CHAPTER 9

Section 2 of the National Parks Overflights Act (P.L. l00-91) set specific minimum altitude requirements for aircraft overflights of Yosemite and Haleakala National Parks. The intent of the law was to determine the extent to which minimum altitudes for overflights could restore natural quiet. Prior to the passage of the Act, aircraft had been reported flying within a few hundred feet of the ground in these two parks. Hence, increasing the minimum distance above the terrain to 2,000 feet in Yosemite, and almost the same amount in Haleakala, should produce a noticeable difference in the aircraft sound environment, even if not fully restoring natural quiet.

Section 3 of the Act discusses the aircraft overflights issue at Grand Canyon National Park (GCNP). That section states that:

(a) "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...”

This section further states that recommendations submitted to the Administrator of the Federal Aviation Administration (FAA) would:

“provide for substantial restoration of the natural quiet and experience of the park and protection of public health and safety from adverse effects associated with aircraft overflight. "

The Section then states that:

"... the Secretary [of Interior] shall submit to the Congress a report discussing -- (A) whether the plan has succeeded in substantially restoring the natural quiet in the park;.... "

The following sections contain the NPS response to the questions posed in Sections 2 and 3 of the Act.

 9.1 Report on Section 2 Requirements - Yosemite and Haleakala National Parks

9.1.1 Yosemite National Park

"Sec. 2(a) Yosemite National Park -- During the study and review periods provided in subsection (c)¹, it shall be unlawful for any fixed wing aircraft or helicopter flying under visual flight rules to fly at an altitude of less than 2,000 feet over the surface of Yosemite National Park. For purposes of this subsection, the term "surface" refers to the highest terrain within the park which is within 2,000 feet laterally of the route of flight and with respect to Yosemite Valley such term refers to the upper-most rim of the valley."

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1. The "study and review periods provided in subsection (c)" encompass the date of the Act (18 August 1987) through a 2-year period following the submittal of this Report to Congress.

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In Yosemite, management reported the major complaints about overflights came from wilderness trail users (at higher elevations of the park), as opposed to visitors on the valley floor. Management felt that the minimum altitude requirement had helped significantly in reducing complaints. This has not meant a restoration of natural quiet. The sample of acoustic measurements made in Yosemite in 1993, Figure 2.17 part 3, showed aircraft to be audible 30-60 percent of the time. Regarding compliance, management felt that pilots who were aware of the restriction generally complied with the law.

Measurements were made at four sites in Yosemite: Rafferty Creek (Judged to be a location where visitors have a high expectation of experiencing quiet) ; Soda Springs Road (higher expectation of quiet than in valley, but fairly congested with a nearby campground store area and concessionaire stables); Mirror Lake (only part of the valley floor that is closed to traffic, but traffic noise from the rest of the valley is audible) ; Glacier Point (one of most popular destinations in park and often crowded with voices audible). In general, high altitude jets were the most commonly audible aircraft heard at the rate of about 10 to 30 per hour. Private propeller airplanes (General Aviation) were heard at the rate of about two to four per hour.

9.1.2 Haleakala National Park

"Sec. 2(b) Haleakala National Park -- During the study and review periods provided in subsection (c), it shall be unlawful for any fixed wing aircraft or helicopter flying under visual flight rules to fly at an altitude below 9,500 feet above mean sea level over the surface of any of the following areas in Haleakala National Park. Haleakala Crater, Crater Cabins, the Scientific Research Reserve, Halemauu Trail, Kaupo Gap Trail, or any designated tourist viewpoint."

Prior to passage of PL l00-91, management reported that commercial tour helicopters flew within the crater down to levels 300 feet above the crater floor. Noise generated by tour helicopter overflights greatly impacted the wilderness users' enjoyment of Haleakala Crater. Management further stated that Haleakala was famous for its natural quiet, but this quality deteriorated as overflights increased.

