was inadvertently left out of the Survey but has requested that it be shown here.


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NPS managers were asked to give their judgments about specific effects of aircraft overflights. First they were asked, if they had concerns about aircraft overflights, what bothered them most about the aircraft activity. Figure 2.15 shows how many park managers rated any of six effects as moderate, serious or very serious problems. Most managers (69 out of 91 responding parks) believed the loudness was a problem, while the particular areas overflown was next most often identified. Fewest managers considered aircraft to be a threat to visitor or stall safety.

NPS managers were also asked to rate five possible impacts on visitors, and Figure 2.16 summarizes the responses. Figure 2.16 shows the number of managers who responded that they rated the impact on the listed qualities as moderate, large or very large. The largest number (63) responded that overflights produced impact on the "ability of visitors to experience natural quiet and the sounds of nature." Forty-three thought aircraft produce moderate to very large impact on the "ability of visitors to appreciate the historical and/or cultural significance of the park." About equal numbers of managers identified impact on the ability of visitors to "hear interpretive programs," or "appreciate park scenery," or to "carry on normal conversations."

Conclusion 2.8:

Loudness and area overflown were the most bothersome aspects of overflights to NPS managers. With respect to impacts on visitors, managers were most concerned about the ability of the visitors to experience natural quiet and the sounds of nature, although other noise impacts were identified. An understanding of the specific park problem will be important to factor into any systematic approach to resolving or mitigating overflight issues.


2.2 Estimates of Overflight Exposure

Development of a logical and rigorous plan for conducting field studies of aircraft overflights required estimates of aircraft overflight exposure for all units of the National Park System and for Forest Service Wildernesses. (Tabachnick et al. 1992) Exposure estimates permit a rank-ordering to insure that all levels of exposure were studied. But the limits of cost and schedule prevented collection of on-site acoustic data or direct observation of overflights, and considerable effort was devoted to collection of information through secondary sources. Maps and charts were used to locate parks and wildernesses. Aeronautical charts used by general, commercial and military aviation planning and routing provided location information for various routes and fright areas. The FAA tried to provide information on use of high altitude jet routes for four seasons, but at the time of the report had only been able to provide partial information. Despite a formal Memorandum of Understanding between the Secretaries of Defense, Agriculture, and Interior, it was not possible to obtain information suitable for inclusion in this effort from the Department of Defense. Telephone interviews of tour operators, used to ascertain fright activity on sightseeing routes, yielded some information, but several operators refused to provide information or referred inquiries to national or state coalitions of air tour operators. Finally, there is no accurate method for estimating the level of general aviation traffic on any route within any airspace. Furthermore, there is no strong correlation of exposure based on these numbers and the numbers of visitors who reported hearing aircraft, or the number of overflights reported by the NPS managers or with the degree of concern expressed by managers.


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Conclusion 2.9:

It is extremely difficult, if not impossible, to obtain accurate determinations of aircraft activity over units of the National Park System without on-site collection of objective information. Moreover, numbers of overflights by themselves are not necessarily indicative of the extent of exposure to aircraft sounds or of the severity of a sound exposure problem.


2.3 Sound Measurement Results

There are many methods for collecting objective data on-site that can be used to quantify aircraft activity over units of the National Park System. One method is to measure aircraft sound levels, and various types of sound measurements have been conducted in many of the national parks over the years. In response to Public Law 100-91, the NPS conducted four sound measurement studies, and three of these provide sound measurement results that can be readily compared and summarized.


Though many metrics are available for quantifying sound, one that the NPS has found has proven useful in examining the sounds produced by aircraft overflights of national parks is the "percent of time audible". NPS studies found this metric to be best correlated with visitors' response to sound (see Chapter 6 and Anderson et al, 1993). This is a measure of the amount of time aircraft can be heard at a specific location by an attentive listener, and it is simply the percent of the time that the listener can hear aircraft. It is a measure that is very easy to compile with no special instrumentation other than a stop watch, a pencil and a sheet of paper. As will be discussed in Chapter 6, "Effects on Visitor Enjoyment," percent of time audible is useful because it can be related to visitor reactions to the sound of aircraft overflights. It is also useful because it accounts for the non-aircraft sound levels. For example, if a site is near flowing water or a parking area, the aircraft may not be heard as easily as in a very quiet location. Hence, percent of time audible is a measure of how long aircraft sound levels protrude above all other sounds. As will be discussed in Chapter 6, percent of time audible is easy to measure but extremely difficult to compute or predict. Hence, once a problem site is identified with percent of time audible data, data collection and analysis needs to be done with an analytically manageable metric (e.g. Equivalent Level or Leq.

