Finally, the Air Tour Association cites its own expert, John R. Alberti, for the proposition that the audibility threshold used by the Park Service in Zone Two, eight decibels below the ambient sound level, is scientifically unsupportable because aircraft noise at that level cannot be heard by a listener with normal hearing. Air Tour Ass'n Br. at 18 (citing Comments of JR Engineering to NPS at 3 (Mar. 20, 1999)). The 2000 NPS Review contains a detailed critique, based on analysis by the agency's own experts, of the Alberti study. 2000 NPS Review app. B, at 5-11. The most serious problem with that study, the Park Service explains, is that it used data "that cannot be characterized ... as representative of the park or its many levels of ambient sound." Id. at 5. In essence, Alberti began with a significantly lower ambient level than that actually measured in the Park's acoustic environments, making his calculation of eight decibels below that level considerably quieter than the actual audibility thresholds the agencies will employ in Zone Two. Id. at 5, 8. Moreover, whatever the theoretical argument, the Park Service reports as a fact that "technicians monitoring the sound environment" in the Park "identified aircraft noise at A-weighted levels of 8-12 decibels below the average A-weighted natural ambient sound levels." Proposed Change in Noise Evaluation Methodology, 64 Fed. Reg. at 3972.

We conclude that the agencies and their experts have presented a satisfactory analytic defense of their model, and therefore reject this challenge from the Air Tour Association. See Marsh v. Oregon Natural Res. Council, 490 U.S. 360, 378 (1989) ("When specialists express conflicting views, an agency must have discretion to rely on the reasonable opinions of its own qualified experts.").

The Air Tour Association also contends that the FAA acted arbitrarily and capriciously by issuing the Limitations Rule without first promulgating a quiet technology rule. As described by the Association, such a rule would establish standards for identifying "quiet aircraft technology" and would implement "incentive" flight paths for aircraft employing such technology. Air Tour Ass'n Br. at 20-21. According to the Association, Congress first ordered the FAA to establish quiet technology standards in the 1987 Overflights Act, and then did so again in the National Parks Air Tour Management Act of 2000, Pub. L. No. 106-181, tit. VIII, 114 Stat. 185 (set out at 49 U.S.C.A. 40128 note). Nonetheless, the FAA's 1996 proposed rule on quiet technology is still not final.

The FAA notes that there is some irony in the air tour operators' new-found interest in a quiet technology rule, as they have previously opposed such a rule. Limitations Rule, 65 Fed. Reg. at 17,714. We also note that the operators have misread both the Overflights Act and the Air Tour Management Act. The Overflights Act does not contain a provision regarding quiet technology. The Air Tour Management Act does direct the FAA, by April 5, 2001, to "designate reasonably achievable requirements" for aircraft to be "considered as employing quiet aircraft technology" for purposes of relief from caps on flight operations. § 804(a), (c). However, the Act also provides that "if the Administrator [of the FAA] ... will not be able to make such designation" by that time, the FAA shall report to Congress "the reasons for not meeting such time period." § 804(a). The FAA submitted the required report. See FAA, U.S. Dep't of Transp., Quiet Aircraft Technology for Grand Canyon (2001). [FN 7]

[FN 7] Another statute, the Airport and Airway Safety, Capacity, Noise Improvement, and Intermodal Transportation Act of 1992, also mentioned "quiet aircraft technology." Pub. L. No. 102-581, § 134, 106 Stat. 4872, 4888 (set out at 16 U.S.C.A. 1-1a note). Like the others, it did not direct the FAA to implement a quiet technology rule; it merely instructed the agency to submit a plan of action to Congress "to manage increased air traffic over Grand Canyon National Park ... to meet the requirements established by [the Overflights Act], including any measures to encourage or require the use of quiet aircraft technology by air tour operators." Id.