Management felt that the altitude restriction of PL l00-91 somewhat reduced the noise levels on the crater floor. However, since the passage of the Act, the number of overflights has increased significantly, thus negating improvements the restriction might have made. Acoustic measurements made at four sites in Haleakala in 1992, Figure 2.17 part 3, showed that aircraft were audible 38-76 percent of the time. Aircraft heard were predominantly helicopters, heard at rates of about eight to ten per hour (Anderson et al. 1993, Horonjeff et al. 1993)

Conclusion 9.1

The lessons learned from Yosemite and Haleakala may be summarized as follows: 

• Raising the minimum altitude to 2,000 feet (the long standing FAA advisory altitude for overflying parks) reduces egregious impacts and may reduce complaints, but does not effectively restore natural quiet. Not only are the lower altitude aircraft audible, but high altitude jets and the sound levels they produce are unaffected by such minimum altitudes. Nevertheless, a 2000 foot minimum altitude can be effective in limiting some adverse effects on natural quiet.

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• Numbers of overflights have an important impact on length of time visitors have an opportunity to experience natural quiet. Unabated increases in numbers can negate gains made through increasing the distance between aircraft and visitors.
   
• As part of their management strategy, park resource managers must carefully consider when and where to preserve natural quiet in devising management strategies for FAA action.
   
• Impacts on natural quiet are likely to be unique at each park, and the solutions equally unique. Approaches to problem solving need to be flexible to produce effective solutions.

9.2 Report on Section 3 Requirements: Grand Canyon National Park

This section provides for the definition of a substantial restoration of natural quiet, an explanation of SFAR 50-2, the description of the studies used in the evaluation, and the conclusions reached. It is important to note that during the course of these evaluative studies, air tours have increased significantly over the Canyon. Existing and forecast operations at the Grand Canyon National Park Airport are a useful indicator of the trends in increasing overflights. For example in 1987, there were 120,180 operations at that airport alone. In 1993, operations at the airport were 187,444, already exceeding those forecast in the airport master plan for the year 2000. By the year 2010, 240,100 operations are forecast. It is vital that this evaluation of the Special Federal Aviation Regulation (SFAR) 50-2 be understood in the context of this growth pattern.

9.2.1 Defining a Substantial Restoration of Natural Quiet

Before overflights began, natural quiet existed over most of park, virtually all of the time. Aircraft sound intrusions are a significant source of mechanical noise that eliminate natural quiet. Since the legislative history of Public Law 100-91 indicates that flight-free zones are to be large areas where visitors can experience the park essentially free from aircraft sound intrusions, and where the sound from aircraft traveling adjacent to the flight-free zone is not detectable from most locations within the zone, the primary measure of restoration is the percentage of time that aircraft are audible. Based on this definition from the legislative history, the policy decision of Grand Canyon National Park (GCNP) is that a substantial restoration requires that 50 percent or more of the park achieve " natural quiet" (i.e.. no aircraft audible for 75-100 percent of the day.

Conclusion 9.2

A substantial restoration of natural quiet in the Grand Canyon will require that there be natural quiet in half or more of the park for most of the day.

9.2.2 Special Federal Aviation Regulations 50-2

The NPS and the FAA have attempted through SFAR 50-2 to accomplish the substantial restoration of natural quiet. The regulation established flight-free zones and specific flight corridors and routes for air tours and general aviation flights. It also established minimum altitude restrictions on all types of flights including air tours, general aviation, high altitude commercial and military aircraft (see Figure 9.1).

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Design and implementation of SFAR 50-2 was a major accomplishment in that it is the first attempt by the FAA to regulate airspace for environmental and safety reasons to such an extent over a national park. Four flight-free zones cover 45% of the park and have a ceiling of 14,499’ MSL (Mean Sea Level). Four flight corridors were established to help aircraft navigate the Special Use Airspace while avoiding the flight-free zones. Approximately 29 aerial tour routes were created by the Federal Aviation Administration's Flight Standards District Office in Las Vegas to allow commercial tour aircraft access to that portion of the Special Flight Rules Area (55%) not restricted by flight-free zones. [At 14.500 feet the entire park is accessible to overflights].

