Opinion for the Court filed by Circuit Judge ROBERTS.
Lеarned Hand once remarked that agencies tend to “fall into grooves, ... and when they get into grooves, then God save you to get them out.” 1 Judge Hand never met the National Transportation Safety Board. In this case, we grant the petition for review because the Board has failed adequately to explain its departures from its own precedent in no fewer than three significant respects.
I.
Petitioner Tilak Ramaprakash was arrested for driving under the influence of alcohol in Doraville, Georgia, in December 1996, and was cоnvicted of that offense on February 25, 1997. As a licensed pilot, he was subject to Federal Aviation Regulation (FAR) § 61.15 (codified at 14 C.F.R. § 61.15 (2003)), which requires pilots to provide the Federal Aviation Administration (FAA) with a written report of any “motor vehicle action” within sixty days. Id. § 61.15(e). The regulation defines “motor vehicle action” to include a “conviction ... for the violation of any Federal or State statute relating to the operation of a motor vehicle while intoxicated by alcohol or a drug, while impaired by alcohol or a drug, or while under the influence of alcohol or a drug.” Id. § 61.15(c)(1).
Ramaprakash admits that he did not file the required report. His violation of FAR § 61.15(e) occurred on April 26, 1997, when the sixty-day period for filing the report ended. Twelve months later, on April 22, 1998, the FAA formally initiated administrative proceedings to suspend his pilot’s certificate by issuing a Notice of Proposed Certificate Action (NOPCA). The NOPCA proposed to suspend his certificate for thirty days in accordance with FAR § 61.15(f)(2), which provides that a violation of § 61.15(e) is grounds for “[s]uspension or revocation of any certificate” issued undеr FAR Part 61. In February 1999, the FAA ordered that Rama-prakash’s license be suspended for thirty *1123 days. He appealed to the National Transportation Safety Board (NTSB or Board).
Before the NTSB, Ramaprakash conceded that he had committed a FAR violation, but moved for dismissal of the FAA’s action against him in light of the Board’s “stale complaint rule” — Rule 33 of the NTSB’s Rules of Practice in Air Safety Proceedings, codified at 49 C.F.R. § 821.33 (2002). That rule provides, in pertinent part:
Where the complaint states allegations of offenses which occurred more than 6 months prior to the [FAA] Administrator’s advising respondent as to reasons for proposed [certificate] action ..., respondent may move to dismiss such allegations pursuant to the following provisions:
(a) In those cases where a complaint does not allege lack of qualification of the certificate holder:
(1) The Administrator shall be required to show by answer filed within 15 days of service of the motion that good cause existed for the delay, or that the imposition of a sanction is warranted in the public interest, notwithstanding the delay or the reasons therefor.
(2) If the Administrator does not establish good cause for the delay or for imposition of a sanction notwithstanding the delay, the law judge shall dismiss the stale allegations and proceed to adjudicate only the remaining portion, if any, of the complaint. 2
It was undisputed that the FAA had failed to meet Rule 33’s six-month deadline: the NOPCA was issued nearly one year after Ramaprakash’s FAR violation. The question before the NTSB was whether “good cause existed for the delay.”
Answering that question requires some understanding of how the FAA monitors compliance with the FAR reporting requirement. One way it does so is by compiling and periodically sending to the National Driver Register (NDR) lists of individuals who seek to obtain or renew their medical certifications. The NDR then matches the names against its own records, which contain information on individuals whose drivers’ licenses have been denied, revoked, suspended, or canceled for cause, or who have been convicted of serious driving offenses. See 49 U.S.C. § 30304(a). The information in the NDR is not detailed enough to show whether the offense involved is one for which а report must be filed under FAR § 61.15; when the FAA receives a computer tape from the NDR with a list of the names that have matched an NDR record, an FAA investigator must then check the National " Law Enforcement Telecommunications System (NLETS) database for details of each airman’s motor vehicle incident. If the NLETS data show that the incident was a reportable offense, the FAA investigator then searches the agency’s records to determine whether the airman filed the required report.