The Regulatory Flexibility Act requires an agency promulgating a final rule to prepare a regulatory flexibility analysis that, inter alia, describes:

the steps the agency has taken to minimize the significant economic impact on small entities consistent with the stated objectives of applicable statutes, including a statement of the factual, policy, and legal reasons for selecting the alternative adopted in the final rule and why each one of the other significant alternatives to the rule considered by the agency which affect the impact on small entities was rejected.

Neither attack is justified. As the FAA explains, it selected the 1997-98 base year because it was "the most accurate and current data available during the period that this rule was being drafted." Limitations Rule, 65 Fed. Reg. 17,718. Indeed, selection of that year actually favors the air tour operators, since subsequent data shows that in the following year the number of air tours declined. Id. Moreover, the agency plainly did consider alternatives to the rule, expressly listing nine and explaining its reasons for rejecting them. Id. at 17,729-30. The Air Tour Association does not point to any alternative that it believes was unreasonably rejected other than the quiet technology rule, which, as we have discussed above, the FAA has reasonably put off for the present. Because the FAA's analysis was reasonable, the Limitations Rule survives the Tour Association's challenge under the Regulatory Flexibility Act. See Allied Local & Reg'l Mfrs. Caucus v. EPA, 215 F.3d 61, 79-80 (D.C. Cir. 2000); Grand Canyon I, 154 F.3d at 470-71.

Finally, the Air Tour Association contends that the Limitations Rule is arbitrary and capricious because it ignores the needs of the elderly and disabled, who find it harder to visit the Park on the ground. The Association asserts that "nearly half of all Grand Canyon visitors never 'visit the Park on the ground,' " and that "[a]s many as half of these 'air only' visitors are elderly, disabled or mobility impaired and have no meaningful way to experience Grand Canyon except by recreational air tour." Air Tour Ass'n Br. at 22-23 (quoting Limitations Rule, 65 Fed. Reg. at 17,716). By "reducing the annual number of recreational air tours" that can be taken by the elderly and disabled, the Association contends that the Limitations Rule fails to accommodate their needs. Id.

Nearly every contention recounted in the preceding paragraph contains a factual error. It is not true, for example, that nearly half of Grand Canyon visitors never visit on the ground. Rather, the authority cited by the Association actually states that over half of air tour visitors also visit the Park on the ground. Limitations Rule, 65 Fed. Reg. at 17,716. Indeed, in another part of its brief, the Air Tour Association states that of the more than 5 million people who visit the Grand Canyon annually, only 750,000 visit by air tour. Air Tour Ass'n Br. at 9. And the Association further concedes that there is no record support for its claim that as many as half of the "air only" visitors are elderly, disabled, or mobility impaired. As the Association notes, "the Administrative Record lacks any reference to the demographics of 'air only' Grand Canyon visitors." Id. at 23 n.5.

Finally, there is also no truth to the contention that the agencies have failed to consider the needs of the elderly and disabled. As the government explains, air tours are not the only means by which these groups may view the Canyon, as the Park has available such accommodations as handicapped-accessible trails, mule rides, and raft trips. See Resp't's Br. at 34. Nor does the Limitations Rule eliminate or even "reduc[e] the annual number ofrecreational air tours"; it merely caps the number of flights at current levels. Limiting the number of visitors at a given time in a national park is a standard measure used to protect park resources, see Limitations Rule, 65 Fed. Reg. at 17,716, and its application to the resource of "natural quiet" is not unreasonable. It is therefore not surprising that the Air Tour Association failed to return to this argument in its reply brief. [FN 8]