9.2.3 Evaluation of Restoration Efforts (SFAR 50-2)

The determination as to whether SFAR 50-2 has been effective in substantially restoring natural quiet is based on a series of studies and modeling exercises. Among the questions asked are: Do its flight-free zones and altitude restrictions substantially restore natural quiet? How effective is the SFAR? Are there areas where improvements are possible?

MANAGEMENT OBJECTIVES 

First, the NPS reviewed its mandates, regulations, policies, and plans related to the protection of natural quiet and the provision of various visitor experience opportunities. From this review, a statement of management goals and objectives was developed to further assist in the evaluation of the effectiveness of the SFAR 50-2. This statement describes the goals and Table 9.1 summarizes the specific management objectives for each of five management zones in the park.

Goals for aircraft overflight management listed in the GCNP'S policy paper are:

1. Substantially restore natural quiet as a natural resource.

2. Provide recreation opportunities and experiences for park visitors, consistent with park policies, where the opportunity for natural quiet is an important component.

3. Mitigate any aircraft-related impacts on other natural and cultural resources.

4. Address issues of health, safety and welfare of on-ground visitors and employees.

EVALUATION

The evaluation of the restoration of natural quiet is based on the following six categories of studies, monitoring, and modeling exercises.

1. GCNP'S MONITORING AND COMPLAINTS: The NPS has considered information provided by visitor complaints and by the park's aircraft monitoring program. Although complaints have been reduced in number since SFAR 50-2, complaints received now focus on specific areas. That is, complaints are usually correlated to areas that are impacted acoustically which are generally located below the rim. Arguments have been made that the reduction in complaints means that natural quiet has been substantially restored. Even though the NPS values visitor complaints and uses them to help confirm

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Table 9.1 Grand Canyon National Park Management Objectives

problem areas, a relationship cannot be drawn between reduction in complaints and the restoration of natural quiet, which must be acoustically determined. Congress did not request a substantial reduction of visitor complaints; the NPS manages parks to protect resources rather than simply respond to visitor complaints.

The monitoring program logs flights on a yearly basis to determine aircraft use along routes and compliance with the regulation. Monitoring data provide additional confirmation regarding areas of heavy aircraft activity over areas with popular on-the-ground use. For instance, the Hermit Trail, which lies under the Dragon Flight Corridor, was found to experience an average of 35 aircraft overflights per hour. With aircraft passing over any number of points along the trail at this frequency, the sound of aircraft can be constant. Other " areas of concern" include Point Sublime, also under the Dragon Corridor; Point Imperial, the Nankoweap trail and the mouth of the Little Colorado River, under the Zuni Point Flight Corridor routes; and the Toroweap Overlook, close to eastbound routes from Las Vegas.

Results of the monitoring confirm an extremely high rate of compliance by air tour operators and other aircraft users flying over the area. The compliance is a compliment to the managers and users of the airspace over Grand Canyon.

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2. ACOUSTIC MONITORING STUDY: Acoustic monitoring was conducted at 23 sites in the Canyon during August and September of 1992 (Horonjeff et al. 1993). Sites were chosen on the north and south rims and along the river, under both flight corridors and flight-free zones, and Figure 9.2 shows where these sound level measurement sites were located.

The data were collected in a manner that yielded not only decibel levels, but information about the amount of time aircraft overflights could be heard at each site. A detailed analysis of the data provides estimates of the percentages of time the aircraft of various operators could be heard (Robert et al. *). Tables 9.2 and 9.3 present the results for sites under flight-free zones and under flight corridors, respectively. At most sites under flight-free zones, aircraft were audible for lower percentages of the time than was the case under the flight corridors. Generally, within flight-free zones (Phantom Ranch, Bright Angel Point, Yaki Point), tour aircraft were audible ten percent of the time or less, while near the edges (Desert View, Lipan Point, Point Sublime), these aircraft were audible for considerably greater amounts of time. Commercial jets (high altitude overflights) were generally audible less than about 15 percent of the time, but could be heard at most locations. Very few general aviation (GA) or military aircraft were heard.

