> ## Documentation Index
> Fetch the complete documentation index at: https://docs.planeconnection.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Fatigue Risk Management
> How fatigue risk management systems (FRMS) work in aviation: Part 117 compliance, bio-mathematical fatigue modeling, K-score calculation, and operational mitigations.
Fatigue is one of the most persistent threats in aviation operations.
Unlike a mechanical failure that can be detected by sensors, fatigue
degrades human performance gradually and invisibly -- the fatigued
individual is often the last person to recognize the impairment. A
Fatigue Risk Management System (FRMS) provides a data-driven,
multi-layered approach to managing this risk beyond the minimum
prescriptive limits of duty time regulations.
This page explains the principles behind fatigue risk management and how PlaneConnection{" "}
implements them. For the regulatory details of flight and duty time limits, refer to 14 CFR Part
117 (applicable to Part 121 operations) and the Part 135 duty time requirements in 14 CFR 135.263
through 135.269. For practical guidance, see [Manage Crew](/en/how-to/ops/manage-crew).
## Why Prescriptive Limits Are Not Enough
14 CFR Part 117 and the Part 135 duty/rest requirements set hard limits
on duty time and minimum rest periods. These limits are essential
guardrails, but they have fundamental limitations:
* **They assume all hours are equal.** A 10-hour duty period starting at
0600 produces very different fatigue outcomes than one starting at
0200, even though both comply with the same duty limit.
* **They do not account for cumulative fatigue.** A pilot who has flown
six consecutive days within daily duty limits may be significantly
more fatigued than one starting a fresh rotation, even though both
are "legal."
* **They cannot measure sleep.** Regulatory rest periods guarantee
opportunity to sleep, not actual sleep. A pilot who spent a "rest
period" in a noisy hotel near an airport may have obtained far less
restorative sleep than the regulations assumed.
* **They are static.** The same duty limit applies regardless of route
complexity, weather conditions, passenger load, or other workload
factors that amplify the effects of fatigue.
An FRMS supplements prescriptive limits with a risk-based approach
that accounts for these variables.
## The Three Layers of FRMS
An effective FRMS operates at three layers, each building on the
previous one.
### Layer 1: Prescriptive Limits
The foundation is full compliance with applicable duty and rest
regulations. For Part 135 operations, this means the duty time and
rest requirements in 14 CFR 135.263 through 135.269. These limits are
non-negotiable -- an FRMS never relaxes prescriptive limits; it adds
protection beyond them.
### Layer 2: Predictive Fatigue Modeling
Bio-mathematical models estimate fatigue levels based on known
factors: time of day (circadian phase), time awake, cumulative sleep
debt, workload, and schedule pattern. These models predict fatigue
before it manifests, allowing proactive scheduling adjustments.
PlaneConnection uses a fatigue scoring model (K-score) that evaluates
crew schedules against these factors and produces a numerical fatigue
risk score for each duty period.
### Layer 3: Reactive Monitoring
Self-reporting, peer observation, and operational data analysis
capture fatigue events that the predictive model missed. Crew members
report fatigue through safety reporting, and the organization tracks
fatigue-related events to validate and improve the predictive model.
## The K-Score Fatigue Model
The K-score is a composite fatigue risk score that considers multiple
factors for each crew member's schedule.
### Scoring Factors
| Factor | Weight | Description |
| ------------------------- | ------ | ---------------------------------------------------------------------------------------------- |
| **Time of day** | High | Circadian rhythm impact. Duty during the WOCL (0200--0600 local) increases risk significantly. |
| **Time since sleep** | High | Hours awake since the last sleep period of 4+ hours. |
| **Cumulative duty** | Medium | Total duty hours in the previous 7 and 28 days. |
| **Rest quality** | Medium | Duration and timing of the most recent rest period. |
| **Consecutive duty days** | Medium | Number of days worked without a full day off. |
| **Workload factors** | Low | Number of legs, sector length, and operational complexity. |
### Score Ranges
| K-Score | Risk Level | Interpretation |
| ------- | ---------- | ------------------------------------------------------------------- |
| 0--20 | Low | Well-rested crew. Normal operations. |
| 21--40 | Moderate | Some fatigue factors present. Monitor during the duty period. |
| 41--60 | Elevated | Multiple fatigue factors compounding. Consider mitigations. |
| 61--80 | High | Significant fatigue risk. Mitigations required before dispatch. |
| 81--100 | Critical | Unacceptable fatigue risk. Do not dispatch without schedule change. |
### The Window of Circadian Low (WOCL)
The WOCL is the period between approximately 0200 and 0600 local time
when the circadian drive for sleep is strongest. Duty periods that
overlap the WOCL carry substantially higher fatigue risk regardless of
how much prior rest the crew member had. The K-score model applies a
significant multiplier to duty time during this window.
## Part 117 and Part 135 Compliance
PlaneConnection tracks compliance with both Part 117 and Part 135 duty
and rest requirements:
| Requirement | Part 117 | Part 135 |
| ------------------------------- | ------------------------------------ | ----------------- |
| Maximum flight duty period | Table B, by rest and start time | 14 CFR 135.267 |
| Minimum rest period | 10 hours (8 hours sleep opportunity) | 14 CFR 135.265 |
| Cumulative flight time (7 day) | 60 hours | N/A (see 135.267) |
| Cumulative flight time (28 day) | 190 hours | N/A |
| Cumulative duty (168 hours) | Limits by table | 14 CFR 135.263 |
The FRMS module flags any schedule that approaches or exceeds these
limits before the assignment is confirmed, giving dispatchers time to
adjust crew pairings.
## Mitigations
When the K-score indicates elevated or higher fatigue risk, the
following mitigations are available:
| Mitigation | Description |
| ----------------------- | --------------------------------------------------------------- |
| **Schedule adjustment** | Delay departure, reassign crew, or add a rest stop. |
| **Augmented crew** | Add a relief pilot to allow in-flight rest on longer segments. |
| **Controlled rest** | Authorize controlled napping during low-workload cruise phases. |
| **Strategic caffeine** | Time caffeine use for maximum alertness during critical phases. |
| **Duty time reduction** | Reduce the planned duty period below the maximum allowed. |
| **Additional rest day** | Insert a day off before a high-demand sequence. |
All mitigations are documented in the FRMS record for the affected
duty period, creating an audit trail of fatigue risk decisions.
FRMS mitigations are operational risk management tools, not waivers of regulatory requirements.
Prescriptive duty and rest limits in 14 CFR Part 117 and 14 CFR 135.263--269 apply regardless of
the K-score. An FRMS adds protection beyond these limits but never replaces them.
## Connecting FRMS to SMS
Fatigue risk management is a component of the broader Safety Management
System. Under 14 CFR Part 5, fatigue-related hazards must be identified,
assessed, and controlled through the Safety Risk Management process
(Sections 5.51--5.55). The FRMS feeds data into:
* **Safety reports** -- fatigue observations submitted by crew.
* **SPIs** -- fatigue-related Safety Performance Indicators (e.g., percentage of duty periods in the WOCL, average K-score by month).
* **Investigations** -- root cause analysis of fatigue-related events.
* **Hazard register** -- identified fatigue hazards with risk assessments and controls.
## Related
Crew scheduling and duty time management.
Flight Risk Assessment Tool that includes fatigue as a risk factor.
How the Flight Risk Assessment Tool scores risk factors.
The SRM process that governs fatigue risk decisions.