A detailed affidavit from FAA official Mark Sweeney desсribed the course of the FAA’s investigation of Ramaprakash’s violation. On May 16, 1997, the FAA received from the NDR a computer tape indicating a motor vehicle incident in Georgia involving Ramaprakash. The tape was *1124 routed to an FAA investigator in May-1997, but apparently no action was taken until September 1997, when the investigator was transferred to a new FAA assignment. A second investigator took possession of the tape, but this investigator too was reassigned in October 1997, and the tape was passed on to a third investigator. After working through a baсklog of prior tapes, this investigator turned to the tape that included Ramaprakash’s record and conducted the NLETS query on February 4, 1998. The NLETS database revealed that the incident disclosed on the NDR was in fact a DUI conviction, and by February 10, 1998, the investigator had searched FAA records and learned that Ramaprakash had failed to report it. See Sweeney Aff. at 2-3, JA 101-02.
The Board, by a 3-2 vote, concluded that the FAA had shown good cause for the delay in issuing the NOPCA. The Board stated that under Rule 33, “the Administrator must show that good cause existed for the delay in discovering thе offense and that, upon discovery, she investigated the matter with due diligence.”
Administrator v. Ramaprakash,
NTSB Order No. EA-4947 (February 7, 2002), at 5,
available at
Ramaprakash sought reconsideration, but the Board refused to reconsider its decision. In its brief order, the Board stated:
As we explained in our оriginal decision, we decline to extend the stale complaint rule under these circumstances, i.e., where the “delay” is non-prejudicial to respondent’s ability to defend against the charges ... and accrued, essentially, because the Administrator chose to delegate her resources in a manner that would not immediately, but eventually, detect airmen’s non-compliance with a mandatory reporting requirement that respondent admits to not adhering to.
Administrator v. Ramaprakash,
NTSB Order No. EA-4984 (July 16, 2002), at 1,
available at
II.
Under the Administrative Procedure Act (APA), a court may set aside agency action found to be “arbitrary, capricious, an abuse of discretion, or otherwise not in accordance with law.” 5 U.S.C. § 706(2)(A). Our review under the APA is highly deferential, but agency action is arbitrary and capricious if it departs from agency precedent without explanation. Agencies are free to change course as their expertise and experience may suggest or require, but when they do so they must provide a “reasoned analysis indicating that prior policies and standards are being deliberately changed, not casually ignored.”
Greater Boston Television
*1125
Corp. v. FCC,
In the orders challenged here, the Board deviated from its precedent in three respects. The first is in the NTSB’s answer to the question whether FAA delays should be more readily excused if the alleged FAR violation is relatively serious. In
Administrator v. U.S. Jet, Inc.,
7 NTSB 246,
We decline the invitation to carve out for the Administrator’s benefit alone a public interest exception to our policy of dismissing appeals that are not prosecuted with due diligence.
We recognize that even cases involving important air safety issues can fall victim to procedural non-feasance.... However, ... we think the public interest and basic principles of fairness favor rules that treat litigants equally over those that, based on presumptions flowing from the seriousness of alleged conduct, create procedural advantage for one party. That circumstance ... counsels against both the necessity and the propriety of modifying the good cause standard in a way that, in effect, would allow the Administrator to escape responsibility for сompliance with rules of practice we strictly apply to all others.
7 NTSB at 246-47.
In a nearly identical case four years later, the Board again refused to find that the public interest in air safety could create an exception to the Board’s requirement that good cause be shown to excuse a delay:
[Procedural decisions should [not] be based on post-default generalities or presumptions about the importance or the desirability of reaching the merits of a case a party ha[s] not handled in accordance with applicable rules. ...
We continue to believe that requiring parties to exercise a high level of diligence in the prosecution of their appeals to us is the best way to ensure that all cases, and especially those that may involve extraordinary air safety concerns, will be heard by the full Board.
Petition of White,
NTSB Order No. EA-4100,
In its Order Denying Appeal in this case, however, the NTSB considered the nature and seriousness of Ramaprakash’s FAR violation in determining whether the FAA had shown, good cause. The Board emphasized that it would be “particulаrly difficult to justify” applying the stale complaint rule to bar the FAA’s complaint, “given the importance to air safety of monitoring the alcohol-related infractions of certificated airmen, and the likelihood that they would go undetected but for the self-disclosure requirements of FAR section 61.15(e).” Order Denying Appeal at 7.
This suggestion that the Board is more willing to find good cause in cases that have serious implications for air safety is inconsistent with U.S. Jet and White, and the Board did not attempt to explain that departure from precedent. Nor did the *1126 Board explain how its statement comports with the text of Rule 33. The rule allows a stale complaint to escape dismissal if the FAA can show good cause for the delay, but it also states that a stale complaint can survive if the FAA can show that “the imposition of a sanction is warranted in the public interest, notwithstanding the delay or the reasons therefor.” 49 C.F.R. § 821.33(a)(1). There would appear to be little need for the public interest to be weighed in any determination of whether good cause exists for delay, when the rule provides an independent and adequate avenue by which stale complaints found to implicate the public interest can proceed.