[FN 8] The Air Tour Association also argues that the Limitations Rule violates the equal protection component of the Fifth Amendment because it exempts flights to and from the Hualapai Indian Reservation from each tour operator's annual allocation. See 14 C.F.R. 93.319(f). The Association's argument is founded upon Adarand Constructors, Inc. v. Pena, 515 U.S. 200, 227 (1995), in which the Supreme Court held that "all racial classifications ... must be analyzed by a reviewing court under strict scrutiny." In Morton v. Mancari, 417 U.S. 535 (1974), however, the Court upheld an employment preference for Indians in the face of an argument similar to that of the Air Tour Association, declaring that, "[a]s long as the special treatment can be tied rationally to the fulfillment of Congress' unique obligation toward the Indians," such treatment must be upheld. Id. at 555. And there is no dispute that the Hualapai exception is at least rationally related to "the government's interest in fulfilling its trust obligation" to the Tribe. Narragansett Indian Tribe v. National Indian Gaming Comm'n, 158 F.3d 1335, 1340 (D.C. Cir. 1998). Although the Air Tour Association contends that Adarand effectively overruled Mancari, the Supreme Court has made clear that the lower courts do not have the power to make that determination. See Agostini v. Felton, 521 U.S. 203, 237 (1997) ("[I]f a precedent of this Court has direct application in a case, yet appears to rest on reasons rejected in some other line of decisions, the Court of Appeals should follow the case which directly controls, leaving to this Court the prerogative of overruling its own decisions." (internal quotation marks omitted)). And this circuit has continued to apply Mancari post-Adarand. See Narragansett Tribe, 158 F.3d at 1340.

The Grand Canyon Trust raises two challenges of its own to the decisions of the FAA. First, the Trust contends that the agency unlawfully altered the Park Service's definition of substantial restoration of the natural quiet: from 50% of the Park experiencing natural quiet for 75% of "any given day," to 50% of the Park experiencing natural quiet for 75% of "the average annual day." Trust Br. at 12. Second, the Trust argues that the FAA's noise methodology is flawed because it only accounts for noise from commercial air tours, while ignoring noise from other types of aircraft. As a consequence of both errors, the Trust contends that the FAA has greatly overestimated the progress that the Limitations Rule makes toward restoring natural quiet.

We consider these two challenges in sections B and C below. Before doing so, we address the government's objections to our considering the Trust's arguments at all.

The Trust filed its petition for review pursuant to 49 U.S.C. 46110(a), which authorizes review in this circuit of certain final "order[s] issued by the ... Administrator of the Federal Aviation Administration." [FN 9] The FAA does not dispute that the Limitations Rule is a final order reviewable under that section. See Limitations Rule, 65 Fed. Reg. at 17,708 ("This final rule constitutes final agency action under 49 U.S.C. 46110."). The agency contends, however, that the Trust's challenges are not attacks on the FAA's order, that the decisions the Trust does attack are not final, and that the Trust's challenges are not ripe for judicial review.

[FN 9] See also Puget Sound Traffic Ass'n v. Civil Aeronautics Bd., 536 F.2d 437, 438-39 (D.C. Cir. 1976) (holding that review under the predecessor to s 46110(a) extended only to final orders).

[FN 10] Two circuit cases cited by the FAA to refute this proposition are inapposite. In Molycorp, Inc. v. EPA, 197 F.3d 543, 545 (D.C. Cir. 1999), we concluded that we were without jurisdiction to review an EPA Technical Background Document. Unlike the FSEA, that document was not issued as part of or in support of a rulemaking, and was "intended solely to provide information to the public and the regulated community." Id. at 545-46. In Florida Power & Light Co. v. EPA, 145 F.3d 1414, 1418-19 (D.C. Cir. 1998), we held that a statement in the preamble of a proposed rule was unreviewable, noting that -- unlike the FAA in this case -- the EPA had yet to take final action on the proposed rule to which the statement was relevant.

Third, the FAA contends that the Trust's challenges are not ripe for review. To determine whether a challenge to final agency action is ripe, we consider "the fitness of the issues for judicial decision and the hardship to the parties of withholding court consideration." General Electric, 290 F.3d at 380 (quoting Abbott Labs. v. Gardner, 387 U.S. 136, 149 (1967)). "In determining the fitness of an issue for judicial review we look to see whether the issue is purely legal, whether consideration of the issue would benefit from a more concrete setting, and whether the agency's action is sufficiently final." Id. We have already determined that the FAA's definition and methodology are sufficiently final. The challenges to those decisions are fit for review in other respects as well.

