AVIATION NOISE LAW
U.S. Air Tour Association et al. v. FAA et al.
Cite as: 298 F.3d 997


U.S. COURT OF APPEALS FOR THE DISTRICT OF COLUMBIA CIRCUIT

UNITED STATES AIR TOUR ASSOCIATION, et al., Petitioners
v.
FEDERAL AVIATION ADMINISTRATION, et al., Respondents

GRAND CANYON TRUST, et al., Intervenors

No. 00-1201
Consolidated with 00-1212

On Petitions for Review of an Order of the Federal Aviation Administration

Argued May 9, 2002
Decided August 16, 2002


COUNSEL:

William Davis Thode and Joseph F. Becker argued the cause for petitioners United States Air Tour Association, et al. With them on the briefs was William Perry Pendley. Lorraine B. Halloway and Timothy M. Biddle entered appearances.

Alexander E. Dreier argued the cause for petitioners Grand Canyon Trust, et al. With him on the briefs were Michael L. Kidney, Catherine S. Stetson, Jeffrey C. Nelson, and Robert Wiygul.

Ronald M. Spritzer, Attorney, U.S. Department of Justice, argued the cause for respondents. With him on the brief was Ellen J. Durkee, Attorney.

Michael L. Kidney, Catherine E. Stetson, Alexander E. Dreier, Jeffrey C. Nelson, and Robert Wiygul were on the brief of intervenors Grand Canyon Trust, et al.

Glenn M. Feldman argued the cause and filed the brief for intervenor Hualapai Indian Tribe.


Before: Edwards, Henderson, and Garland, Circuit Judges.

Opinion for the Court filed by Circuit Judge Garland.

Garland, Circuit Judge: As part of an ongoing effort to reduce aircraft noise in Grand Canyon National Park, the Federal Aviation Administration (FAA) promulgated a rule limiting the number of air tours permitted to fly over the Park. Two groups of petitioners, one led by the United States Air Tour Association and the other by the Grand Canyon Trust, challenge that rule. We reject the challenges brought by the Air Tour Association, but conclude that the challenges brought by the Trust raise issues that require further consideration by the FAA.


I

The history of regulation of aircraft overflights at Grand Canyon National Park is set out in Grand Canyon Air Tour Coalition v. FAA, 154 F.3d 455, 460-64 (D.C. Cir. 1998) [hereinafter Grand Canyon I]. We recount some of that story here and explain subsequent developments to the extent necessary to give context to the present controversy.


A

In 1987, Congress enacted the National Parks Overflights Act, Pub. L. No. 100-91, 101 Stat. 674 (set out at 16 U.S.C.A. 1a-1 note). Section 3 of the Act declared that "[n]oise 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." Overflights Act § 3(a). To address this problem, Congress required the Secretary of the Interior to submit to the Administrator of the FAA:

recommendations regarding actions necessary for the protection of resources in the Grand Canyon from adverse impacts associated with aircraft overflights. The recommendations shall 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 overflights.
Id. § 3(b)(1) (emphasis added). Congress also required the FAA to "prepare and issue a final plan for the management of air traffic in the air space above the Grand Canyon." Id. § 3(b)(2). That plan, the Act declared, "shall ... implement the recommendations of the Secretary without change unless the [FAA] determines that implementing the recommendations would adversely affect aviation safety." Id. Finally, Congress directed the Secretary to submit, within two years of the effective date of the plan, "a report discussing (A) whether the plan has succeeded in substantially restoring the natural quiet in the park; and (B) such other matters, including possible revisions in the plan, as may be of interest." Id. § 3(b)(3).

In response to the Overflights Act, the Secretary of the Interior submitted recommendations to the FAA in December 1987. In May 1988, the FAA implemented those recommendations in the form of Special Federal Aviation Regulation (SFAR) 50-2. See Special Flight Rules in the Vicinity of the Grand Canyon National Park, 53 Fed. Reg. 20,264 (June 2, 1988). The regulation applied to aircraft flying below 14,500 feet and established, inter alia, flight free zones (areas into which aircraft may not fly), minimum altitudes, and other rules constraining flight paths within the Park. It remained in effect through 1997.