Conclusion 9.3

Flight-free zones can limit the areas when aircraft, especially tour aircraft, are audible high percentages of the time. But aircraft of all types may still be heard for some percent of the time at virtually all areas when sound data were collected, notably at the edges of some of the flight-free zones. These results suggest that a substantial restoration of natural quiet has not been achieved for large segments of the Canyon.

3. DOSE-RESPONSE STUDIES : In order to quantify how visitors feel about the sound of aircraft overflights, the NPS conducted data collection of a type never before done in a recreational setting: "dose-response" measurements. Simultaneous measurement of aircraft sound levels and surveys of visitors permitted development of "dose-response" relationships that estimate what percent of people are affected by a given level of aircraft overflight sound (Anderson et al, 1993). Only in residential communities have such data ever been acquired, and never with such close coordination between the measured "dose" of sound that was present and the individual responses. Extreme care was taken in the data collection and analysis, and some specific conclusions are possible.

For overflights of tour aircraft, the measure of sound that best predicts visitors' reactions is the percent of time aircraft are audible. The results show that visitors have very different sensitivity to aircraft sound, depending upon the site where data are collected. At the two frontcountry "overlook" sites, Lipan Point and Point Imperial, for a given level of aircraft sound, considerably fewer visitors reported annoyance or interference with natural quiet than reported these effects at the three "short-hike" sites of Hermit Basin in the Grand Canyon and Sliding Sands and Wahaula Temple in Hawaii. Though many factors likely influence this sensitivity, it is reasonable to conclude that as visitors pursue activities that take them away from their cars and other visitor activities, they are likely to be more sensitive to the sound of aircraft overflights - tour aircraft in the case of this study. For visitors to the short-hike sites, roughly 30 to 40 percent can be expected to report moderate to extreme interference with their appreciation of natural quiet when aircraft are audible ten percent of the time².

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2. See (Anderson, G.S., et al, 1993) Figure E.3 or Figures H.8 and H.9.

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Table 9.2 Percent of Time Aircraft were Audible Under Flight-Free Zones

* The measured percent of time audible will not always equal the sum of percents by operator because aircraft of different operators were sometimes audible at the same time.

Conclusion 9.4

The percent of time aircraft are audible correlates with how visitors feel about aircraft sound. Even when aircraft are audible for relatively low percentages of time, a percentage of the visitors can notice the aircraft, and believe that the sound has interfered with their appreciation of natural quiet. Further, it is likely that visitors who hike away from auto accessible locations are more sensitive to intruding aircraft sounds than are visitors who do not. Hence, the NPS concludes that preservation of natural quiet is of significant value to visitors, especially for the backcountry, river corridor and Cross Canyon Corridor trail system use zones at GCNP.

Table 9.3 Percent of Time Aircraft were Audible Under Flight Corridors

4. BENNETT-COX STUDY: The Air Access Coalition (an association of air tour operators) retained Bennett / Cox, Consultants, to sample sound exposure at 22 sites before (in 1988) and after (in 1993) SFAR 50-2 was implemented over the Grand Canyon (Bennett et al. 1994). The consultants used methods that permitted separate identification of maximum sound levels for different aircraft types and ranges of non - aircraft sound levels. One of the measurement sites chosen, Point Sublime, was also a site where NPS had acoustic information collected in September 1992³. The Bennett / Cox data show considerable reduction of (maximum) A-weighted sound levels from 1988 to 1993, attributable to SFAR 50-2. In fact,

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3. Site 19.0 reported in NPOA 93-4, page 150 ff.

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the Bennett / Cox 1993 aircraft sound levels are entirely consistent with the NPS NPOA Report No. 93-4 levels for Point Sublime, and the NPS acknowledges that SFAR 50-2 has produced significant reductions in aircraft sound levels for this location. The Air Access Coalition would like to use 1988 sound levels as a baseline to compare changes in sound levels; l988 was already too noisy for the NPS which uses natural quiet as the baseline for comparison.

