Opinion for the Court filed by Circuit Judge BROWN.
This case features two natural gas pipeline companies, Petal Gas Storage, L.L.C., and High Island Offshore System, L.L.C., that challenge ratemaking orders from the Federal Energy Regulatory Commission. The chief issue — the only one Petal and *698 HIOS share in common, and the one that brings the Interstate Natural Gas Association of America (representing most of North America’s natural gas transportation companies) into this case as interve-nor — is whether the Commission erred in its selection of the “proxy groups” used to calculate petitioners’ gas transmission rates, along with its placement of petitioners within those proxy groups. We hold that the Commission did err, by failing to explain how its proxy group arrangements were based on the principle of relative risk. In addition, HIOS presents three claims of its own, arguing the Commission should have approved the settlement it presented; selected a faster depreciation rate for its pipeline system; and awarded it a higher management fee. We reject all three claims, concluding that the Commission was well within the considerable deference we show it in ratemaking cases.
I
We begin with the question of whether the Commission erred in its selection of proxy groups and placement of petitioners within those proxy groups. The Administrative Procedure Act’s arbitrary and capricious standard governs our review, entitling the Commission to substantial deference, particularly in the rate-making context,
E. Ky. Power Coop. v. FERC,
The Commission has a duty under § 4 of the Natural Gas Act to ensure “just and reasonable” rates in the natural gas industry. 15 U.S.C. § 717c. Proxy groups are a tool the Commission uses, by policy, to determine just and reasonable rates. Since the Supreme Court has held that rates “should be sufficient to assure confidence in the financial integrity of the enterprise, so as to maintain its credit and to attract capital,”
FPC v. Hope Natural Gas Co.,
For natural gas pipelines, the Commission has traditionally relied on a proxy group of publicly traded companies with a high proportion of their business in pipeline operations.
Transcon. Gas Pipe Line,
90 F.E.R.C. ¶ 61,279, at 61,933 (2000). But the industry is changing. Acquisitions, financial mishaps, and other factors have left, by one count, just three companies that fit the old requirements (too few for a proxy group),
Williston Basin Interstate Pipeline Co.,
104 F.E.R.C. ¶ 61,036, at 61,
Petal and HIOS dispute the design and implementation of the Commission’s proxy groups in their cases on three grounds. First, they claim the Commission improperly included in their proxy groups low-risk, diversified natural gas companies with most of their business in distribution rather than pipelines — essentially LDCs (local gas distribution companies). Second, they claim the Commission improperly excluded risk-comparable MLPs (master limited partnerships) with a high proportion of their business in natural gas pipelines. Finally, given a proxy group of companies mainly engaged in gas distribution, Petal and HIOS object to the Commission placing them in the middle rather than the high end of the range of returns.
That proxy group arrangements must be risk-appropriate is the common theme in each argument. The principle is well-established.
See Hope Natural Gas Co.,
On the record before us, we do not find adequate support for the contention that the Commission’s proxy group arrangements were risk-appropriate. In its HIOS ruling, the Commission states that changes in the gas pipeline industry compel a new approach to proxy groups.
High Island Offshore Sys., L.L.C.,
110 F.E.R.C. ¶ 61,043, at 61,157-58 PP 131-32 (2005). We accept this. The Commission rejects the use of MLPs on the ground that they issue distributions rather than dividends (and thus might provide returns
of
equity as well as returns
on
equity), and justifies the use of what are primarily gas
*700
distribution companies as “the best [option] available ... on the current record.”
Id.
at 61,
We therefore vacate the Commission’s orders with respect to the proxy group issue. On remand, we do not require any particular proxy group arrangement. Perhaps it would be best to include gas distribution companies and exclude MLPs, but to put petitioners’ rates of return on equity at the top of the range. Or perhaps including MLPs and excluding gas distribution companies, while putting Petal and HIOS in the middle of the range, would be best. The Commission might even acceptably return to this court with just the same arrangements it has chosen, albeit explained and justified in very different terms. What matters is that the overall proxy group arrangement makes sense in terms of relative risk and, even more importantly, in terms of the statutory command to set “just and reasonable” rates, 15 U.S.C. § 717c, that are “commensurate with returns on investments in other enterprises having corresponding risks” and “sufficient to assure confidence in the financial integrity of the enterprise ... [and] maintain its credit and ... attract capital,”
Hope Natural Gas Co.,
II
We turn now to HIOS’s three individual claims. In 2002, HIOS sought to raise its shipping rates. An administrative law judge rejected the rate increase in 2004, and HIOS appealed. While the appeal was pending, HIOS worked out a settlement with at least some of its shippers and filed the settlement with the Commission for approval. The Commission rejected the settlement and, on the merits, depreciated HIOS’s pipeline system at a slower rate than HIOS favored, awarded HIOS a smaller management fee than HIOS requested, and, of course, used a proxy group to determine HIOS’s rate of return on equity. Besides the proxy group argument (addressed above), HIOS claims the Commission should have accepted its settlement proposal. Failing that, HIOS disputes the Commission’s choice of depreciation rate and management fee. As before, the arbitrary and capricious standard governs our review.
