The Air Tour Coalition also maintains that the FAA's Final Rule should be remanded because the FAA committed a series of errors under the Administrative Procedure Act. Specifically, the Coalition contends the FAA failed to: permit comment on the definition of "substantial restoration of the natural quiet," respond to comments on that definition, adequately justify the definition, consider the interests of the air tour industry, explain its departure from prior regulations, and respond to comments in connection with the Regulatory Flexibility Act, 5 U.S.C. 601 et seq. We reject all of these challenges for a mix of factual and legal reasons.

The APA requires agencies to provide notice and an opportunity to comment on proposed rules. See 5 U.S.C. 553(c); see also McLouth Steel Prod. Co. v. Thomas, 838 F.2d 1317, 1322-23 (D.C. Cir. 1988). The FAA did that here. The Final Rule was proposed in a Federal Register notice on July 31, 1996, and numerous comments were submitted. See Final Rule, 61 Fed. Reg. at 69,305-18 (summarizing and responding to comments). The Air Coalition's true complaint is not that it lacked an opportunity to comment, but that it was not permitted to comment meaningfully because the FAA viewed itself as bound to adopt the Park Service's 1995 definition, and so neither took the Coalition's comments into consideration nor responded to them. See generally Final Rule, 61 Fed. Reg. at 69,306 ("[T]he terms do not need additional comment under the Administrative Procedure Act."). Ordinarily, this would be a potentially winning administrative law argument. An agency is required to provide a meaningful opportunity for comments, which means that the agency's mind must be open to considering them. See McLouth, 838 F.2d at 1323. An agency must also demonstrate the rationality of its decision-making process by responding to those comments that are relevant and significant. See Professional Pilots Fed'n v. FAA, 118 F.3d 758, 763 (D.C. Cir. 1997); Home Box Office, Inc. v. FCC, 567 F.2d 9, 35 (D.C. Cir. 1977).

But the Overflights Act is not the ordinary statute. It envisions a regulatory program that is the product of two agencies and clearly divides the institutional responsibilities between them. It instructs that the "Secretary [of the Interior] shall submit to the Administrator [of the FAA] recommendations ... [which] shall provide for substantial restoration of the natural quiet and experience of the park." Overflights Act 3(b)(1). And it directs that the Administrator, after providing notice and an opportunity to be heard, "shall ... implement the recommendations of the Secretary without change unless the Administrator determines that implementing the recommendations would adversely affect aviation safety." Id. 3(b)(2) (emphasis added). Under this statutory scheme, the FAA was correct in believing that it had no choice but to adopt the Park Service's recommendations (except for any safety concerns the FAA might have), and hence that it did not need to provide additional comments of its own on the Interior Department's definition of the statutory terms. See Final Rule, 61 Fed. Reg. at 69,306.

We reached the same conclusion with respect to a similar statute in Bangor Hydro-Electric Co. v. FERC, 78 F.3d 659 (D.C. Cir. 1996). In that case, a hydroelectric producer, licensed by the Federal Energy Regulatory Commission (FERC) to build a dam, challenged a FERC order requiring it to adopt a fish passage plan conforming to one prescribed by the Department of the Interior. FERC issued the order pursuant to a statute providing that "[t]he Commission shall require the construction ... of such fishways as may be prescribed by the Secretary of the Interior." Id. at 661 (quoting 16 U.S.C. 811). In doing so, FERC declined to consider Bangor's arguments concerning the need for the fishway, concluding that under the statute it had no choice but to require Bangor to construct it. And in light of that statute, we held, as we hold here, that it was not the agency's "role to judge the validity of Interior's position -- substantially or procedurally." Id. at 663; see also Escondido Mut. Water Co. v. La Jolla Band of Mission Indians, 466 U.S. 765, 778 & n.20 (1984). [FN 11]

[FN 11] The Coalition's reliance on McLouth Steel Products Co., 838 F.2d 1317, is misplaced. In denying a petition filed by McLouth under the Resource Conservation and Recovery Act, 42 U.S.C. 6901 et seq., the EPA used a computer model it had developed previously. The agency refused to entertain or respond to comments on the model, despite never having previously exposed the model to comment. In so doing, the EPA failed to provide the opportunity to comment required by 553 of the APA. See 838 F.2d at 1322-23. But as this recitation indicates, McLouth is not like either this case or Bangor: McLouth involved decision-making by a single agency, and no statute required it to adopt the computer model at issue.