However, the Bennett / Cox presentation fails to report two other important aspects of the Point Sublime sound environment, one an omission, one probably an error. First, the presentation gives no account of how much of the time aircraft could be heard or how many were heard. Over approximately a five-hour period, the NPS data report roughly 20 to 30 aircraft per hour were heard, and aircraft were audible an average of 76 percent of the time (See Table 9.2). Second, Bennett / Cox report the non-aircraft background sound levels as between approximately 20 and 40 dBA. Though such background levels are certainly possible, particularly if there was fairly continuous wind or bird and insect sounds, it is likely that the equipment used was not capable of accurately measuring the low sound levels present at this location. During the NPS sponsored measurements, non-aircraft sound levels were between approximately 10 and 20 dBA roughly 75 percent of the time when aircraft were not audible. Standard sound measuring instrumentation used in community noise measurements will not accurately measure below 20 to 25 dBA. Special "low-noise" instruments were acquired and designed and constructed for the NPS measurements (Horonjeff et al. 1993)⁴. Thus, though the Bennett / Cox presentation reasonably depicts the changes in (maximum) aircraft sound levels, it fails to address two aspects of the sound environment that are critical for judging the restoration of natural quiet: the extreme quiet present at some locations, and the amount that this quiet is disrupted by the sound of aircraft overflights.

Conclusion 9.5

The Air Access Coalition sponsored data demonstrate that SFAR 50.2 has reduced aircraft sound levels significantly at some locations. However, these data do not address restoration of natural quiet, since no information is given about how much of the time aircraft can be heard, and reported non-aircraft sound levels are probably inaccurately high.

5. VISITOR SURVEY: A mail survey was conducted of randomly sampled visitors to the Grand Canyon. These visitors were separately identified in five categories: frontcountry visitors, summer and fall backcountry visitors, river users in motorized boats, and river users in oar-powered boats (Baumgartner et al. 1994). Figures 9.3 and 9.4 show how these visitors ranked various reasons for their trip to the Canyon. Visitors were asked to rate (not important at all, slightly important, moderately important, very important, extremely important) eight different reasons⁵. Five of these categories (representing the range of responses) are shown in Figures 9.3 and 9.4. Figure 9.3 shows the percent who rated the reasons as moderately, very or extremely important. Clearly, all these reasons are important in this figure.

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4. See Page 9 and NPOA Report 93-6, Appendix c for descriptions of the instrumentation used.

5. View the natural scenery, enjoy the natural quiet, appreciate the history and/or cultural significance of the park, do things with family, experience piece and quiet, see new and different things, learn about things in the park, get some physical exercise.

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However, Figure 9.4 shows just those who rated the reasons as extremely important. Natural quiet, for river and fall backcountry visitors is almost as important as viewing the scenery, while the other reasons are less important.

Conclusion 9.6

For some categories of visitors, specifically river users and fall backcountry visitors, natural quiet is almost as important a reason for visiting the Grand Canyon as is viewing the scenery. Enjoying natural quiet is extremely important to many visitors to the Grand Canyon.

Of these different categories of visitors, how many reported hearing aircraft? Figure 9.5 gives the percentages. For all categories, half or more than of the surveyed visitors remembered hearing aircraft. Figure 9.6 (identical to Figure 3.4) shows what percentages of these visitors reported moderate, very or extreme impacts from the overflights.

Conclusion 9.7

Different categories of visitors report different degrees of adverse effects, but for all categories sampled, more visitors report that aircraft interfere with their appreciation of natural quiet than report interference with enjoyment or annoyance.

Figure 9.7 shows how inappropriate the five categories of visitors thought six different types of overflights to be when within hearing or sight of visitors. Clearly, most visitors find military training and private aircraft somewhat or very inappropriate over National Park areas. Tour aircraft overflights and "transporting commercial passengers between cities " are judged inappropriate by roughly comparable percentages of visitors, depending somewhat upon visitor category. Finally, few visitors judge park "management, research, and maintenance" or "emergency services, like fire fighting or search and rescue" as inappropriate.

Conclusion 9.8

Except for park management and emergency related overflights, large percentages of Grand Canyon visitors regard aircraft overflights within sight or hearing of visitors on the ground as somewhat or very inappropriate over National Park areas.

