Fatigue is one of aviation's most dangerous and least visible hazards. A fatigued pilot rarely recognizes the extent of their own impairment — reaction time slows, decision-making degrades, and situational awareness collapses, yet the individual often reports feeling only slightly tired. To close this safety gap in scheduled air carrier operations, the FAA enacted 14 CFR Part 117, a comprehensive, physiology-based framework governing flight time limitations, flight duty periods, and rest requirements for Part 121 certificate holders. Commercial pilot applicants are expected not only to know Part 117's key numbers but to understand the reasoning behind them and how the regulation differs from older, simpler clock-watching approaches.
Scope and Applicability
Part 117 applies to certificate holders conducting domestic, flag, and supplemental operations under 14 CFR Part 121 whenever they use flight crew members in those operations. This is an important boundary: Part 117 is not the default rule for every commercial operation. Commuter and on-demand operators certificated under Part 135 follow separate flight-time and duty-time rules found in 14 CFR 135.261 through 135.273. Part 91 general aviation operations have no prescribed rest requirements beyond the pilot's own self-assessment obligations under 14 CFR 61.53. Confusing these regulatory buckets is one of the most common errors on the FAA Commercial Pilot knowledge test.
The regulation's fundamental philosophy shift matters for the exam and for the cockpit: rather than setting a single universal daily limit regardless of circumstances, Part 117 ties permissible duty time to circadian rhythm science — specifically to what time of day a flight crew member begins their duty, how many flight segments they will fly, and whether they are physiologically acclimated to their reporting time zone. These variables directly affect cognitive performance in ways a simple 8-hours-on, 8-hours-off rule cannot capture.
Essential Definitions
Flight Time
Under Part 117 (and consistent with the definition used throughout the FARs), flight time begins the moment an aircraft first moves under its own power for the purpose of flight and ends when it comes to rest at the next point of landing. Taxi time counts; gate-hold time with engines running counts. This is the measure used for the cumulative flight time caps.
Flight Duty Period (FDP)
The flight duty period is broader than flight time. It begins when a flight crew member is required to report for duty with the intention of conducting flight operations — meaning the clock starts at the airport check-in or briefing, not at engine start. The FDP ends when the aircraft is parked after the last flight of that duty period. Pre-flight paperwork, ground delays, time between segments, and post-flight shutdown all fall inside the FDP window. This distinction is heavily tested: a pilot who reports two hours before their first departure has already consumed two hours of FDP before the wheels ever leave the ground.
Rest Period
A rest period under Part 117 is a continuous, uninterrupted block of time completely free from all duty, during which the certificate holder places no obligations on the flight crew member. The quality of rest matters — the regulation specifically requires that the rest period be long enough to provide an opportunity for at least 8 hours of uninterrupted sleep. Travel time, commuting, and positioning flights that require the pilot's services do not qualify as rest.
Core Numerical Limits
Cumulative Flight Time Caps
- 100 hours of flight time in any 672 consecutive hours (28 days).
- 1,000 hours of flight time in any 365 consecutive calendar days.
Both limits apply simultaneously. A pilot must remain within whichever constraint is more restrictive at any given moment. These are flight time caps only — there is no equivalent single-number cumulative cap on FDP hours in the regulation; FDP is controlled on a per-duty-period basis through the tables in Part 117.
Flight Duty Period Maximums
Maximum FDP is determined by two variables: the scheduled start time expressed in the pilot's acclimated time (the local time at the location where the pilot has spent the prior 72 hours) and the number of flight segments planned. The regulation presents this as a table in 14 CFR 117.13. As a representative benchmark: for an unaugmented operation with a report time between 0700 and 1259 acclimated local time and 1–2 flight segments — near the peak of the circadian alertness window — the maximum FDP is 13 hours. That same pilot flying three segments in the same time window still has a maximum FDP of 9 hours under the table's segment-based reductions; further reductions occur only at higher segment counts (such as 7 or more segments) or in less favorable report-time windows. Report times that fall in the overnight window (roughly 0100–0559 local) carry the shortest allowable FDPs because circadian performance is at its nadir. Augmented crews using in-flight rest facilities can qualify for extended FDPs under specific conditions outlined in 14 CFR 117.17 and 117.19, but augmentation does not eliminate the caps — it modifies them within defined limits.