Figure 2.17 summarizes the percent of time aircraft were audible at specific locations in eight units of the National Park System.

GRAND CANYON: Data were collected in Grand Canyon National Park during three time periods, including shoulder (Spring and Fall) and high visitor use seasons. The first two data collection efforts, in October / November of 1989 and in April / May of 1990, were conducted by observers who made continuous digital sound level tape recordings, and who pressed a button that recorded a tone on the tape whenever an aircraft was heard. (Fidell et al, 1994) The 1989 and 1990 data were collected at each of the locations listed in Figure 2.17 for periods of several hours per day for four or more days. In general, the average time for data collection at these 1989 and 1990 sites was 16 hours. Thus, for example, at Sanup Plateau, where data were collected a total of about 24 hours, aircraft could be heard for about 14 hours or 58 percent of the time.


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The third period of data collection occurred between August / September of 1992, and data was acquired by observers who used sound level monitors that collected and stored sound level data once a second, and who used palm-top computers to key in the times aircraft were heard, the type of aircraft, and the type of non-aircraft sounds that could be heard when no aircraft were present. Data were collected for about 4 hours at each location, except at four sites where visitors were also interviewed (see Chapter 6). At the four interview sites, sound level data were collected an average of 15 hours. All data were collected during daylight hours.

Though these measurements provide far more data than is displayed in Figure 2.17 (see the following section "decibel based data"), the percent of time audible metric provides a means for a simple and quick comparison of the extent to which aircraft are audible. Parts 1 and 2 show aircraft audibility in various parts of the Grand Canyon. For the sites measured, aircraft were heard from a low of about 5 percent of the time to a high of almost 80 percent of the time. The sites were not randomly chosen, so should not be thought of as representing all possible aircraft sound exposures in the Canyon. Rather, the data should be taken to show the general range of aircraft sound exposures and to show that there are many locations where aircraft could be heard for moderate to high percentages of the time.

HAWAII PARKS: The techniques used in 1992 in the Grand Canyon were also applied in Hawaii. (Horonjeff et al. 1993) Data were taken over 4 to 6 hour periods at four of the sites (Kalahaku, Overlook, Pu'u Mamane, Pu'u O'o and Halemaumau Crater) and for periods of 21 to 25 hours at the other four locations. The results are presented in part 3 of Figure 2.17.

FIVE OTHER PARKS: Park personnel were trained in the use of a simplified method to collect sound level data. The method requires a sound level meter and limited training in use of a specialized data form. (Miller et al. 1994) It yields several sound metrics, including percent of time aircraft are audible. In general, the method requires that sound level samples be taken every 15 seconds for 20 to 30 minute periods with the observer keeping a log of which samples include audible aircraft sound. The method was used by park personnel in the five parks shown in Figure 2.17, parts 3 and 4.

CUMBERLAND PARKS: Measurements were conducted eight times at the four locations, so that each site was monitored for about 4 hours. Thus, for example, out of about 4 hours of listening at the Sea Camp Campground, aircraft could be heard more than 55 percent of the time.

MOUNT RUSHMORE: Measurements were conducted more than 20 times over nearly two weeks time at the three sites, and each site was measured for eight to ten hours total. Hence, over approximately 10 hours of monitoring at Borglum View Terrace, aircraft could be heard more than 40 percent of the time.

YOSEMITE: Measurements were conducted twelve times over one to three days at the four sites for a total measurement time of about six hours at each ate. Thus, out of 6 hours of listening, aircraft were audible more than half the time at three of the four locations: Rafferty Creek, Soda Springs and Mirror Lake Road.

PETROGLYPH: measurements were conducted for approximately 20 minutes at each location. Thus, measurements here provide information over more locations, but with less certainty at any single location. The results in Figure 2.17, part 4, show the wide variation in aircraft sound that can be experienced in this park. Of 21 locations where measurements were made, aircraft could be heard more than half the twenty minute listening period at 15 sites.


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Figure 2.17 Sound Measurement Results Acquired in Eight National Parks


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GLACIER: Measurements were conducted seven times over three days at Logan Pass, and three times over two days at Middle Fork. This small sample is not believed to be representative of the situation in Glacier. Of 31/2 hours of listening at Logan Pass, for example, aircraft could be heard slightly more than 10 percent of the time.

Conclusion 2.10:

“Percent of time audible” provides a simple method for the NPS to use in quantifying how much of the time aircraft can be heard at locations in a given national park.

Conclusion 2.11:

“Percent of time audible” data taken in eight national park units, though limited for some of these areas, suggests that there are locations and times in many of these parks when aircraft can be heard for significant portions of time.