Whether the FAA may use an average annual day as part of its definition of substantial restoration, and whether it may ignore the noise of non-tour aircraft, are purely legal issues. See Fox Television Stations, Inc. v. FCC, 280 F.3d 1027, 1039 (D.C. Cir. 2002) (finding an issue fit for review because whether an agency determination is arbitrary and capricious or contrary to law is a "purely legal" question). And they arise in the concrete setting of the Limitations Rule. Moreover, we see no reason to believe that our consideration of these issues would benefit from postponing review. As we have already noted, there is nothing "tentative or interlocutory" about the FAA's definition and methodology, Appalachian Power, 208 F.3d at 1022, and there is no prospect that they will change in the near future unless questioned by this court. At oral argument, the government suggested that the Trust should wait until the FAA claims natural quiet has been restored to 50% of the Park, and then appeal if it disagrees. But the Trust's argument is that "under FAA's interpretation of the [Overflights] Act it will never achieve that mandate." Trust Br. at 23-24 (citing our statement in Grand Canyon I, 154 F.3d at 477, that it would be arbitrary and capricious for the FAA to have no intention of achieving the Act's goal). If we assume as we must for purposes of this ripeness analysis that the Trust's challenges are valid, [FN 11] then its argument is well-founded. For the FAA to substantially restore natural quiet to 50% of the Park on "any given day" (as the Trust contends is required), the agency would have to restore quiet to significantly more than 50% of the Park on the "average annual day." See infra Part III.B. And the FAA has never indicated that it has any plans to achieve such a result.

[FN 11] See, e.g., Better Gov't Ass'n v. Dep't of State, 780 F.2d 86, 94 (D.C. Cir. 1986).

[FN 12] See City of Houston v. HUD, 24 F.3d 1421, 1431 n.9 (D.C. Cir. 1994) ("[W]here there are no institutional interests favoring postponement of review, a petitioner need not satisfy the hardship prong.").

[FN 13] As we discuss in Part III.B below, the Trust points to evidence in the record that, using an "any given day" rather than an "average annual day" standard, only 19% of the Park will be substantially restored to natural quiet. Moreover, as we discuss in Part III.C, even that figure is based on a model that ignores noise from non-tour aircraft, and record evidence indicates that including that noise may well further reduce the percentage of the Park to which natural quiet will be substantially restored.

As we have discussed, section 3(b)(1) of the Overflights Act gives the Secretary of the Interior the responsibility to develop a definition of "substantial restoration of the natural quiet," a responsibility that the National Park Service has performed on behalf of the Secretary. See Grand Canyon I, 154 F.3d at 468. In its 1994 report, the Park Service defined the term as requiring that 50% of the Park experience natural quiet at least 75% of "the day." See 1994 NPS Report at 182. In its 2000 Final Supplemental Environmental Assessment, the FAA stated that it interpreted the phrase "the day" in the Park Service's definition to mean the "average annual day." FSEA at 4-18 to 4-19; see id. at 4-12.

The Trust contends that the FAA's interpretation is unlawful because it substitutes a new FAA definition, "the average annual day," for what the Trust believes to be the Park Service's meaning, "any given day." The Trust further contends that the FAA's interpretation results in a definition of the statutory term, "substantial restoration of the natural quiet," that is arbitrary and unreasonable. The FAA replies that because the phrase "the day" is ambiguous, this court should defer to its interpretation.