Indeed, the Board in the past has found the seriousness of a violation to be a reason to be less, rather than more, lenient in finding good cause for delay. The Board noted in
Administrator v. Dill,
NTSB Order No. EA-4099,
The Board’s second departure from its precedent lies in its analysis of the role that prejudice plays under the stale complaint rule. For more than twenty years, the NTSB has explained that a party seeking dismissal of a stale complaint is not required to show prejudice from the delay. In
Administrator v. Zanlunghi,
3 NTSB 3696,
The Board’s approach here is a complete about-face. In the Order Denying Appeal, the Board stated:
[Ramaprakash] does not assert that, had the complaint been filed soоner, he would have answered differently or been better equipped to defend against the Administrator’s allegations. ...
In these circumstances, specifically, where a respondent’s ability to defend against a charge has not been compromised by the passage of time between the admitted violation and the action to sanction it, it would be arbitrary to dismiss the complaint under a rule designed to forestall evidentiary difficulties that can arise because of prosecutorial delay.
*1127 Order Denying Appeal at 7. The Board rеiterated this holding in the Order Denying Reconsideration: “As we explained in our original decision, we decline to extend the stale complaint rule under these circumstances, i.e., where the ‘delay’ is nonprejudicial to respondent’s ability to defend against the charges (having admitted all factual allegations).... ” Order Denying Reconsideration at 1. This language is impossible to square with Zanlunghi, Parish, and their progeny. Those cases make it clear that prejudice is presumed when a complaint alleges violations that occurred more than six months bеfore the NOPCA. Applying the stale complaint rule to Rama-prakash’s case would not “extend” the rule at all — unless the Board is no longer adopting a presumption of prejudice.
The FAA argues that the Board did not impose a requirement of prejudice — that it simply concluded that the presumption of prejudice had been overcome in Rama-prakash’s case. FAA Br. at 41. The NTSB has indeed indicated that the presumption of prejudice is rebuttable.
See, e.g., Dill,
The third, and perhaps most consequential, respect in which the Board departed from its precedent involves the longstanding requirement of prosecutorial diligence in stale complaint cases. In denying Ra-maprakash’s appeal, the Board noted that the stale complaint rule is “designed to forestall evidentiary difficulties that can arise because of prosecutorial delay,” Order Denying Appeal at 7, but avoiding such prejudice is not the only purpose of the stale complaint rule. At least as important, the Board has emphasized for decades, is the incentive that the rule provides for the FAA to improve air safety by promptly investigating and punishing those who violate the FAR. As long ago as 1974, the NTSB declared that “[i]t is the purpose of [the stale complaint rule] to assure that the Administrator’s investigation and prosecution of alleged regulatory violations is pursued with reasonable diligence and that prospective charges not be held over an airman’s head for an unreasonable period.... ”
Stewart, 2
NTSB at 1142;
see also Dill,
In its prior cases interpreting Rule 33, the Board has repeatedly stated that diligent investigation of
possible
violations is essential to a finding that good cause exists for a delay in issuing a NOPCA. In
Zanlunghi,
for example, the NTSB noted that a finding of good cause is warranted when there is evidence that “reasonable diligence [was] exercised following the FAA’s non-contemporaneous receipt of information concerning the potentially actionable conduct.” 3 NTSB at 3697. The Board elaborated that it knew that “the FAA may not always immediately learn of conduct which may have been violative of the FAR.”
Id.
In the same vein, the Board stated in
Brea
that “belated awareness” of “the possibility of an FAR violation” may serve as good cause for a delay in issuing a NOPCA, “provided that reasonable prosecutorial diligence is exercised” after the FAA receives “information concerning the act(s) or omission(s) which may be indicative of such a violation.”
Zanlunghi, Brea, and Dill speak of potentially actionable conduct, of possible violations, of conduct that may have violated the FAR, or of acts or omissions that may indicate a violation. None of the cases suggests that the FAA can wait until it has confirmation of a violation before beginning to work diligently on issuing a NOP-CA. This choice of language makes sense: if diligence is required, it should begin as soon as the ball is in the FAA’s court. It would make little sense to apply a requirement of diligence to only part of the period during which a case demanded nothing other than FAA attention. The Board in these cases quite reasonably recognized that in some situations the FAA may be completely ignorant of a potential violation for some time, but insisted that once the FAA is tipped off to a potential violation, it must act diligently if it intends to show good cause for the overall dеlay.