Moreover, in determining whether the Final Rule is arbitrary or capricious, we may consider only the regulatory rationale actually offered by the agency during the development of the regulation, and not the post-hoc rationalizations of its lawyers. See Citizens to Preserve Overton Park, Inc. v. Volpe, 401 U.S. 402, 420 (1971); SBC Communications, Inc. v. FCC, 138 F.3d 410, 418 (1998); see also Bangor, 78 F.3d at 662 (holding that the FERC licensing requirement would have to be supported by the record before the agency). Hence, although the FAA would not have violated any APA procedural or quasi-procedural requirement by failing to respond to comments about the Park Service definition, the government would have risked the possibility that the justification for the definition previously offered by the Park Service (and submitted to the FAA) might not satisfy the APA's substantive requirement of agency rationality. See Motor Vehicles Mfrs. Ass'n v. State Farm Mut. Auto. Ins. Co., 463 U.S. 29, 43 (1983). For example, if any of those comments exposed a previously unrecognized irrationality in the Park Service definition, it would now be too late for the agency's lawyers to plug the gap.

Fortunately for the government, the Park Service did offer an adequate and reasonable justification for the definitions it chose. See NPS Report at 60, 78, 182. Moreover, notwithstanding the FAA's legal position that it need not offer anything in addition, the Final Rule further elaborated on that explanation. See Final Rule, 61 Fed. Reg. at 69,305-10. Most of that explanation has already been discussed at several places above. The Park Service noted that under the statute it was required to make a recommendation that would provide for "substantial" restoration of the natural quiet, and it concluded that a reasonable definition of "substantial" was one that restored natural quiet in at least half the park for most (75%) of the day. NPS Report at 182. The agency also reasonably read the statute's requirement that "natural" quiet be restored, to mean it should look to an increment above the ambient, natural sounds of the Park. See id. at 60. Finally, the agency reasonably chose to measure that increment based on the smallest sound "an attentive listener" could hear, because that measure could be easily "related to visitor reactions to the sound of aircraft overflights," id., and because that measure best protected the experience of the back-country users, see Final Rule at 69,309-10; NPS Report at 13.

The Coalition further contends that, in formulating its rule, the FAA failed to take into consideration "the two policy goals to be accommodated in the Overflights Act -- 'the substantial restoration of the natural quiet and experience of the park' ... and maintenance of viable air tourism in the GCNP." Coalition Br. at 14. But it is not by chance that the Coalition could put only the first of those goals in quotation marks. The second -- maintenance of viable air tourism -- is not mentioned in the Overflights Act. For evidence of this "goal," the Coalition is forced to look to a colloquy on the Senate floor between Senators Matsunaga and McCain. But again, even if we were to accord weight to a floor colloquy, the colloquy the Coalition has chosen shows only that "it was not our intent to eliminate the so-called air tour industry." 133 Cong. Rec. at S10800 (emphasis added). All the Senators agreed to was that "when the essential values for which the park was created can accommodate such use, air tours are perfectly appropriate." Id. (emphasis added). And Senator McCain further noted that "when it comes to a choice between the interests of our park system and those who profit from it, without a doubt, the interests of the land must come first." Id. at S10799.

But this argument is again beside the point. Contrary to the Coalition's suggestion, the FAA did consider the impact its regulation would have on the air tour industry. As the FAA explained, "[t]he primary policy reason for adopting this rule, is that it is the best compromise the FAA has been able to formulate to achieve the mandate of [the Overflights Act] and maintain a viable air tour industry serving GCNP." Final Rule, 61 Fed. Reg. at 69,328. Hence, whether it was required to do so or not, the Park Service did in fact consider the interests the Coalition represents. [FN 12]

[FN 12] We also reject the Coalition's suggestion that the President's Earth Day memorandum, directing the agencies to complete their rulemaking by the end of 1996 and to achieve the substantial restoration of natural quiet by 2008, renders the FAA's decision suspect. The Coalition does not argue that the President's direction itself violated any statutory rule, but rather that as a consequence of the "haste" that it engendered, the FAA was unable to offer a reasoned explanation for the Final Rule, and was otherwise unable to satisfy the requirements of the APA. Because we hold that the FAA's explanation for its Final Rule is reasonable, and that the promulgation of the rule satisfied the APA, the President's memorandum does not affect our analysis. See generally Sierra Club v. Costle, 657 F.2d 298, 407-08 (D.C. Cir. 1981).