On September 12, 1994, the National Park Service (the Park Service or NPS), on behalf of the Secretary of the Interior, submitted the report to Congress required by section 3 of the Overflights Act. See NPS, U.S. Dep't of the Interior, Report on the Effects of Aircraft Overflights on the National Park System (published in 1995) [hereinafter 1994 NPS Report]. In that report, the Park Service made several foundational determinations. First, it decided that the appropriate measure for quantifying aircraft noise was the percentage of time that aircraft are audible. See id. at 60. Second, the Park Service concluded that the key statutory phrase, "substantial restoration of the natural quiet," required that "50% or more of the park achieve 'natural quiet' (i.e., no aircraft audible) for 75-100 percent of the day." Id. at 182. Subsequently, the agencies determined that an aircraft was audible at three decibels above the average natural ambient sound level. See FAA, U.S. Dep't of Transp., Environmental Assessment: Special Flight Rules in the Vicinity of Grand Canyon National Park, 4-4 to 4-5 (1996) [hereinafter 1996 Environmental Assessment].

Applying these principles, the agencies concluded that, under SFAR 50-2, only 31% of the Park enjoyed "a substantial restoration of natural quiet" -- by which they meant that only 31% of the Park experienced natural quiet for at least 75% of the day. Special Flight Rules in the Vicinity of Grand Canyon National Park, 61 Fed. Reg. 69,302, 69,317 (Dec. 31, 1996) [hereinafter 1996 Final Rule]. [FN 1] Moreover, the agencies predicted that without revisions to the existing regulations, projected growth in the number of air tours would cause the percentage of the Park enjoying substantial restoration of natural quiet to drop to less than 10% by the year 2010. Id.

[FN 1] When the agencies discuss their current progress toward "a substantial restoration of natural quiet," they refer to the percentage of the Park experiencing natural quiet for at least 75% of the day. When they discuss the overall statutory goal of "substantial restoration of the natural quiet," however, they refer to a situation in which at least 50% of the Park achieves natural quiet for at least 75% of the day. Depending upon the context, we will use the phrase in the same two ways in this opinion.
On December 31, 1996, the FAA issued a final rule that adopted the definitions contained in the 1994 NPS Report, including the definition of substantial restoration of the natural quiet. See 1996 Final Rule, 61 Fed. Reg. at 69,305-10. Among other things, the 1996 Final Rule also established new flight free zones, instituted flight curfews, and set a cap on the number of aircraft that could fly over the park -- although not on the number of flights. See id. at 69,317, 69,332. In addition to the 1996 Final Rule, the FAA proposed two further rules: one to modify flight paths in the Park; the other to require operators to use quieter aircraft. See Proposed Air Tour Routes for the Grand Canyon National Park, 61 Fed. Reg. 69,356 (Dec. 31, 1996); Noise Limitations for Aircraft Operations in the Vicinity of Grand Canyon National Park, 61 Fed. Reg. 69,334 (proposed Dec. 31, 1996). The FAA predicted that the 1996 Final Rule, in conjunction with the two proposed rules, would meet the statutory goal of substantial restoration of the natural quiet by the year 2008. See Noise Limitations for Aircraft Operations, 61 Fed. Reg. at 69,329.

In October 1997, the FAA discovered that it had significantly underestimated the number of tour aircraft operating in the Park, and that as a consequence the 1996 Final Rule would be less effective than it had thought. See Special Flight Rules in the Vicinity of Grand Canyon National Park, 62 Fed. Reg. 58,898, 58,899 (Oct. 31, 1997). After oral argument in Grand Canyon I, the FAA informed the court that it was considering placing a cap on the number of flights, in addition to the 1996 Final Rule's cap on the number of aircraft. See Grand Canyon I, 154 F.3d at 464.


B

In Grand Canyon I, several groups of petitioners challenged provisions of the 1996 Final Rule. The principal challenges came, as they do here, from a group of air tour operators (the Air Tour Coalition) that included members of petitioner Air Tour Association, and from a group of environmental organizations led by petitioner Grand Canyon Trust. The air tour operators argued that the rule did "too much, too soon," while the Trust argued that it did "too little, too late." Grand Canyon I, 154 F.3d at 459-60. We upheld the rule against both challenges.