The mail survey asked:

"Considering the advantages and disadvantages of aircraft flying over National Park areas, what do you think the National Park policy should be for aircraft activity for the following aircraft flight purposes?"

Figure 9.8 presents the responses for "sightseeing tour flights". The great majority of respondents neither support "reasonable growth" nor "do nothing. " They want to see the activity stay at current levels or be reduced / eliminated. Virtually all of the river/oar and fall backcountry visitors support reduction or elimination of sightseeing tour flights.

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Conclusion 9.9

There is little support among the five categories of Grand Canyon visitors for a "do nothing" policy or a "reasonable growth" policy. Maintenance of the current level, or reduction / elimination are preferred policies.

Finally, the survey asked:

"If aircraft activity got to the point where restrictions were thought to be necessary on aircraft flights over a National Park, how much would you support each of the following .., restrictions on sightseeing tour flights"

Figure 9.9 shows percentages of visitors, by visitor category, who support or strongly support seven types of limitations. Visitor support varies with visitor category, but over half of all visitors would support encouraging the use of quieter aircraft, restricting the number of flights that are permitted to fly over the park, establishing times of the day when aircraft are not permitted to fly over the park, establishing areas in the park where aircraft are prohibited, as well as areas where they are allowed to fly.

Conclusion 9.10

A majority of visitors to the Grand Canyon would support several specific types of limitations on air tour overflights.

6. ACOUSTIC MODELING/QUIET AIRCRAFT STUDY: Computerized acoustic modeling is a commonly used approach to depict sound levels or sound exposure over large geographic areas. On-site sound measurements are generally difficult and time consuming, and can be conducted at only a few specific locations, hence computer modeling of sound exposure is necessary in order to understand sound levels area-wide. The NPS sponsored development of a computer model (the National Park Service Overflight Decision Support System or NODSS) (Reddingius 1994) that can calculate various sound metrics across parks, including time-above a specified threshold (e.g., natural quiet). The program computes sound levels for large areas of a park, using information about types, numbers and altitudes of aircraft flown, locations of flight tracks and geographic terrain information. The results of the computations included in this report can be interpreted in terms of "natural quiet" (green), "substantial restoration of natural quiet (white), " and "remainder of park (red), " and hence provide a visualization of the status of natural quiet in GCNP.

Using the numbers, routes, altitudes, and equipment types indicated in a 1989 FAA survey conducted on behalf of the NPS, the NODSS modeling software produced Figure 9.10 (1989 Levels). The combined colors of white and green cover about 34% of the park (white is 0.49% and green is 33.94%), If no quiet aircraft are introduced, and operations continue to increase as forecast in the Grand Canyon National Park Airport Master Plan, Figure 9.11 results (2010 No Action). This figure graphically depicts the progress lost in the restoration of natural quiet that could occur by the year 2010 if conditions remain unchanged. The combined colors of white and green cover only about 10% of the park (white is 0.39% and green is 9.97%), This clearly suggests that increasing use will result in a degradation of natural quiet in the GCNP; this is clearly unacceptable.

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Conclusion 9.11

Computer modeling supports the conclusion that natural quiet has not been substantially restored, that very few areas currently experience natural quiet, and that the areas of natural quiet will diminish considerably if no quiet aircraft are introduced and if tour operations are permitted to increase. The acoustic profiles tend to verify the computed results.

9.2.4. Summary of Section 3 Requirements

Though significant reductions in aircraft sound have occurred for areas of the Grand Canyon, and though compliance with SFAR 50-2 has been excellent, natural quiet is not yet substantially restored to GCNP. This lack of natural quiet affects some visitor groups much more than others, with backcountry users and river /oar users more affected and frontcountry visitors less affected. All studies point to these same conclusions, and the NPS is obligated, in pursuit of both its Congressionally mandated and defined management responsibilities, to seek a further restoration of natural quiet. Most visitors support keeping air tour operations numbers at current levels or reducing them.

Conclusion 9.12

There has not been a substantial restoration of natural quiet in Grand Canyon, although the NPS acknowledges the value of the SFAR and the improvement it has brought.

