Opinion for the Court Per Curiam.
Concurring Opinion filed by Circuit Judge SCALIA.
PER CURIAM.On August 9, 1983, this court issued its opinion in Interstate Natural Gas Association of America v. FERC, 716 F.2d 1 (D.C.Cir.), cert. denied, — U.S. -, 104 S.Ct. 1615, 1616, 80 L.Ed.2d 144 (1983) (Interstate). We held that under the Natural Gas Policy Act of 1978,15 U.S.C. § 3301, et seq. (1982), the Federal Energy Regulatory Commission’s “dry” method of measurement of the Btu content of natural gas for wellhead pricing purposes was improper and resulted in prices in excess of those permitted under the statute. We instructed FERC to vacate its Order designating the “dry” method. Two consequences of our decision were inexorable: implement the proper pricing method for future pricing decisions, and remedy the effects of past use of the improper pricing method at the earliest possible moment.
The Interstate parties are before us again, in response to the motion of petitioner Associated Gas Distributors to enforce our mandate in Interstate. For the reasons stated below, we direct FERC to implement the refund procedures set forth in Order No. 399.
I.
Following the issuance of our mandate, FERC moved to comply with its terms. FERC issued a rule requiring wet pricing henceforth, 49 Fed.Reg. 3072 (1984). In further implementation of our holding in Interstate, FERC issued Order No. 399 which directed producers to refund the excessive costs arising from use of the improper “dry” method of calculation:
The Federal Energy Regulatory Commission (Commission) is amending and finalizing its regulations that establish refund procedures for overcharges resulting from adjustments to the calculation of energy content of natural gas ... sold pursuant to the Natural Gas Policy Act of 1978. In so doing, the Commission is *148implementing [this court’s] decision in Interstate Natural Gas Association of America v. Federal Energy Regulatory Commission.
49 Fed.Reg. 37735 (1984) (footnote and citation omitted).
Order No. 399, issued pursuant to the requisite notice and comment, established a system of refunds that fulfilled the premises of our decision. Under the Order, producers of natural gas were to refund to pipelines the overcharges made under the improper “dry” method. These refunds were to be paid over by the pipelines to natural gas customers who had, in the first instance, paid these excessive charges. Under the Order, “large producers” were to have refunded their overcharges on November 5, 1984, while “small producers” were scheduled to issue their refunds on May 3, 1985. To date, no refunds have been made.
FERC explicitly prohibited producers from offsetting their refund obligations against any monies owed them by the pipelines. Order No. 399, 49 Fed.Reg. 37738-39. This prohibition had particular reference to claims the producers had made for additional monies due them under Section 110 of the Natural Gas Policy Act. FERC has issued Order No. 94-A allowing producers to collect these “Section 110 costs” from pipelines. 48 Fed.Reg. 5152 (1983). Order No. 94-A is now under review in the United States Court of Appeals for the Fifth Circuit. Texas Eastern Transmission Corp. v. FERC, No. 83-4390 (5th Cir. filed 1983).
In rejecting the use of offsets, FERC recognized not only that many of the Section 110 costs owed had not been paid, but also that tying these unrelated and disputed costs to refund of the improper “dry” charges would delay and disrupt the implementation of our decision. FERC stated in Order 399:
The Commission also believes that permitting offsets of section 110 costs and Btu refunds would complicate an already difficult process and would make Commission monitoring of Btu refunds more difficult. In addition, the Commission is concerned that permitting pipelines and firstsellers to offset section 110 costs and Btu refunds could prevent the Btu refunds from reaching as many of the customers actually overcharged as possible. Considering that the section 110 orders are also subject to judicial review, the Commission believes it is more appropriate to segregate the collection of section 110 costs from the Btu refunds.
49 Fed.Reg. 37739.
Two months after issuing Order No. 399, FERC abruptly reversed its position on offsets, and, in Order No. 399-A, 49 Fed.Reg. 46353 (1984), FERC, without addressing the weighty rationale behind its earlier Order, authorized producers to offset their refund obligations by deducting monies due them under Section 110.
II.
Associated Gas Distributors has moved this court for an order commanding FERC to comply with our mandate by directing producers to make appropriate refunds to pipelines immediately, as set forth in Order No. 399. A petition for review of Orders No. 399 and 399-A is currently pending in the Fifth Circuit. See Mobil Oil Exploration & Producing Southeast Inc. v. FERC, No. 84-4775 (5th Cir. filed Nov. 20, 1984). The Fifth Circuit, however, has issued an order delaying filing of the administrative record in Mobil until after this court issues its ruling in the present case. Accordingly, jurisdiction in the instant matter properly lies in this court. See 15 U.S.C. § 3416(a)(4).
The availability of mandamus to confine FERC to the terms of this court’s mandate is fully supported by precedent. See, e.g., Briggs v. Pennsylvania R.R., 334 U.S. 304, 306, 68 S.Ct. 1039, 1040, 92 L.Ed. 1403 (1948); Yablonski v. UMW, 454 F.2d 1036, 1038-39 (D.C.Cir.1971), cert. denied, 406 U.S. 906, 92 S.Ct. 1609, 31 L.Ed.2d 816 (1972). FERC protests, however, that our mandate did not reach so far as to preclude the offset authorized by Order No. 399-A. We need not enter the debate over the *149propriety of resort to mandamus in this ease, because, in any event, Associated Gas Distributors’ motion to enforce the mandate may be treated as a petition to review that order.