In Grand Canyon I we held that the statutory term was ambiguous, and we therefore deferred to the Park Service's definition. 154 F.3d at 466-67 (citing Chevron, 467 U.S. at 841-43). There is no question that the phrase, "the day," in the Park Service's definition is also ambiguous, and that the Park Service is entitled to deference for its interpretation of its own definition. See Auer, 519 U.S. at 461. The problem for the FAA, however, is that it is not the Park Service, and "deference is inappropriate when [an agency] interprets regulations promulgated by a different agency." Office of Pers. Mgm't v. FLRA, 864 F.2d 165, 171 (D.C. Cir. 1988); see Dep't of the Treasury v. FLRA, 837 F.2d 1163, 1167 (D.C. Cir. 1988). Nor is the FAA entitled to deference for its own interpretation of "substantial restoration of the natural quiet," as Congress expressly reserved for the Secretary of the Interior the authority to interpret that statutory term. Overflights Act s 3(b)(2); see Grand Canyon I, 154 F.3d at 468; see also Dep't of the Treasury, 837 F.2d at 1167 ("Under the law of this circuit, when an agency interprets a statute other than that which it has been entrusted to administer, its interpretation is not entitled to deference.").

As far as we can determine, the National Park Service has addressed the meaning of "the day" in its definition of substantial restoration on only one occasion: the January 2000 review of its Change in Noise Evaluation Methodology, issued just three months before the FAA issued the Limitations Rule. [FN 14] In that review, the Park Service stated that it had defined the statutory term, "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. This definition is a threshold not to be exceeded on any given day ... and refers to the 12 hour daylight period.... The NPS believes that it is a reasonable and rational definition.

[FN 14] The FAA points us to the "inside cover page" of the FSEA as demonstrating Park Service approval of the FAA's average annual day interpretation. Resp't's Br. at 49 n.8. The FSEA, however, is a lengthy FAA document, the inside cover of which says nothing more than that the Department of the Interior is a "cooperating agency." The FAA also argues that the Park Service adopted an "average annual day" standard in the Change in Noise Evaluation Methodology, in which the Service declared that "the time period of interest is the day (i.e., the average 12 daylight hours)." 64 Fed. Reg. at 38,007. But that declaration merely states that substantial restoration of natural quiet must be achieved during daylight, which on average lasts 12 hours, and that quiet at night (when air tours do not fly) does not count in the calculation. See Grand Canyon I, 154 F.3d at 475 n.19. It says nothing about averaging noise over an entire year.

[FN 15] Although the government contends that the 2000 NPS Review is merely a "technical document" that cannot be taken to represent the views of the agency, Resp't's Br. at 51, the review gives every appearance of being authoritative. Indeed, the introduction states that "the National Park Service (NPS) has prepared the following report" in "response to language in [a] House Appropriations Bill." 2000 NPS Review at 4. In any event, whether or not the review is decisive affirmative evidence of the Secretary's view, it creates more than enough doubt to justify a remand.

The FAA's "average annual day" interpretation appears to be inconsistent not only with the 2000 NPS Review, but also with the premise of the Park Service's definition of substantial restoration, a premise that the FAA itself has repeatedly endorsed. That premise is that aircraft noise should be regulated to enhance the experience of Park visitors. As we said in Grand Canyon I, both the 1994 NPS Report and the notice announcing the 1996 Final Rule were replete with concern for the manner in which aircraft noise affected visitors' experiences. 154 F.3d at 465-66; see id. at 466 (noting that "in the Park Service's view, natural quiet is a resource because it is relevant to visitor enjoyment"). And as we made clear in Part II.A above, the same is true of the Park Service's explanation of the methodology underlying the Limitations Rule. Indeed, this premise derives in part from the Overflights Act itself, which directed the Secretary of the Interior to issue recommendations "that shall provide for substantial restoration of the natural quiet and experience of the park." Overflights Act s 3(b)(1) (emphasis added).