Conclusion 9.13

If no changes are made to the SFAR, progress to date in the restoration of natural quiet will be lost. Projections suggest that without further improvements, the loss of natural quiet will accelerate to an unacceptable level.

Conclusion 9.14

The NPS recommends that SFAR 50-2 be revised to effect a greater restoration of natural quiet.

9.3 What are the Opportunities for Solutions?

Overflights that impact natural quiet may be divided into two types:

• Flights of very low altitude (altitudes of a few hundred feet or less, flown primarily by helicopters) which go well beyond the issue of natural quiet and enter areas of personal safety, appropriateness in a park setting, concern for effects on wildlife, and so forth.
   
• Flights of somewhat higher altitude or greater lateral distance from the visitor which primarily impact natural quiet or the acoustic/ aesthetic character of the setting, but little else. 

It is the latter of these two types which is addressed here.

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9.3.1 Realistic Expectations

The findings of many different studies all strongly indicate that if aircraft fly over a national park where quiet is a resource, there will never be complete natural quiet at all times and in all places within the park. Instituting "flight-free" zones, such as was done under SFAR 50-2 at Grand Canyon National Park, may create some areas where aircraft are not audible, but to be effective in restoring natural quiet to large areas, flight-free zones will need to be very large - on the order of 20 to 30 miles, in minimum dimension. Directly below authorized flight corridors no reasonable minimum altitude either for tour aircraft or for higher altitude commercial jet traffic can completely restore natural quiet. Quieter aircraft may help restore natural quiet to greater land areas to the side of the flight corridor, but not directly beneath it. Beneath the corridor, quieter aircraft can reduce the degree of impact, but not eliminate it.

Reducing the numbers of overflights or increasing the number of passengers per aircraft reduces the frequency of the intrusions, and in turn increases the uninterrupted periods of time visitors may experience natural quiet. It is likely that very large reductions would be required at seriously affected parks before visitors within one to two miles of flight corridors would experience only infrequent impacts to the natural quiet of the environment.

Moving the locations of flight corridors needs to be carefully analyzed so that the noise burden is not simply shifted to another area that is just as valued by another segment of the visitor population. Natural quiet may be improved in one area of the park, but degraded in another. To the extent that visitors are geographically distributed over large areas of the park, gains and losses from moved flight corridors must be carefully considered. Commercial interests may be economically affected by changes in air corridors, and there may be safety and regulatory implications as well.

9.3.2 Realistic Opportunities

If the FAA and NPS act cooperatively, there are opportunities for problem solving. Where aircraft overflights are over or immediately adjacent to park boundaries, management must define where it is important to preserve natural quiet and what opportunities they seek to provide park visitors. They must also ascertain the critical areas of the park and times of day these opportunities can be provided, and work with the affected parties to reach compromise on achieving their goals. At a minimum, the affected parties will include the FAA, air tour operators, and park management.

SEPARATION OF VISITORS AND OVERFLIGHTS. Dedicating certain areas of the park for tour overflights is likely to be the first step. In so doing, natural quiet under and to the side of corridors will be degraded. The loss of natural quiet is the consequence of accommodating aircraft overflights. Mitigation opportunities in the land areas adjacent to flight areas or corridors will be park specific, and may take advantage of natural attenuation opportunities.

EXPLOITING NATURAL ATTENUATION. To the extent that altitudes can be minimized (without going below reasonable minimums), park terrain can sometimes be used to acoustically shield flight-free areas from aircraft noise. If hills or ridges are available, lowering the aircraft altitude should be considered. By lowering altitudes, areas directly beneath flight corridors that are already impacted will have impacts intensified, but if local terrain features are present, land areas where natural quiet is protected may be increased. Breaking the line-of-sight between the visitor and aircraft can reduce maximum noise levels by an amount that would otherwise be gained only by a near doubling of the distance between aircraft and the visitor.