Federal appellate courts have broadly recognized that the filing of a paper substantially equivalent to one that formally. inaugurates the normal review process may well suffice for that purpose. Thus the need for a notice of appeal may be satisfied- by any of a variable host of filings evincing unequivocably an intention to appeal. See, e.g., Belton v. United States, 259 F.2d 811, 814 (D.C.Cir.1958) (en banc) (letter requesting leave to appeal in forma pauperis) (citing cases); Riffle v. United States, 299 F.2d 802 (5th Cir.1962) (letter to judge of court of appeals). The reason is that the content requirements of a notice of appeal, see Fed.R.App.P. 3(c), may easily be met by a document of similar import, and the interests of justice may demand its acceptance as an effective substitute. See 6 C. Wright, A. Miller, E. Cooper & E. Gressman, Federal Practice § 3949 at 356-57. Indispensably, any such filing must take place within the period allowed for initiation of an appeal. Alley v. Dodge, 501 F.2d 880, 881-82 (D.C.Cir.1974); see also Johnson v. United States, 405 F.2d 1072, 1073 n. 6 (D.C.Cir.1968).
These considerations exert equal force in the situation before us. A petition for review need only “specify the parties seeking review and designate the respondent and the order or part thereof to be reviewed,” Fed.R.App.P. 15(a), and undeniably Associated Gas Distributors’ motion did that. The motion, filed the day following issuance of Order No. 399-A, was well within the time petitions for review are indulged, and the relief sought is as much obtainable on direct review of the agency’s action as it is in a mandamus proceeding. Indeed, precisely because direct review and mandamus may provide identical relief, these two judicial actions may be sought in the alternative. See, e.g., United States v. Green, 499 F.2d 538, 539-40 & nn. 4, 5 (D.C.Cir.1974); Chase v. Robson, 435 F.2d 1059, 1060, 1062 (7th Cir.1970). We have heretofore held that a petition for mandamus can do service as a petition for review. National Organization for Reform of Marijuana Laws v. Ingersoll, 497 F.2d 654, 656 n. 3 (D.C.Cir.1974). Similarly, petitions for mandamus have been treated as notices of appeal, United States v. Green, 499 F.2d at 540 n. 5; Jordan v. United States District Court, 233 F.2d 362, 365, (D.C.Cir.), vacated on other grounds, 352 U.S. 904, 77 S.Ct. 151, 1 L.Ed.2d 114 (1956), and vice versa, Dellinger v. Mitchell, 442 F.2d 782, 789 (D.C.Cir.1971); Coastal Steel Corp. v. Tilghman Wheelabrator Ltd., 709 F.2d 190, 197 (3d Cir.), cert. denied, — U.S. -, 104 S.Ct. 349, 78 L.Ed.2d 315 (1983). Because the parties to the instant case have amply addressed the merits, no one will be prejudiced by considering Associated Gas Distributors’ motion to enforce the mandate as a petition for review.
III.
Turning then to the substance of the dispute before us, we conclude that the offset provision contained in Order No. 399-A must be set aside. As stated above, FERC issued Order No. 399, in which offsets were explicitly prohibited. Shortly thereafter, FERC issued Order No. 399-A, in which offsets were permitted. While an agency may change its course, it can do so only with a full and complete explanation that would withstand judicial review. FERC’s Order No. 399-A does not meet that requirement. Without such an explanation, the agency’s decision may be arbitrary and capricious. Dunlop v. Bachowski, 421 U.S. 560, 577, 95 S.Ct. 1851, 1862, 44 L.Ed.2d 377 (1975). More fundamentally, Order No. 399-A defies the teaching of the Supreme Court in FPC v. Tennessee Gas Transmission Co., 371 U.S. 145, 83 S.Ct. 211, 9 L.Ed.2d 199 (1962) (Tennessee Gas). In that case, the Court held that *150refunds must be implemented expeditiously:
[I]t is the duty of the Commission ..., where refunds are due, to direct their payment at the earliest possible moment consistent with due process.
Id. at 155, 83 S.Ct. at 216-17. The rationale for prompt ordering of refunds is clear: “to afford consumers a complete, permanent and effective bond of protection from excessive rates and charges.” Atlantic Refining Co. v. Public Service Commission, 360 U.S. 378, 388, 79 S.Ct. 1246, 1253, 3 L.Ed.2d 1312 (1959). The Court in Tennessee Gas required immediate refunds even though delaying refunds would have allowed a more accurate allocation of the burden of payment to the gas pipeline, and the immediate payment ordered might have resulted in some loss to the pipeline due to the inaccurate allocation.
FERC’s prohibition of offsets was well-considered. The controversy over production-related costs to be recouped by producers under Section 110 constitutes unsettled litigation wholly separate from the instant case. As such, permitting producers to offset their refund obligations would bring delay and confusion into an already complex area. Moreover, the principles of law and the customers involved in the refund arrangement are far from identical to those implicated in the Section 110 costs proceedings. By prohibiting offsets, FERC designed the system most likely to assure that customers will receive refunds due them. Involving disparate issues in a matter on appeal to another court can only complicate, delay, and obfuscate the refunds at issue here.
The law requires the ordering of refunds at the earliest possible moment. That moment has come and gone. The offset scheme embodied in Order No. 399-A will only further delay refund of the overcharges, circumventing our mandate in Interstate and the clear instruction of the Supreme Court in Tennessee Gas. We accordingly grant the relief sought by petitioner Associated Gas Distributors. We hereby direct FERC to vacate the offset portion of Order No. 399-A, for the reasons set forth in its own Order No. 399.
It is so ordered.