Report NPOA 93-4 presents detailed sound level data collected in 1992 in the Grand Canyon and in Haleakala and Hawaii Volcanoes National Parks. Figures 2.18 and 2.19 present samples of these "acoustic profile" data measured in the Grand Canyon and in the Hawaiian Islands, respectively. The form is a graphic one (though the data are also tabulated in NPOA 93-4), and shows both non-aircraft background levels, and maximum aircraft produced levels for the entire day's measurement, a period of 4 to 7 hours for the locations shown.

The vertical axis of the graph shows the A-weighted sound level in decibels. The curve on the left-hand portion of the graph describes the background sound level; the " + " signs on the right hand side describe the maximum sound levels of individual aircraft overflights. The curve on the left shows how background levels varied over the measurement. At Separation Canyon (site number 9, see Figure 9.2 in Chapter 9), background levels were between approximately 31 dB and 12 dB; ten percent of the time they were above 26 dB and 90 percent of the time they were above roughly 16 dB. In comparison, the aircraft that were measured produced maximum levels, Lmax, between about 28 and 57 dB. When the aircraft levels are about the same as the background levels, at least for some of the time, aircraft sounds will tend to be less audible or "masked"; aircraft levels that are above all background levels are easily heard.

Separation Canyon is in an area where the minimum flight altitude is 5000 feet above sea level (MSL). The elevation of the site is about 1300 MSL, so aircraft should be a minimum of 3700 feet above the site. Bright Angel Point, on the other hand, is well within a flight-free zone created by SFAR 50-2 (See Chapter 9, Figure 9.1) and is one of the quietest areas with respect to aircraft sound. As shown, at Bright Angel Point many of the aircraft produced levels are at the level of the background sound levels, at least part of the time. Tour aircraft are audible less of the time at Bright Angel Point (about 6 percent of the time) than at Separation Canyon (about 16 percent of the time, see Chapter 9, Tables 9.2 and 9.3).

Toroweap Overlook is within, but at the edge of another SFAR 50-2 flight-free zone. Aircraft stay over a mile from this location. The figure shows clearly, however, that aircraft sound levels considerably exceed the background levels, which are quite low. Point Sublime is also within a flight-free zone, and though aircraft levels are lower than at Toroweap, aircraft are audible more than 70 percent of the time.


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Figure 2.18 Acoustic Profile Data from Grand Canyon National Park


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The data from the Hawaiian parks, Figure 2.19, suggest the effectiveness of setting minimum altitude restrictions. Over Haleakala (Sliding Sands and Kalahaku Overlook) P.L. 100-91 restricts flight to 9,500 MSL or higher; this restriction is intended to keep aircraft above the rim where the elevation is about 9,600 MSL. Both the Sliding Sands and Kalahaku sites are at about 9,400 MSL. On the other hand, there are no altitude restrictions for Wahaula Temple or Pu'u O'o. Some aircraft flew quite low over Wahaula Temple. .

Conclusion 2.12

Flight-free zones can be designed to effectively limit aircraft sound levels, but they must be very large if natural quiet is to be restored or substantially restored.

Conclusion 2.13

Minimum altitude restrictions can help limit aircraft sound levels, but the restrictions should be carefully chosen, considering location and elevation of the park areas of concern.


2.4 Summary

It is likely that there could be as many as 50 to 100 units of the park system where overflight problems are likely or certain to exist. NPS managers have consistently, for nearly a decade, identified 30-40 parks as priorities for research and problem solving. NPS managers believe about 30 percent or approximately 100 of the National Park System units (excluding Alaska) experience some level or type of aircraft overflights that constitute a problem. In one-third of this set of parks, managers are very or extremely concerned about overflights. More than half of the affected parks are overflown by military, commercial passenger aircraft and general aviation aircraft. Somewhat fewer are overflown by sightseeing aircraft. The primary impact of these overflights is believed by park managers to be the sound produced, and that the sound impact produced by aircraft is more of a problem than sound from any other sources.

Because of the variations in numbers and types of overflights, a systematic method is needed to objectively determine the degree of the overflight problem. However, such a method must be integrated with management objectives. Park managers differ in their concern about aircraft overflights. Local conditions and management objectives play an important role in determining management concern. One objective method that can be used is measurement of "percent of time audible" which answers the question of how often aircraft can be heard. Sound level data in decibels, of the type presented in Section 2.3, graphically show the relation of aircraft to background sound levels.


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Figure 2.19 Acoustic Profile Data from Haleakala and Hawaii Volcanoes National Parks


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