We reject this argument for two reasons. First, the characterization of the FAA's 1996 Final Rule as a "departure" is somewhat of an overstatement, since the 1988 rule initially was set to expire in 1992. See 53 Fed. Reg. at 20,264. Hence, we cannot say that the 1988 rule expressed the government's final position on how to achieve the substantial restoration of the natural quiet; the Park Service did not adopt a final definition of that phrase until its 1994 Report to Congress.

But second, and contrary to the Coalition's contention, the government did explain why something more than the 1988 rule was necessary. The Park Service explained that although under SFAR 50-2, 34% of the Park enjoyed substantial restoration of natural quiet, without revisions to the regulation the percentage would drop to less than 10% by the year 2010. See NPS Report at 13. The Park Service noted that "air tours have increased significantly over the Canyon" in the years since the 1988 regulation, increasing from 120,180 in the year prior to the regulation to more than 187,000 in 1993, and that the number was expected to continue to increase still further. Id. "It is vital that this evaluation of [SFAR 50-2] be understood in the context of the predicted growth in the number of flights," the Park Service said. Id. Accordingly, the Service found that "natural quiet is not yet substantially restored," id. at 195, and that it was "obligated, in pursuit of both its Congressionally mandated and defined management responsibilities, to seek a further restoration of natural quiet," id. at 198. This is more than sufficient explanation for the government's decision to revise the 1988 rule.

Finally, the Coalition contends that the FAA failed to respond to comments on the inadequacy of its analysis under the Regulatory Flexibility Act, and failed to consider alternatives to the rule it adopted. We reject both challenges as factually inaccurate. The FAA did a lengthy analysis of the economic impact of the proposed rule on small businesses, as required by the Regulatory Flexibility Act, and responded to comments submitted by the Small Business Administration and other commenters. See Final Rule, 61 Fed. Reg. at 69,318-28. It also considered alternatives to the rule. The Coalition claims that the FAA flatly rejected its obligations to consider alternatives, stating that such consideration was " 'beyond the scope of this analysis.' " Coalition Br. at 15 (quoting 61 Fed. Reg. at 69,327-28). But that quotes the FAA too selectively. What the FAA said was that, "[t]o recount all the alternatives that were considered would be beyond the scope of this analysis." Final Rule, 61 Fed. Reg. at 69,328 (emphasis added). The FAA made clear, however, that it did consider alternatives, expressly listing seven that were recommended and noting that "[m]any combinations of all of these alternatives or recommendations were considered in developing this rule." Id.; see also Environmental Assessment at 2-1 to -14 (identifying and analyzing alternatives).

The FAA thus satisfied the requirements necessary to demonstrate a rational decision-making process -- that is, that it respond to relevant comments and consider reasonable alternatives. See State Farm, 463 U.S. at 51; Professional Pilots Fed'n, 118 F.3d at 763. The Coalition does not describe any particular response as inadequate, nor does it point to any alternative that the agency irrationally rejected -- other than the alternative of routing tours away from concentrations of visitors which, as we noted above, the FAA reasonably could reject.

The gravamen of Clark County's petition is that the FAA promulgated its flight free zones too soon. The FAA should not have done so, the County maintains, until it was also ready to promulgate the associated flight corridors and tour routes. Nor should the FAA have issued the flight free zones until it had more adequately assessed their environmental impact.

The County's first contention is that it was unreasonable for the FAA to promulgate expanded flight free zones without at the same time promulgating final routes, because that made it impossible to assess the effect of the flight free zones either on noise or on the viability of air tours. Without defined routes, Clark County says, it is "forced to guess where FAA might place routes amongst the almost infinite options left by the flight free zones." County Br. at 19. Underlying this dispute is the County's fear that the expanded flight free zones appear to have the effect of closing the lucrative Blue 1 route out of Las Vegas -- the principal city in the County -- without providing a viable alternative.