In the course of our decision, we affirmed -- against challenges from both the Coalition and the Trust -- the Park Service's definition of "substantial restoration of the natural quiet" as "50% or more of the park achiev[ing] 'natural quiet' (i.e., no aircraft audible) for 75-100 percent of the day." That definition, we said, was a reasonable construction of an ambiguous statutory phrase. Id. at 466-67 (citing Chevron U.S.A. Inc. v. Natural Res. Def. Council, 467 U.S. 837, 841-43 (1984)). We also upheld as reasonable the agencies' three-decibels-above-ambient measure of audibility, and we rejected the Air Tour Coalition's contention that the agencies had ignored their statutory obligation to consider the actual experience of Park visitors. Id. at 465-67, 469. We did not need to decide whether there was such a statutory obligation, because it was clear that the agencies' definition and audibility measure were specifically developed to address and to enhance the experience of Park visitors. Id. Grand Canyon I also observed that the Overflights Act "clearly divides the institutional responsibilities between" the FAA and the Park Service. Id. at 468. Because the Act directs the FAA to "implement the recommendations of the Secretary without change" unless they would adversely affect aviation safety, we held that the FAA "had no choice but to adopt the Park Service's recommendations" regarding substantial restoration of the natural quiet. Id.

Finally, we noted the Trust's complaint that, using the Park Service's definition and the FAA's recent reevaluation of its data, neither the 1996 Final Rule nor the two additional proposed rules would achieve Congress' goal of substantially restoring natural quiet in the Park. We agreed that it would be arbitrary and capricious for the government not to intend to achieve the congressional goal on any timetable at all. Id. at 477. We accepted, however, the FAA's assurance that it still "anticipates meeting the goal of substantial restoration by 2008" through implementation of the two proposed rules and consideration of a cap on the total number of overflights. Id. at 478. In so doing, we emphasized that the Trust could raise its claim again if "the FAA does not issue additional regulations reasonably promptly, or if those regulations do not appear likely to achieve the statutory goal on a reasonable timetable." Id.


C

On April 4, 2000, the FAA published two additional rules governing flights over the Grand Canyon. One of those rules (the Airspace Rule), not at issue here, modifies air flight paths in the Park. [FN 2] The second rule, the Limitations Rule, is the subject of the petitions for review filed in this case. That rule imposes a cap on the total number of commercial air tours that operators may run in the Park. See Commercial Air Tour Limitation in the Grand Canyon National Park Special Flight Rules Area, 65 Fed. Reg. 17,708 (April 4, 2000) (codified at 14 C.F.R. ss 93.303-.325) [hereinafter Limitations Rule]. Under the Limitations Rule, an air tour operator may not conduct more flights in the Park than it conducted during the base year of May 1, 1997 through April 30, 1998. 14 C.F.R. 93.319(a), (b).

[FN 2] See Modification of the Dimensions of the Grand Canyon National Park Special Flight Rules Area and Flight Free Zones, 65 Fed. Reg. 17,736 (April 4, 2000). On October 17, 2001, this court severed challenges to the Airspace Rule from those to the Limitations Rule at issue here, and held the former challenges in abeyance pending ongoing FAA administrative proceedings.
In developing the Limitations Rule, the FAA and the Park Service issued three associated documents that detailed the methodology they used to quantify noise levels in the Park and to measure progress toward the goal of substantial restoration of the natural quiet. First, the Park Service announced in July 1999 that it was changing the threshold at which it would regard aircraft noise as audible in part of the Park. See Change in Noise Evaluation Methodology for Air Tour Operations Over Grand Canyon National Park, 64 Fed. Reg. 38,006 (July 14, 1999) [hereinafter Change in Noise Evaluation Methodology]. As discussed above, the agencies had previously adopted a threshold of three decibels above the average natural ambient sound level, a measure of audibility we affirmed in Grand Canyon I. In the Change in Noise Evaluation Methodology, the Park Service divided the Park into two zones: Zone One, encompassing about one-third of the Park, includes the more developed areas; Zone Two, encompassing two-thirds, contains the backcountry. The Park Service announced that in Zone One it will continue to consider aircraft audible at three decibels above the average natural ambient level. 64 Fed. Reg. at 38,006-08. For Zone Two, however, the Park Service determined that aircraft noise is audible if it is eight decibels below the average natural ambient level. Id.