As the Trust points out, the use of an annual average does not correspond to the experience of the Park's actual visitors. People do not visit the Park on "average" days, nor do they stay long enough to benefit from averaging noise over an entire year. For the typical visitor, who visits the Grand Canyon for just a few days during the peak summer season, the fact that the Park is quiet "on average" is cold comfort. Indeed, the FAA acknowledges that, "[b]ecause many park visitors typically spend limited time in particular sound environments during specific park visits, the amount of aircraft noise present ... can have great implications for the visitor's opportunity to experience natural quiet in those particular times and spaces." Limitations Rule, 65 Fed. Reg. at 17,712. The problem with an annual average is that it gives equal weight to summer and winter days, notwithstanding that there are many more visitors during the former than the latter. Thus, theoretically the use of an annual average could permit the statutory standard to be met despite an incessantly noisy summer, assuming that the other seasons were relatively quiet. On the FAA's view, it could then declare that it had achieved substantial restoration of natural quiet and cease any further efforts to restrict aircraft noise.

Nor are these consequences of using an annual average merely theoretical. The FAA's Final Supplemental Environmental Assessment projects that the 2000 Final Rules will restore natural quiet in 43.6% of the Park on the "average annual day." See FSEA at F-4. The same document, however, reveals that on the average summer day, natural quiet will be substantially restored in only 31.3% of the Park, and that on the day with the most air tour overflights the natural quiet will be substantially restored in only 19.0% of the Park. Id. This result appears inconsistent with the rationale that the Park Service offered for its 1994 definition of substantial restoration, and that we found reasonable in Grand Canyon I: "Protecting 50% of the Park for 75% of the day gives [backcountry visitors] at least a reasonable chance of seeing the less-traveled areas in peace." 154 F.3d at 467; see also 1994 NPS Report at 184.

The FAA's only response to the seeming unreasonableness of its definition is that its standard guidelines for evaluating the impact of aircraft noise employ an annual average, and that the courts have generally deferred to those guidelines. Resp't's Br. at 54-56 (citing, e.g., City of Bridgeton v. FAA, 212 F.3d 448, 459 (8th Cir. 2000); Morongo Band of Mission Indians v. FAA, 161 F.3d 569, 578-79 (9th Cir. 1998)). But those guidelines were developed to evaluate aircraft noise at airports, see 14 C.F.R. pt. 150; id. § 150.9(b), and it is in that context that the courts have deferred. See, e.g., Bridgeton, 212 F.3d at 459; Morongo Band, 161 F.3d at 578-79. None of the cited cases involved a statute that required the FAA to achieve substantial restoration of the natural quiet, or to employ a Park Service definition that demands quiet during 75% of "the day."

In sum, we conclude that the FAA's use of an annual average day for measuring "substantial restoration of the natural quiet" appears inconsistent with both the Park Service's definition of the term and with the premise upon which that definition was based. As noted in our discussion of ripeness above, this is not merely a technical dispute. It has a significant impact both on the FAA's estimate of the progress its current and planned rules make, and on its determination of the end point at which it will have fulfilled the statutory objective. We must therefore remand this issue for further consideration.

We recognize that, on remand, the Park Service may declare that it did in fact choose what seems to us to be the FAA's less reasonable reading of the Service's definition of "substantial restoration." If it does, however, the Park Service must understand that "the Overflights Act ... did not relieve the government ... of its obligation not to promulgate a rule that is 'arbitrary, capricious, an abuse of discretion, or otherwise not in accordance with law.'" Grand Canyon I, 154 F.3d at 469 (quoting 5 U.S.C. 706(2)(A)). To survive review under that standard, the Service will have to explain why its definition is reasonable in light of the premise upon which it was assertedly based. At this point, the government has offered no explanation at all.

The Trust's second challenge is to an aspect of the FAA's noise methodology. The Trust argues that, in projecting the amount of noise experienced by different parts of the Park, the FAA's noise model only considers noise emitted from air tour flights. By failing to account for noise from other aircraft that fly over the Grand Canyon -- for example, from commercial jets, general aviation, and military flights -- the model arbitrarily overstates how quiet the Park really is. The Trust asserts that if non-tour aircraft noise were included in the calculation, the percentage of the Park in which natural quiet would be substantially restored -- even on the FAA's average annual day -- would be significantly less than the projected 43.6%.