We should note that, ordinarily, agencies have wide latitude to attack a regulatory problem in phases and that a phased-attack often has substantial benefits. See City of Las Vegas v. Lujan, 891 F.2d 927, 935 (D.C. Cir. 1989); General Am. Transp. Corp. v. ICC, 872 F.2d 1048, 1058 (D.C. Cir. 1989). Indeed, at oral argument the Air Tour Coalition conceded that it would benefit from an early resolution of the definition of "substantial restoration," because that would enable it to "negotiate" an acceptable route structure with the government and the Trust. See Oral Arg. Tr. at 135-39; id. at 135-36 (advising that "[t]here is a national negotiated rulemaking that's currently pending" regarding air tours over other national parks).

But events have largely overtaken this dispute. Although the FAA did not originally make clear whether it would stay the implementation of the flight free zones until it issued the final corridors and route structure, thereafter the FAA represented to this court that it would extend the effective date of the flight free zones until that time. See id. at 97; see also FAA Supp. Br. at 11 (filed after oral argument, making same representation). Since then, the agency has formally extended the effective date to January 31, 1999. See Special Flight Rules in the Vicinity of Grand Canyon National Park, 62 Fed. Reg. 66,248, 66,248 (1997). Accordingly, whatever its merits, Clark County's contention that it was irrational for the agency to implement flight free zones until it issued final corridors and routes is now moot.

On the other hand, the County's underlying concern, that the flight free zones will have a negative impact on Las Vegas-based flights, is not moot. But it is also not yet ripe. We follow a two-pronged test in determining whether a challenge to a final rule is ripe for review. First, we consider the "fitness of the issues for judicial decision." This involves an inquiry into "whether the court or agency would benefit from postponing review until the policy in question has sufficiently crystallized." Florida Power & Light Co. v. EPA, 145 F.3d 1414, 1421 (D.C. Cir. 1998) (internal quotations omitted). "The court's interests in avoiding unnecessary adjudication and in deciding issues in a concrete setting militate in favor of postponing review if, for example, the court finds that resolution of the dispute is likely to prove unnecessary or that the court's deliberations might benefit from letting the question arise in some more concrete form." Id. (internal quotations omitted). Second, if a challenged decision is not "fit" for review, we must consider whether postponing review will cause the petitioner "hardship." Id.

In light of the fact that the FAA is still working on corridors and routes, the County's challenge to the flight free zones is not fit for review at this time. As the County's own argument makes clear, neither it nor we can assess whether the flight free zones will hurt the County, or how much they will do so, until we know which new routes and corridors through the flight free zones the FAA will authorize. "The effects of the Final Rule," the County correctly notes, "depend on where FAA places flight tracks for air tour operations...." County Br. at 18. Waiting until those new routes and corridors are issued may make "resolution of the dispute ... unnecessary." Florida Power, 145 F.3d at 1421, because they may accommodate the Las Vegas flights that are the County's principal concern. At a minimum we will "benefit from letting the question arise in [a] concrete form." Id. Moreover, given the FAA's stay of the flight free zones pending promulgation of new routes and corridors -- which permits the Blue 1 route to continue to operate in the interim -- the County will not suffer hardship as a result of the postponement. Accordingly, this challenge by the County is not currently ripe for review.

The same is true of the County's contention that the FAA violated the National Environmental Policy Act (NEPA), 42 U.S.C. 4321 et seq., by concluding in its Environmental Assessment that the Final Rule would have no significant environmental impact. See Environmental Assessment (J.A. 151-52). [FN 13] The County contends that if the effect of the Final Rule is to close Blue 1 without providing a viable alternative, it would cause significant environmental effects by shifting tourists from air to ground transportation. Without considering the legal merits of this argument, it is clear that we cannot evaluate it factually without knowing whether the final list of routes and corridors will leave air tour operators without a viable alternative. [FN 14]

[FN 13] Based on that Assessment, the agency determined that the Final Rule warranted a "finding of no significant impact." See Final Rule, 61 Fed. Reg. at 69,318. If correct, this finding means that the FAA was not obligated under NEPA to prepare an environmental impact statement. See 40 C.F.R. 1501.4, 1508.13 (1997); see also Public Citizen v. National Highway Traffic Safety Admin., 848 F.2d 256, 265-68 (D.C. Cir. 1988); Sierra Club v. United States Dep't of Transp., 753 F.2d 120, 126 (D.C. Cir. 1985).