Second, in January 2000, the Park Service issued a review of its Change in Noise Evaluation Methodology. NPS, Dep't of the Interior, Review of Scientific Basis for Change in Noise Impact Assessment Method Used at Grand Canyon National Park (2000) [hereinafter 2000 NPS Review]. The review explained in detail the acoustic model used in assessing noise impacts in the Park. It also reaffirmed the Park Service's 1994 definition of "substantial restoration of the natural quiet" as: "Fifty percent or more of the Park achieving 'natural quiet' (i.e., no aircraft audible) for 75-100 percent of the day." Id. at 16. And it stated that "[t]his definition is a threshold not to be exceeded on any given day ... and refers to ... the 12 hour daylight period ... during which air tours occur." Id.; accord id. at 4-5.

Third, in February 2000, the FAA issued a Final Supplemental Environmental Assessment in which it analyzed the effects that it expected the Limitations Rule to have on noise in the Park. See FAA, U.S. Dep't of Transp., Final Supplemental Environmental Assessment: Special Flight Rules in the Vicinity of Grand Canyon National Park (2000) [hereinafter FSEA]. Of particular importance here, the FSEA stated that the FAA intended to use an "average annual day" standard when determining the percentage of "the day" that is restored to natural quiet at a given location. See id. at 4-12, 4-18, F-4. The assessment also made clear that the FAA's noise model only accounts for noise from tour aircraft, and does not consider noise from other aircraft that fly over the Grand Canyon, including commercial jets, general aviation, and military flights. Id. app. G, at 40.

Applying these standards, the FAA concluded that the Limitations Rule would make "significant steps towards substantially restoring natural quiet," Limitations Rule, 65 Fed. Reg. at 17,713, although the combined effect of all of the agency's rulemakings would still not achieve the goal of having 50% of the Park experience natural quiet for at least 75% of the day, id. at 17,711. The FAA estimated that only 32% of the Park currently achieved that mark, and that if no further action were taken, future air tour growth would reduce that to 25% of the Park in nine to ten years. Id. at 17,724. Adding the new 2000 rules, the FAA predicted, would increase the percentage of the Park experiencing the substantial restoration of natural quiet to above 41% and maintain that level in the future. Id.; see FSEA at 4-18 (predicting substantial restoration of 43.6% of the Park through 2003 and 43.5% in 2008). But the agency recognized that additional steps, including implementation of the still-pending quiet technology rule, would be necessary to achieve Congress' goal in the Overflights Act. Limitations Rule, 65 Fed. Reg. at 17,714.


D

As noted above, two groups of petitioners have filed challenges to the Limitations Rule. The first is led by the United States Air Tour Association, a trade organization whose members fly air tours over the Park. The second, led by the Grand Canyon Trust, is a group of six environmental organizations. Petitioners level a number of challenges at the rule, raising questions of statutory construction, regulatory interpretation, and the rationality of the agencies' methodologies and policy choices.

As to questions of statutory construction, where legislation is "silent or ambiguous with respect to [a] specific issue," we are obligated to defer to an agency's interpretation as long as it is "based on a permissible construction of the statute." Chevron, 467 U.S. at 843. Similarly, we defer to an agency's reading of its own regulation, unless that reading is "plainly erroneous or inconsistent with the regulation." Auer v. Robbins, 519 U.S. 452, 461 (1997) (internal quotation marks omitted). We also examine agency regulations to determine whether they are "arbitrary, capricious, an abuse of discretion, or otherwise not in accordance with law." Administrative Procedure Act, 5 U.S.C. 706(2)(A). In that regard, the question for the court is whether the agency has considered the relevant factors and articulated a "'rational connection between the facts found and the choice made.'" Motor Vehicle Mfrs. Ass'n v. State Farm Mut. Auto. Ins. Co., 463 U.S. 29, 43 (1983) (quoting Burlington Truck Lines, Inc. v. United States, 371 U.S. 156, 168 (1962)). Finally, by statute the FAA's findings of fact "are conclusive" if they are "supported by substantial evidence." 49 U.S.C. 46110(c).