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In flat or open areas where terrain shielding cannot effectively be used, distance (either in altitude or laterally) is a mitigation option. Very large distance changes may be necessary to achieve natural quiet, however. To a first approximation, 10 decibels of reduction can be expected for every doubling of distance between the visitor and aircraft at its closest point of approach. Thus, to obtain the first 10 decibels of reduction, the existing distance between aircraft and the nearest visitor would have to be doubled. To obtain 20 decibels of reduction the distance would have to be quadrupled, and to obtain 30 decibels of reduction, the distance would have to be increased by a factor of eight.

ENCOURAGING NOISE REDUCTION AT THE SOURCE. Another mitigation measure is encouraging and phasing in quieter aircraft, or retrofitting existing aircraft. Aircraft speed, power, and propeller pitch on fixed wing aircraft, and flight regimes which eliminate blade slap for helicopters are also effective mitigation measures to be taken at the source of the noise. Relationships between these variables and aircraft noise levels will be aircraft specific, and may require additional study. The NPS believes that quiet aircraft will, over time, contribute to the mandated substantial restoration of natural quiet at GCNP. Figure 9.12 shows a comparison between observer-based audibility contours for quieter aircraft (deHavilland DHC-6- 300 Twin Otter and the McDonnell Douglas MD-900 NOTAR Helicopter) vs. louder aircraft (Cessna 207/402 and Bell Jet Ranger helicopter). This graphic shows clearly the critical need to reduce noise at the source.

REDUCING DURATION OF NOISE INTRUSIONS. Limiting times of day may be another mitigation alternative, but this measure may result in a greater intensity of flying during other portions of the day. This alternative may not be met with enthusiasm from air tour operators, however, since their investment in aircraft could remain unproductive for periods of time.

ENCOURAGING USE OF GREATER PAYLOAD AIRCRAFT. Tour aircraft which can accept greater numbers of passengers without substantial increases in noise level emissions may be an attractive step toward mitigation in some circumstances. With larger numbers of people per flight, and fewer flights, the percentage of time that natural quiet is compromised would be reduced.

9.3.3 Environment Needed for Effective Comprehensive Solutions

Some parks have unique resources of natural quiet to be protected. These resources may be concentrated in limited areas of some parks, or distributed over large areas of others. Each park also has unique air tour attractions. In some parks, these attractions are fixed in location. In others, such as Hawaii Volcanoes NP, areas of air tour interest change weekly or even daily. Parks also have unique mitigation options which affect how sound propagates from aircraft to areas where natural quiet is to be protected. These parks need unique airspace management plans approved by the FAA.

For these reasons, effective solutions to restoring the natural quiet in parks must be sought at a local level. A national-level framework or process is necessary to facilitate this. Tools for identifying opportunities for improvements, and tools for evaluating the effectiveness of improvements must be provided to local park management. They must also be equipped with a set of procedures for using these tools that will guide them through the process to satisfactory solutions. As part of the 1992 park manager's survey, management was asked how strongly they would support a formal set of procedures to resolve overflight issues. Over 80 percent of the managers indicated a moderate to extreme support for formal procedures.

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Conclusion 9.15

The uniqueness of individual park units, their visitation areas, and their opportunities for mitigation to improve natural quiet strongly suggests that in a systematic approach to problem solving, flexibility will be essential to get to productive park-level solutions.

Conclusion 9.16

Interagency airspace management coordination guidelines are needed to outline agency responsibilities and interfaces on overflight issues.

9.4 Summary

Simple limitations on altitude can diminish the impacts of overflights, but it is unlikely that those limitations alone will effectively restore natural quiet. In the case of Grand Canyon, even SFAR 50-2 could not produce a substantial restoration of natural quiet, although its value is recognized. Visitors are sensitive to the diminution of natural quiet, and many rate experiencing natural quiet as an important reason for visiting the Grand Canyon. Visitors to other parks also rate experiencing natural quiet as an important part of their visits.

The failure to substantially restore natural quiet should not be interpreted as an indication of the failure of any group to take appropriate action. It is an indication of the difficulty of finding solutions that will make the effective use of airspace and preserve park resources simultaneously. The NPS recommends that SFAR 50-2 be revised by the FAA to contribute to a more effective restoration and maintenance of natural quiet.

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