[FN 14] In light of this resolution, we do not consider whether Clark County would have standing under NEPA based on its assertion that, because the FAA's action will cause tourists to travel to the Park by ground rather than by air, the County will be injured by an increase in vehicular emissions within the County. See generally Florida Audobon Soc'y v. Bentson, 94 F.3d 658, 665 (D.C. Cir. 1996) (en banc).

The Hualapai Tribe also makes what amounts to an argument that the FAA issued its Final Rule too soon, because it failed to consider first whether the establishment of the expanded flight free zones would push aircraft noise off the Park and onto the Hualapai Reservation. The consequences of such a shift, the Tribe contends, would be harm to the Tribe's traditional cultural properties, sacred sites, ongoing religious and cultural practices, natural resources, and economic development. In the Tribe's view, the FAA's failure to consider these consequences, and to consult with the Tribe about them, violated the National Historic Preservation Act, 16 U.S.C. 470 et seq., NEPA, the APA, and the United States' trust obligation to the Tribe.

We find these arguments unripe for consideration for the same reason we found the County's arguments unripe. Until we know what routes the air tours will take, we simply cannot assess whether, or how much, they will affect the Reservation. Holding off that assessment until the routes are concrete may make our resolution of the dispute unnecessary. See FAA Br. at 39 ("The FAA has committed to ensuring that any new routes that are located above the Hualapai Reservation avoid historic, cultural and religious sites."); id at 45 ("The final routes may well meet many of the Tribe's anticipated [environmental] concerns."). Such a postponement surely will facilitate any review that is necessary. And since the flight free zones have been stayed in the interim, postponement will not injure the Tribe. [FN 15]

[FN 15] The same analysis applies to the Tribe's allegation that overflights that "directly and substantially impair the use of" reservation lands would constitute an unlawful taking of those lands. Until the routes and corridors are established, it is not possible to tell whether there will be overflights that impair the Tribe's use of its lands. And as long as the FAA continues to stay the effective date of the flight free zones, such overflights will not occur.

[FN 16] In its brief, the Tribe contended that under its trust obligations, the United States was required, but failed, to consult with it on a government-to-government basis while developing the Final Rule. The FAA, however, cited considerable evidence that consultations have occurred. See, e.g., Final Rule, 61 Fed. Reg. at 69,305-07 (outlining consultations with Indian tribes); Environmental Assessment at 4-19 to -21, 4-23 (outlining meetings with Hualapai and other tribes to review impact on historical sites and socio-economic interests of tribes). At oral argument, the Tribe reformulated its argument, conceding that there had been consultations, but contending that they had not been meaningful. See Oral Arg. Tr. at 50-51.

We now turn to the arguments of the Grand Canyon Trust, which attack the FAA's Final Rule from the opposite side -- not as constituting too much, too soon; but as being too little, too late. The Trust has little quarrel with the individual elements of the Final Rule -- the flight free zones, curfews, aircraft caps or reporting requirements. But it argues that they are not enough to achieve Congress' goal, and that the agency has delayed action for far too long. The Final Rule is too little, the Trust contends, because the government's definition of substantial restoration does not restore as much natural quiet as the statute requires. The Final Rule is too late, the Trust charges, because a rule that will not achieve substantial restoration until the year 2008 is inconsistent with the statutory goal.

The Trust argues that for four reasons, the Final Rule's definition of substantial restoration of the natural quiet -- that 50% of the Park achieve natural quiet for at least 75% of [SOMETHING MISSING] day -- does not satisfy the Overflights Act.

First, the Trust contends that Congress intended more than half of the Park to be free of aircraft noise 100% of the time, a percentage the government's 75% figure will not necessarily achieve in any area. [FN 17] But the statute does not say that a substantial area of the Park must be quiet 100% of the time. The statutory goal is simply the "substantial restoration of the natural quiet," a phrase too broad and ambiguous to read as "address[ing] th[is] precise question." Chevron, 467 U.S. at 843. The Act does require the designation of "flight free zones," but even if the Final Rule permits noise to leak into those zones, [FN 18] the statutory language still requires only that the zones be "flight free," not "noise free."