We apply these principles to our analysis of the Air Tour Association's challenges to the Limitations Rule in Part II below, and to those of the Grand Canyon Trust in Part III.


II

The Air Tour Association asks this court to hold the Limitations Rule unlawful for five principal reasons: (i) it was prompted by an improper change in the definition of "natural quiet"; (ii) the acoustic methodology that justifies the rule is scientifically flawed; (iii) the FAA arbitrarily and capriciously issued the rule without first promulgating a quiet technology rule; (iv) in promulgating the rule, the FAA violated the Regulatory Flexibility Act, 5 U.S.C. 601 et seq.; and (v) the rule arbitrarily and capriciously ignores the needs of the elderly and disabled. These five contentions are considered in the following sections.


A

In determining the need for the Limitations Rule, as well as its predicted impact on noise in the Park, the FAA employed the standards announced in the Park Service's 1999 Change in Noise Evaluation Methodology. Although the new methodology continues to use the three-decibels-above-ambient threshold for measuring audibility in the more-developed areas of the Park, it substitutes an eight-decibels-below-ambient threshold for use in backcountry areas. The Air Tour Association contends that this reflects a new interpretation of the statutory term "natural quiet." It argues that while the previous interpretation was based on "noticeability," measuring sounds that a person not engaged in active contemplation of the Park likely would notice, the new interpretation substitutes a "detectability" standard, measuring any sound that is detectable to a vigilant observer.

The Air Tour Association further asserts that this change is unlawful because it is inconsistent with our opinion in Grand Canyon I, which upheld the three-decibels-above-ambient threshold. The new threshold for Zone Two is contrary to Grand Canyon I, the tour operators argue, because it is unrelated to how visitors actually experience aircraft noise in the Park. According to the Air Tour Association, visitors to the Grand Canyon do not attentively listen for such sounds, and would not notice below-ambient-level noise. Because the new standard is inconsistent with Grand Canyon I, and because it represents a change from the prior standard, the Air Tour Association urges us to accord it less deference than we would ordinarily extend to an agency determination.

We must begin with the last point, because it misperceives the scope of our review. The Supreme Court "has rejected the argument that an agency's interpretation 'is not entitled to deference because it represents a sharp break with prior interpretations' of the statute in question." Rust v. Sullivan, 500 U.S. 173, 186 (1991) (quoting Chevron, 467 U.S. at 862). An agency is not required to establish "rules of conduct to last forever," but rather "must be given ample latitude to adapt [its] rules and policies to the demands of changing circumstances." State Farm, 463 U.S. at 42 (internal quotation marks omitted). Nor does the fact that we previously affirmed an agency interpretation fix that interpretation in stone. In Grand Canyon I we held that the standards employed in the 1996 Final Rule reflected a reasonable construction of the Overflights Act. 154 F.3d at 469. That does not preclude a new standard, promulgated pursuant to notice and comment as this one was, from being reasonable as well. What the Park Service must do to sustain its decision is justify the change in course with a "reasoned analysis." State Farm, 463 U.S. at 57 (quoting Greater Boston Television Corp. v. FCC, 444 F.2d 841, 852 (D.C. Cir. 1970)); see Rust, 500 U.S. at 186-87.

The Park Service has provided that reasoned analysis. First, it contends that it has not changed the underlying definition of natural quiet. See Change in Noise Evaluation Methodology, 64 Fed. Reg. at 38,006. The original 1994 NPS definition, applied in the 1996 rule, was "no aircraft audible," 1994 NPS Report at 182; 1996 Environmental Assessment at 4-2, which is the same definition we upheld as reasonable in Grand Canyon I, see 154 F.3d at 461-62. According to the agency, it continued to employ that definition during the Limitations Rule rulemaking. See Change in Noise Evaluation Methodology, 64 Fed. Reg. at 38,007 ("[A]udibility is the basis for assessing progress toward the legislatively mandated goal of substantially restoring natural quiet."); id. at 38,011 ("Natural quiet remains the same as 'no aircraft audible.'").