Minimum Rest Requirements
- Before beginning any FDP, a flight crew member must receive a minimum of 10 consecutive hours of rest.
- That 10-hour period must provide an opportunity for at least 8 hours of uninterrupted sleep.
The 10-and-8 relationship trips up many students. The 10 hours is the minimum total rest window; the 8 hours is the protected sleep opportunity nested inside it, leaving at least 2 hours for hygiene, meals, and transit within the rest facility. Both numbers must be satisfied — meeting one does not satisfy the other.
Short-Call Reserve
A pilot on short-call reserve (SCR) is not on a formal FDP but must be reachable and available to report within a defined time window specified by the carrier. Part 117 protects SCR pilots by ensuring they receive the required pre-FDP rest before being assigned to a flight duty period. The FDP clock does not start before the beginning of the rest period used to satisfy the pre-FDP rest requirement. Carriers must give adequate notice to allow the pilot to obtain qualifying rest before the reserve window begins.
Fatigue Risk Management Systems
Beyond the prescriptive table-based limits, Part 117 requires covered certificate holders to develop and implement a Fatigue Risk Management System (FRMS) as part of their overall Safety Management System. An FAA-approved FRMS uses scientific data, operational experience, and continuous monitoring to identify and mitigate fatigue risks that the standard tables may not capture — for example, chronic partial sleep deprivation across a sequence of early-morning report times. Critically, an FRMS is not a blanket waiver tool. It must be FAA-approved, data-driven, and subject to ongoing audit. Certificate holders cannot simply assert an FRMS exists to exceed the standard limits without demonstrated compliance with the program's specific requirements.
The Regulatory History and Safety Rationale
Part 117 was adopted as a new rule following the February 2009 Colgan Air Flight 3407 accident near Buffalo, New York, in which fatigue was identified as a contributing factor. Before Part 117, flight time and duty rules for Part 121 operations were found in Part 121 subparts Q, R, and S. The accident prompted Congress to mandate stronger crew rest standards through the Airline Safety and Federal Aviation Administration Extension Act of 2010, which led the FAA to issue the new Part 117 rule (published December 2011), taking effect in January 2014. Understanding this history reinforces that the numbers in Part 117 are not arbitrary administrative hurdles — they represent minimum safety thresholds derived from sleep research and accident investigation.
For the commercial pilot, the practical obligation extends beyond knowing the numbers. Under 14 CFR 61.53 and the broader airman fitness standards, a pilot has both the right and the legal duty to decline flight when fatigued, regardless of scheduling pressure or economic incentive. Part 117 provides the structural framework; pilot self-assessment and professional integrity provide the final safety layer.
Common Test Traps
- Part 117 vs. Part 135: Part 117 governs Part 121 air carrier operations only. Part 135 commuter and on-demand operations use the separate rules in 14 CFR 135.261–135.273. The two rule sets are not interchangeable on the exam or in practice.
- FDP clock starts at report time: The flight duty period begins when a pilot reports for duty, not at engine start or takeoff. A two-hour early report consumes two hours of the permissible FDP before the aircraft moves.
- 10 hours rest ≠ 8 hours sleep: Both figures must be independently satisfied. The 10-hour window is the total rest period; the 8-hour sleep opportunity is a protected subset within it.
- Cumulative limits are simultaneous: A pilot must comply with both the 100-hour/672-hour limit and the 1,000-hour/365-day limit concurrently. Meeting one does not excuse exceeding the other.
- FDP tables are variable: There is no single universal maximum FDP. The correct value depends on report time (acclimated) and number of flight segments. Memorizing only one figure without understanding the table structure will produce errors on scenario-based questions.
- Augmentation modifies but does not eliminate limits: Augmented-crew provisions extend FDPs within defined bounds; they do not make FDP limits disappear.