[FN 17] That is not to say there will be no such areas. There are a number of back-country areas of the Park that experienced almost a complete absence of aircraft sound even under the previous rule. See NPS Report at 187-88; Environmental Assessment at 4-13 to -15 (tables describing three locations at which aircraft are audible for only 0-4 % of the day, and one where they are audible 0-9%).

[FN 18] The FAA found that because of the way aircraft sound carries in the Canyon, it was able, for some part of the time, "to fully penetrate to the center of every flight-free zone created by" the previous rule. See Final Rule, 61 Fed. Reg. at 69,309. The record does not disclose whether the same will result for the expanded flight free zones created by the new rule.

The Trust contends, second, that even if the statute does not require the agency to create completely noise-free areas, the Park Service definition still does not provide "substantial" restoration. It argues that the "dictionary meaning" of "substantial" is "more than half." It then argues that a rule requiring that 50% of the Park be quiet for 75% of the day, is mathematically equivalent to one yielding a "restoration" value of only 37.5%, because 50% x 75% = 37.5%. Neither the statute nor the legislative history compels acceptance of either part of this argument.

"Substantial" may well be defined as meaning "more than half." See Webster's Third New International Dictionary 2280 (1993) ("being that specified to a large degree or in the main") (4th meaning). But it also has a host of much vaguer dictionary meanings, ranging from "not seeming or imaginary," id. (1st meaning), to "considerable in amount," id. (2nd meaning). See Victor v. Nebraska, 511 U.S. 1, 19 (1994) ("[O]n the one hand, 'substantial' means 'not seeming or imaginary'; on the other, it means 'that specified to a large degree.' "). Indeed, in the administrative law context, we refer to "substantial" evidence as meaning "more than a scintilla, but less than a preponderance." Burns v. Office of Workers' Compensation Programs, 41 F.3d 1555, 1562 n.10 (D.C. Cir. 1994) (internal quotations and citation omitted). In short, the term is simply too ambiguous to compel the "plain meaning" claimed by the Trust, and more than sufficiently elastic to support the agency's definition as reasonable.

But even if "substantial" does mean "more than half," the agency's definition of substantial restoration effectuates that meaning by requiring that more than half of the Park be silent more than half -- indeed, more than three quarters -- of the time. The Trust's mathematical equation, while creative, does not persuade us otherwise. [FN 19] There is no support in the statute or legislative history for requiring that kind of numeric calculation, nor for its unstated premise: that "substantial" pertains to a combination of space and time, rather than to each variable considered separately.

[FN 19] Clark County has an equally imaginative mathematical reply. The Final Rule, it points out, requires that 50% of the Park be quiet 75% of a 12-hour day -- not 75% of a 24-hour day. When this is combined with 100% silence during the 12-hour night, the correct calculation is: (50% x 75%) + (50% x 100%), which results in a "restoration" of 87.5%. See Clark County Intervenor Br. at 10-11. This calculation may be a bit too creative. Because air tours have never flown at night, it is hard to see how silence during that period can be considered part of any "restoration." On the other hand, the curfew, which extends the period of 100% silence beyond 12 hours, may well raise the "combined" percentage above the Trust's calculation of 37.5%.

We see nothing in the Overflights Act that forbids the government from considering the impact of its regulations on the air tour industry. Congress, after all, required "substantial restoration of the natural quiet," not total restoration. The statute's provisions for flight free zones and restrictions on flight altitudes, see Overflights Act 3(a)(1), and for a plan to "manag[e] air traffic in the air space above the Grand Canyon," id. 3(a)(2), indicate that Congress contemplated some overflights would continue. See also 133 Cong. Rec. at S10799 (statement of Sen. McCain) ("I believe this bill will enable the air tour industry to continue to thrive."). The FAA's statement does not indicate that the agency considered maintenance of a viable air tour industry in derogation of its statutory responsibility to issue a plan that would provide for substantial restoration of the natural quiet. To the contrary, the agency said the Final Rule was a compromise that still would "achieve the mandate" of the Act. Final Rule, 61 Fed. Reg. at 69,328; see also id. ("It is the intent of the rule adopted to permit the continuation of aerial viewing of the canyon ... in a manner consistent with the stated purposes of section 3 [of the Overflights Act] to substantially restore the natural quiet of the Grand Canyon ....") (emphasis added). As long as that is so, we do not find anything in the statute that would bar the agency from considering this issue in the course of promulgating its regulatory plan. [FN 20]