What has changed, the Park Service says, is the threshold it uses to measure audibility in Zone Two. The agency agrees with the tour operators that the 1996 rule used a noticeability threshold for determining when sounds become audible, based on "the level at which visitors engaged in activities other than contemplation of the national park are likely to hear aircraft noise." Change in Noise Evaluation Methodology for Air Tour Operations Over Grand Canyon National Park, 64 Fed. Reg. 3969, 3971 (proposed Jan. 26, 1999) [hereinafter Proposed Change in Noise Evaluation Methodology]; see also Change in Noise Evaluation Methodology, 64 Fed. Reg. at 38,007. That, it says, is what led to the original three-decibels-above-ambient standard. But the Park Service contends that since 1996 it has gathered more data and performed additional research. Id. That data, collected in Grand Canyon National Park, shows that "an active listener could hear aircraft when their sound levels were between 8 and 11 [decibels] below the A-weighted ambient." FSEA at 4-5; see Change in Noise Evaluation Methodology, 64 Fed. Reg. at 38,007; Proposed Change in Noise Evaluation Methodology, 64 Fed. Reg. at 3971-72. [FN 3]

[FN 3] The A-weighted level of a sound is a single number determined by combining the sound levels in all frequencies. This combin- ing de-emphasizes the low and high frequencies in a manner similar to the sensitivities of human hearing. The A-weighted level is widely accepted as one of the best over-all sound level metrics for analysis of transportation noise. It has been shown to correlate well with human assessment of the loudness or noisiness of a sound.

Change in Noise Evaluation Methodology, 64 Fed. Reg. at 38,001.

The new studies resulted in the Park Service's decision to divide the Park into two zones, and to use different noise thresholds for each zone. The agency continued to apply the three-decibels-above-ambient threshold in the more-developed areas of the Park, where visitors were often engaged in activities other than contemplation. But it employed the new, eight-decibels-below-ambient standard in the remaining, mostly backcountry areas. In those areas, the Park Service was concerned about the experience of visitors "sitting quietly but actively seeking to experience the natural quiet and solitude of the park." Proposed Change in Noise Evaluation Methodology, 64 Fed. Reg. at 3971. The Park Service's evidence is that such visitors are able to perceive aircraft noise at the lower decibel level. Id. As the agency explains, "the threshold for Zone Two is set at 8 decibels below the average ambient sound levels" because it is "a threshold which reflects the point at which aviation noise can be heard (i.e., audible) by ground visitors seeking to experience the natural and cultural soundscapes of national parks." Id. at 3972. Thus, far from representing a lack of concern for visitors' experiences, the agency adopted the distinction between the two zones precisely because visitors experience sound differently in different areas.

Nor does the use of a threshold below the ambient indicate, as the Air Tour Association contends, that aircraft noise is banned even though it cannot be heard above the natural ambient sound. Rather, it reflects the Park Service's new understanding that audibility depends not just on volume (loudness), but also on frequency (pitch). Change in Noise Evaluation Methodology, 64 Fed. Reg. at 38,011. As the agencies explain, "studies conducted in the [Park] for the NPS ... have shown that individuals who are actively listening can hear aircraft at lower levels than the ambient A-weighted sound levels ... because aircraft sound often contains tones that are not present in the natural ambient sound." FSEA at 4-5 (emphasis in original); see Change in Noise Evaluation Methodology, 64 Fed. Reg. at 38,012. [FN 4] Hence, in order to preserve the definition of natural quiet as "no aircraft audible" in the backcountry, the Park Service concluded that it was necessary to change the threshold of audibility to below the ambient level.

