Petitioners make two arguments on the merits. They contend that the FAA's No Hazard determinations are arbitrary and capricious because they depart from the agency's own internal guidelines. They also argue that the FAA failed to fulfill its obligations under 49 U.S.C. § 44718(b). We need reach only the first of these arguments because we agree with petitioners that, in light of the FAA's improper application of its own handbook, the FAA did not "adequately explain its result." Public Citizen v. FAA, 988 F.2d 186, 197 (D.C. Cir. 1993).
According to the handbook, see Procedures for Handling Airspace Matters, FAA Order 7400.2G (Apr. 10, 2008) (hereafter "handbook"), the FAA can find a hazard if the proposed structure would have a "substantial adverse effect." Id. § 7-1-3(e). A "substantial adverse effect" is defined to include one that would have an "[a]dverse effect" on a "significant volume of aeronautical operations." Id. § 6-3-5 (defining "Substantial Adverse Effect"); see also id. § 6-3-4 (noting that the volume of flights is significant "if one or more
aeronautical operation per day would be affected"). We will return shortly to the concept of "adverse effect."
After discussing the adverse effects the turbines would have on nearby radar facilities, the FAA's Determination addressed the impact on VFR operations, purporting to find no adverse effect on such operations. In so doing, the FAA relied solely on § 6-3-8(c)1 of the handbook, which says:
A structure would have an adverse [aeronautical] effect upon VFR air navigation if its height is greater than 500 feet above the surface at its site, and within 2 statute miles of any regularly used VFR route.
Handbook, § 6-3-8(c)1 (accurately paraphrased in Determination at 7, J.A. 7). After acknowledging that a regularly used VFR route would be affected, and correctly reciting § 6-3-8(c)1, the FAA leapt to the conclusion that the turbines would not have an adverse effect because they would not exceed the 500-foot threshold. Id. ("Therefore, . . . , the wind turbines . . . do not meet the criteria to have an adverse effect.").
But under any reasonable reading of the handbook, § 6-3- 8(c)1 simply identifies one circumstance in which a structure could have an adverse effect, potentially one among many. A different part of the handbook, § 6-3-3 (including subsections (a) through (f)), introduces the concept of "adverse effect":
6-3-3. Determining adverse effect.
A structure is considered to have an adverse aeronautical effect if it first exceeds the obstruction standards of part 77, and/or is found to have physical or electromagnetic radiation effect on the operation of air navigation facilities. A proposed or existing structure, if not amended, altered, or removed, has an adverse effect if it would:
. . .
b. Require a VFR operation, to change its regular flight course or altitude.
§ 6-3-3 (emphasis added). It is undisputed that the project turbines would (i) have the threshold "physical or electromagnetic radiation effect on the operation of air navigation facilities" (per the first sentence), and would (ii) "[r]equire a VFR operation, to change its regular flight course or altitude" (per the second sentence, together with § 6-3-3(b)).1 See Determination at 5, 7. The FAA's complete reliance on § 6-3-8(c)1 is therefore inconsistent not only with the language of that provision (reading into it a non-existent "only"), but with the organization of the handbook, which anticipates that structures qualifying under either segment of § 6-3-3's first sentence are to be assessed for the harms identified in the second sentence's subsections (a) through (f).
Improperly relying solely on § 6-3-8(c)1, the FAA failed to supply any apparent analysis of the record evidence concerning the wind farm's potentially adverse effects on VFR operations. A study by a consulting firm, MITRE, commissioned by the FAA, charted how many flights flew through a three-dimensional zone around the project, the boundaries of which were 500 feet to the side and 1000 feet above the turbines. The study found that over the course of a 90-day period 425 VFR flights flew through the immediate vicinity of the project site and that 94.1% of these 425 were flying at an altitude of 1000 feet or less. J.A. 381, 391-92. The 425 flights would be, of course, more than four and a half times the one flight per day that § 6-3-4 sets as the threshold of significance.
Once the turbines are built, many of these flights may be forced to be rerouted or to proceed in violation of the FAA's own regulation, 14 C.F.R. § 91.119, which requires a 500-foot distance between an aircraft and any structure. Further, the FAA's own weather compressibility study concluded that, during instances of inclement weather, "VFR aircraft could potentially be compressed to a lower altitude" to avoid cloud cover, such that they also would come within 500 feet of the turbines in violation of § 91.119. J.A. 469. Indeed, § 6-3-8(b)2 of the handbook says that any structure "that would interfere with a significant volume of low altitude flights by actually excluding or restricting VFR operations in a specific area would have a substantial adverse effect and may be considered a hazard to air navigation." The FAA may ultimately find the risk of these dangers to be modest, but we cannot meaningfully review any such prediction because the FAA cut the process short in reliance on a misreading of its handbook and thus, as far as we can tell, never calculated the risks in the first place.
The FAA repeatedly notes in its brief that the handbook "largely consists of criteria rather than rules to follow." Respondent's Br. at 40. We agree. Any sensible reading of the handbook, and of § 6-3-8(c)1 in particular, would indicate there is more than one way in which the wind farm can pose a hazard to VFR operations. Indeed, other sections of the handbook, especially when read in light of some of the evidence noted above, suggest that the project may very well be such a hazard. Here, by abandoning its own established procedure, see D&F Alfonso Realty Trust v. Garvey, 216 F.3d 1191, 1197 (D. C. Cir. 2000), the FAA catapulted over the real issues and the analytical work required by its handbook.
Whether in fact an application of the handbook's guidelines to the studies discussed above will cause the FAA to find the project a hazard, and if so, of what degree, we obviously cannot tell at this stage. But it surely is enough to trigger the standard requirement of reasoned decision-making, i.e., to require the FAA to address the issues and explain its conclusion. Public Citizen, 988 F.2d at 197. The FAA's misplaced reliance on § 6-3-8(c)1 is no substitute.
 In assuming that elements (i) and (ii) are both necessary, we give the benefit of the doubt to the FAA, reading the "first" of § 6-3-3's first sentence as implying that structures qualify as having adverse effects only if they satisfy the criteria of both the first sentence and the second (through one or more of its subsections).