[FN 20] We should note that the agency's concern for the tours was at least as much for air tour passengers as for air tour operators. See, e.g., Final Rule, 61 Fed. Reg. at 69,309 ("[V]iewing the canyon from the air is a legitimate and valuable means of appreciating the beauty of the Grand Canyon."); Proposed Final Rule, 61 Fed. Reg. at 40,134 ("[C]ommercial sightseeing operators provide a valuable public service by creating a unique way [for] all to view the Grand Canyon and provide an effective means for elderly and handicapped individuals to enjoy the park.").

Finally, we address the Trust's argument that the Final Rule achieves a substantial restoration "too late," and its request that we "(1) require the agencies within 60 days to issue regulations that will immediately achieve the substantial restoration of natural quiet ...; (2) direct that the regulations ..., at a minimum, establish[ ] flight-free zones sufficiently large that 50% of the Park is noise-free; and (3) retain jurisdiction over this matter to ensure compliance...." Trust Br. at 17-18 (emphasis in original). As an "interim measure," the Trust asks us to order "an immediate cap of 40,000 annual air tour overflights." Id. at 18.

Although the APA gives courts the authority to "compel agency action unlawfully withheld or unreasonably delayed," 5 U.S.C. 706(1); Telecommunications Research & Action Ctr. v. FCC, 750 F.2d 70 (D.C. Cir. 1984) [hereinafter "TRAC"], we are acutely aware of the limits of our institutional competence in the highly technical area at issue in this case. As a court we have no idea what the unintended consequences of immediately imposing an expansion of the flight free zones -- which under the Final Rule already will cover 87% of the Park -- might be. In addition to air safety concerns, it is possible that implementing such zones would do no more than shift the flights and their noise from the Park to the Hualapai Reservation. Nor do we know what the consequences of ordering a cap on flights would be, or whether there might be other regulations that could better do the job. That, of course, is why such considerations normally are the province of expert agencies rather than courts -- and, as noted above, the FAA currently is considering such options. Moreover, although the Trust's frustration with the agencies' slow and faltering pace is understandable, we cannot say it has made out a case for the immediate imposition of so drastic a remedy. [FN 21]

[FN 21] When deciding whether to grant a petition for mandamus on the ground of agency action unreasonably delayed, this court is guided by the following criteria:

(1) the time agencies take to make decisions must be governed by a rule of reason; (2) where Congress has provided a timetable or other indication of the speed with which it expects the agency to proceed in the enabling statute, that statutory scheme may supply content for this rule of reason; (3) delays that might be reasonable in the sphere of economic regulation are less tolerable when human health and welfare are at stake; (4) the court should consider the effect of expediting delayed action on agency activities of a higher or competing priority; (5) the court should also take into account the nature and extent of the interests prejudiced by delay; and (6) the court need not find any impropriety lurking behind agency lassitude in order to hold that agency action is unreasonably delayed.

TRAC, 750 F.2d at 80 (D.C. Cir. 1984); see also Action on Smoking & Health v. Dep't of Labor, 100 F.3d 991, 994 n.1 (D.C. Cir. 1996); DiCola v. FDA, 77 F.3d 506, 509-10 (D.C. Cir. 1996).

The statute's timing provisions do not, however, support the Trust's contention that Congress "intended the job to be done in 120 days." Trust Br. at 14. What Congress demanded within 120 days was the issuance of a regulatory plan that would achieve the goal of substantial restoration; it did not direct that substantial restoration actually be in place on the 121st day. Indeed, the provision for a report, which was to discuss whether the plan had succeeded and suggest revisions, makes clear Congress contemplated that the agencies' first plan might not succeed and might have to be revised -- as the agencies have done in the regulatory plan at issue here.

