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.
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.
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
Score Ranges
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:
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:
All mitigations are documented in the FRMS record for the affected
duty period, creating an audit trail of fatigue risk decisions.
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
Manage Crew
Crew scheduling and duty time management.
Conduct a FRAT
Flight Risk Assessment Tool that includes fatigue as a risk factor.
FRAT Methodology
How the Flight Risk Assessment Tool scores risk factors.
Understanding Risk Management
The SRM process that governs fatigue risk decisions.