The Board departed from this precedent in the Order Denying Appeal, adopting a different trigger for the diligence requirement. Preliminary indications were once adequate, but now the discovery of the violation itself is the triggering event. See Order Denying Appeal at 5 (holding that the Administrator must show that “upon discovery, she investigated the matter with due diligence”). As the dissenting Board members noted, the Board's holding meant that “so long as the Administrator proceeds with due diligence after she discovers the violation, she may wait an indefinite amount of time ... to discover that violation and save her complaint from the stale complaint rule.” Order Denying Reconsideration at 4.
The Board majority found that although the NDR tape of May 16, 1997, fisted Ramaprakash’s name, “the Administrator did not have an indication of a possible section 61.15(e) violation until her NLETS query” on February 4,1998. Order Denying Appeal at 6. This statement purports to hew more closely to the language of Zanlunghi and Brea, but it does not stand up to scrutiny because it fails to account for why the investigator conducted the NLETS query. No NLETS query would have been necessаry unless the FAA already had an indication of a possible violation. The NDR tape provided that indication, and the tape meets the definitions in Zanlunghi, Brea, and Dill that emphasize preliminary information as the trigger for the prosecutorial diligence requirement. The NLETS information increased the probability that a violation had occurred (which ripened to a certainty when the FAA agent determined on February 10, 1998, that Ramaprakash had not reported the motor vehicle action); but if the FAA can take as long as it pleases to move from onе level of certainty to another — when the only constraint is the FAA’s own allocation of its investigative resources — then the prosecutorial diligence requirement so clearly established in the Zanlunghi line of cases has all but disappeared. The Board cannot undertake such a departure from its precedent without providing a reasoned explanation for its decision, and it provided none here.
The Board’s heavy reliance on
Administrator v. Ikeler,
NTSB Order No. EA-4695,
The FAA argues that
Ikeler
(and, by extension, the orders challenged here) is not a departure from Board precedent at all, suggesting that it is consistent with the Board’s decisions in
Administrator v. Gotisar,
NTSB Order No. EA-4544,
III.
After refusing to dismiss the FAA’s complaint as stale, the Board said that it was nonetheless troubled by the length of time between the FAA’s receipt of the NDR tape and the NLETS query, and concluded:
[B]ecause our ruling in Ikeler sustained a suspension order which involved a similar delay, the Administrator had no reason in this case to anticipate that we might view the issue differently. Whether Ikeler is followed in future cases may well depend on the magnitude of the delay, for at some point, we are inclined to believe, the Administrator’s interest in prioritizing her enforcement efforts will not outweigh the negative impact of forcing an airman to answer a charge long after the conduct giving rise to it.
*1130 Order Denying Appeal at 8 (footnotes omitted).
The Board revisited this point when it denied reconsideration, stating:
[W]e have ... placed the Administrator on notice that in future cases we will look more closely at the time that elapses between the time the Administrator could have, but did not, learn of the violation by comparing readily-available evidence. As we inferred in our original decision, however, this analysis, and our continued adherence to [Ikeler], will depend on the specific facts of future cases and arguments pertaining to the stale complaint rule.
Order Denying Reconsideration at 1-2 n.l.
This court has observed that “the core concern underlying the prohibition of arbitrary or capricious agency action” is that agency “ad hocery” is impermissible.
Pacific N.W. Newspaper Guild, Local 82 v. NLRB,
Notes
. Hearings to Study Senate Concurrent Resolution 21 Before a Subcommittee of the Senate Committee on Labor and Public Welfare, 82nd Cong., 1st Sess. 224 (1951) (quoted in Henry J. Friendly, Benchmarks 106 (1967)).
. Rule 33 provides a separate procedure for cases in which the FAA alleges a lack of qualification — typically a more serious charge. Even if some or all of the allegations in the complaint are stale, the case may proceed if the administrative law'judge "deems that an issue of lack of qualification would be presented by any of all of the allegations, if true.” 49 C.F.R. § 821.33(b)(2).
See Administrator v. Stewart,
2 NTSB 1140, 1142,
. The two Board members who dissented from the orders in’Ramaprakash’s case concurred in the Board's unanimous order in Ilteler, this suggests that, at least by their lights, Ikeler is distinguishable from the instant case.