[FN 4] The Park Service offers this example:

[D]uring a concert if we listen for a high note on the piccolo, only the portion of the background sound ... that is of nearly the same pitch can interfere with our ability to hear the piccolo. The base violins can play as loudly as they like without the piccolo becoming inaudible.... Hence, the relative overall "loudness" of the background and the target sounds is not the key factor in the detection process. For a target sound to be audible, it must ... be louder than ... background sounds in the same frequency band.

Change in Noise Evaluation Methodology, 64 Fed. Reg. at 38,011.

We conclude that the Park Service's explanation for its change in methodology -- as well as its explanation of why that change remains true to the agency's original principles -- are reasonable, and that the change is neither arbitrary nor capricious.


B

We next consider the Air Tour Association's "scientific" challenges to the "noise model and related sound data" used by the FAA and the Park Service in crafting the Limitations Rule. Air Tour Ass'n Br. at 15. Again, our standard of review is important. As a general matter, we "defer to the agency's reasonable exercise of its judgment and technical expertise" in the area of "aircraft noise." Grand Canyon I, 154 F.3d at 460; see also Sierra Club v. Dep't of Transp., 753 F.2d 120, 129 (D.C. Cir. 1985). When an agency uses a computer model, it must "explain the assumptions and methodology used in preparing the model and, if the methodology is challenged, must provide a complete analytic defense." Small Refiner Lead Phase-Down Task Force v. EPA, 705 F.2d 506, 535 (D.C. Cir. 1983) (internal quotation marks omitted). Ultimately, however, the "scientific nature" of a model "does not easily lend itself to judicial review," and our review "proceeds with considerable deference to the agency's expertise." Appalachian Power Co. v. EPA, 135 F.3d 791, 802, 814 (D.C. Cir. 1998). The principal question for us is whether the agencies' explanation of the model's assumptions and methodology is reasonable. See generally Small Refiner, 705 F.2d at 535. [FN 5]

[FN 5] Any agency model that satisfied this standard would also satisfy the directive in the National Parks Air Tour Management Act of 2000, passed the day after the publication of the Limitations Rule, that the agencies employ "reasonable scientific methods" when assessing aircraft sound levels within the national parks. Pub. L. No. 106-181, tit. VIII, s 808, 114 Stat. 185, 194 (set out at 49 U.S.C.A. 40128 note).
The agencies used a noise model developed by the FAA, known as the Integrated Noise Model (INM), to assess noise impacts in the Park. The INM is "the most widely used civilian software program for analyzing noise effects relating to changes in airspace use." 2000 NPS Review at 9. The Air Tour Association does not challenge the use of the model itself, but rather the way in which it was applied to the noise problem in the Grand Canyon.

First, the Air Tour Association cites excerpts from the report of a Park Service consultant, to the effect that "a more complex metric is required to achieve satisfactory accuracy." Air Tour Ass'n Br. at 17 (quoting 2000 NPS Review app. F, at 23). But as the government points out, the "more complex metric" recommended by the consultant was the one ultimately used by the agencies, see 2000 NPS Review app. F, at 23, and the consultant's overall conclusion was that "the science being used by NPS for noise modeling is grounded on extensive and valid scientific data," id. at Executive Summary 4; see id. at 1, 8. The Air Tour Association also contends that the INM overestimates aircraft noise because the agencies disabled an algorithm that adjusts for "lateral attenuation" -- sound absorption by the ground and the air near the ground. But the FAA reasonably responds that the algorithm was removed because it is used to account for the effect of "flat, acoustically soft terrain, such as grass, as would be found in the vicinity of most major airports." FSEA at 4-7. As the FAA explains, for visitors near the rim of the Canyon "there is effectively no ground surface between the source and receiver," and in the "vast majority of other locations ... the ground surface is made up of acoustically hard rock and packed dirt." Id. [FN 6]

[FN 6] The Air Tour Association further criticizes the Park Service for excluding from its analysis data recorded at a number of collection sites. The Service's experts explain, however, that this data was collected before the plan for measuring sound levels was finalized, and that the excluded sites were not representative of the predominant acoustic environments in the Park. 2000 NPS Review app. C, at 8.


Continued in Part Two