There is more force to the Trust's argument that, even if Congress had no specific timetable in mind, it was unreasonable for the FAA to wait ten years to issue a regulation requiring substantial restoration, and then to issue one that permits another ten years to pass before substantial restoration is achieved. But although the FAA was tardy, it is unfair to characterize it as doing nothing during those first ten years. It issued SFAR 50-2, which went part of the way toward restoration. As Congress directed, the government then evaluated progress under that regulation, found it wanting, and eventually proposed the current rule. Although it was undeniably slow in doing so, this is the first time any party has challenged the agency's delay in court. That is not to say, as the FAA implies, that this somehow estops the Trust from complaining. But this is not a case where an agency has been contumacious in ignoring court directions to expedite decision-making.

Nor can we accept the Trust's argument that issuing a rule that does not contemplate final achievement of Congress' goal for ten years is inherently unreasonable. The issues involved here are complex. It is clear from the record that achieving substantial restoration will require a multitude of agency actions, including the entry into service of quieter aircraft. Nothing in the Trust's submissions demonstrate that this can be achieved "immediately." Similarly, as we have noted above, the interrelationship between the flight free zones and the routes and corridors is complicated, as is the effect these together will have on the surrounding land, including the Hualapai Reservation.

Finally, we also note the Trust's complaint that even using the Park Service's own definition, the Final Rule will not achieve substantial restoration of natural quiet; and that the FAA's latest reevaluation of the data indicates that not even that rule plus the two proposed rules will achieve Congress' goal. We agree that it would be arbitrary and capricious for an agency simply to thumb its nose at Congress and say -- without any explanation -- that it simply does not intend to achieve a congressional goal on any timetable at all. Indeed, counsel for the FAA conceded as much in oral argument. Oral Arg. Tr. at 85-86, 89-90, 116.

But the FAA has not taken that course here. It has never defended the Final Rule as the sole means for restoring the natural quiet, but only as the first of three steps. Its contemplation was that the three rules together would achieve that goal by 2008. See Final Rule, 61 Fed. Reg. at 69,306; Noise Limitations Rule, 61 Fed. Reg. at 69,338. For the same reason that we questioned the validity of Clark County's contention that the FAA should have held up promulgation of the Final Rule until it had all three rules ready, we reject the Trust's contention that the FAA must give birth to all three today. See City of Las Vegas v. Lujan, 891 F.2d at 935 (finding that "agencies have great discretion to treat a problem partially" and holding that court will not strike down agency action "if it were a first step toward a complete solution"); General Am. Transp. Corp., 872 F.2d at 1058.

The FAA acknowledges that the new data on the number of aircraft overflying the Park renders its original three-part plan less effective than originally assumed. The FAA has represented, however, that it still anticipates meeting the goal of substantial restoration by 2008. See Oral Arg. Tr. at 82, 90. To do this, "[q]uiet aircraft technology will obviously have to make up the gap in 2008, together with the route structure." Id. at 82. The FAA also will consider using a cap on the number of overflights. See FAA Letter at 2. The FAA has assured this court that it still believes that "the quiet technology rulemaking and the finalization of the air tour routes, when completed, will result in attainment of the statutory goal." FAA Supp. Br. at 4.

We will take the government at its word. See Orion Communications Ltd. v. FCC, 131 F.3d 176, 182 (D.C. Cir. 1997). 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, the Trust may petition to compel agency action unlawfully withheld or unreasonably delayed. But we are not at that point yet, and hence can do no more than affirm the rule currently before us.

For the foregoing reasons, the petitions for review of the Final Rule are denied. We note, however, that we have held unripe those of petitioners' challenges that specifically arise out of the interrelationship between the Final Rule's flight free zones, and the still-uncertain flight corridors and routes. Accordingly, those challenges may be raised again when the corridors and routes finally are promulgated. [FN 22]

[FN 22] This reservation applies only to challenges specifically arising out of the interrelationship between the zones and the routes and corridors. It does not, for example, apply to a challenge to the agency's interpretation of the statutory phrase, "substantial restoration of the natural quiet," as we have found the present challenges to that interpretation ripe and have upheld the agency's interpretation.