*701 A
The Commission’s stringent, merits-based rejection of its uncontested settlement, HIOS argues, violated FERC’s settlement regulations and departed without explanation from the precedent of
Stingray Pipeline Co., 101
F.E.R.C. ¶ 61,365 (2002). In the proceedings below, some controversy swirled over whether to regard the settlement as uncontested (as HIOS contended it should be). But we need not join the fray: The opinion under our review for this issue proceeds on the “assumption that] the settlement may be treated as uncontested,”
High Island Offshore Sys.,
110 F.E.R.C. ¶ 61,043, at 61,
Under FERC’s regulations for uncontested settlements, the Commission “may” give a settlement its approval “upon a finding that the settlement appears to be fair and reasonable and in the public interest.” 18 C.F.R. § 385.602(g)(3). The implication—as we have stated in dicta before and affirm now—is that the Commission may adopt an uncontested settlement
only
after finding it “fair and reasonable and in the public interest”; that is, the Commission has a duty to disapprove uncontested settlements that are unfair, unreasonable, or against the public interest.
Mobil Oil Corp. v. FPC,
Here, the Commission rejected the settlement for two reasons. First, it dictated rates half again as high as the administrative law judge had approved. Second, it awarded a three-million-dollar payout from HIOS to the active parties to the settlement—a payout that the Commission concluded would, in the circumstances of this case, undermine the usual assumption that a settlement’s active parties will protect the interests of its inactive parties. HIOS complains this level of scrutiny goes to the merits in a way properly reserved for contested settlements, 18 C.F.R. § 385.602(h)(l)(i), but we see only the independent consideration of fairness, reasonableness, and the public interest the Commission is duty-bound to give. HIOS claims the Commission approved a very similar settlement proposal in
Stingray.
But in
Stingray,
the rates dictated in the settlement were
lower
than those the administrative law judge had approved. True,
Stingray
featured a payout provision like the one here. Faced with the discrepancy, the Commission explains that “upon further reflection” it is “increasingly concerned about the unduly discriminatory nature of such arrangements.”
High Island Offshore Sys.,
110 F.E.R.C. ¶ 61,043, at 61,
*702 B
Determining a gas pipeline’s depreciation rate requires forecasting “the probable useful life of the specific pipeline systems in question,” based both on wear and tear and on the exhaustion of natural resources.
Memphis Light, Gas & Water Div. v. FPC,
That is not, however, the issue facing us. Our question is whether the Commission’s choice was rational or arbitrary. The context gives further shape to our arbitrary and capricious standard of review. First, “[t]he finding of the Commission as to the facts, if supported by substantial evidence, shall be conclusive.” Natural Gas Act § 19(b), 15 U.S.C. § 717r(b). Second, where an agency is “making predictions, within its area of special expertise, at the frontiers of science ... as opposed to simple findings of fact, a reviewing court must generally be at its most deferential.”
Balt. Gas & Elec. Co. v. Natural Res. Defense Council, Inc.,
C
Typically, a pipeline receives a return on its net investment, which is measured by the pipeline’s rate base. In the anomalous situation where a pipeline’s assets have become fully depreciated, FERC policy provides that the pipeline may receive a “management fee” in lieu of such a return.
See Tarpon Transmission Co.,
57 F.E.R.C. ¶ 61,371, at 62,240 (1991). Here, both HIOS and the Commission agree that HIOS should be awarded a management fee on the
Tarpon
model, and agree that the
Tarpon
formula strictly applied would produce an unreasonably low result. The issue for the Commission, then, was how to appropriately modify the
Tarpon
formula. The issue for us — again, at the apex of our deference under
Baltimore Gas,
*703
HIOS argues its approach to modifying the
Tarpon
formula is “more reasonable” and “straightforward” than the Commission’s, and would “best match[] the return that the fee is designed to substitute for.” Petitioners’ Reply Br. 31-32. But FERC is not required to choose the best solution, only a reasonable one.
See Deaf Smith County Grain Processors, Inc. v. Glickman,
For the reasons above, we vacate and remand on the issue of the proxy groups, and otherwise affirm.
So ordered.