The FAA does not dispute that it excludes non-tour aircraft from its model, and the Trust's description is in accord with that of the Final Supplemental Environmental Assessment. FSEA app. G, at 40. Nor does the FAA dispute that, at least theoretically, the exclusion of non-tour aircraft could cause it to overestimate the periods when, and number of places where, Park visitors substantially experience natural quiet. The FAA's only response to the Trust's challenge is that excluding non-tour aircraft is reasonable because they "contribute minimal noise compared to commercial air tour aircraft.... [The FAA] did not believe the minimal amount of noise they contribute would affect the accuracy of its estimates." Resp't's Br. at 59.

The agency cites no direct evidence that the noise from non-tour aircraft is minimal, basing that conclusion solely on the fact that general aviation flights account "for about 3 percent of all aircraft" in the Park. FSEA app. G, at 40. What is at issue here, however, is not the number of aircraft that fly over the Grand Canyon, but rather the percentage of time that they are audible. Although we might ordinarily defer to the FAA's expert assurance that the number of non- tour aircraft is too small to affect the accuracy of the latter calculation, we cannot defer when the government's own data suggests the contrary. Here, the record evidence indicates that the exclusion of non-tour aircraft from the calculation of the percentage of time that aircraft are audible may well have a significant impact on the results.

The 1994 NPS Report provides a chart, based on actual observations, that breaks down the percentage of time that different types of aircraft were audible at different points in the Park. 1994 NPS Report at 187. For example, it shows that at Lipan Point, total aircraft noise -- including noise from air tours, commercial jets, general aviation, and military flights -- was audible during 43% of the observation period. Air tours, however, could only be heard 30% of the time. Hence, at that location, excluding non-tour aircraft from the calculation would overestimate by 13 percentage points the amount of time during which natural quiet prevailed. Other locations reveal similarly significant differentials between the amount of time that some type of aircraft was audible and the amount of time that air tours could be heard: a 14% differential at Bright Angel Point, 10% at Toroweap Overlook, and 12% at Phantom Ranch Overlook. Id. When asked at oral argument, the government could offer no explanation for why these differentials were immaterial in calculating the areas or periods in which the Park substantially experiences natural quiet.

The FAA also seeks to defend the exclusion of non-tour aircraft from its noise model on the ground that "FAA's regulations were not required to address every conceivable type of aircraft." Resp't's Br. at 60. That is true, and the FAA can certainly choose to achieve the substantial restoration of natural quiet by regulating air tours alone. But the FAA cannot dispute that whether or not non-tour aircraft are regulated, natural quiet does not exist when the sound they make is audible. [FN 16] Nor does the Overflights Act provide any basis for ignoring noise caused by such aircraft. [FN 17] Hence, the fact that the FAA has chosen not to regulate certain categories of aircraft does not justify ignoring the sound those aircraft make when deciding how extensively to regulate other categories. And in the absence of any reasonable justification for excluding non-tour aircraft from its noise model, we must conclude that this aspect of the FAA's methodology is arbitrary and capricious and requires reconsideration by the agency.

[FN 16] See, e.g., 2000 NPS Review at 6 (defining "natural quiet" as "the sound levels associated with a given acoustic environment, absent any mechanical or manmade noise sources" (emphasis added)); 1994 NPS Report at 182 (defining "natural quiet" as "no aircraft audible").

[FN 17] See Overflights Act § 3(a) (declaring 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").

For the reasons discussed in Part II, the Air Tour Association's petition for review is denied. For the reasons discussed in Part III, we grant the Grand Canyon Trust's petition and remand the case to the FAA for further proceedings consistent with this opinion.