1. Introduction
The Aviation Fatigue Calculator estimates a pilot’s fatigue level and impairment using scientific models that correlate fatigue with performance degradation similar to Blood Alcohol Content (BAC). This document explains the algorithms behind:
- Fatigue Score Calculation
- BAC Equivalent Estimation
- Risk Assessment & Recommendations
2. Fatigue Score Algorithm
The fatigue score (0–100) is computed using weighted factors from:
A. Base Factors
| Factor | Calculation | Max Contribution | Rationale |
|---|---|---|---|
| Duty Duration | DutyHours × 4 (capped at 40) | 40 | Longer flights increase fatigue linearly. |
| Night Landing Penalty | +15 if landing between 00:00–06:00 | 15 | Circadian disruption worsens alertness. |
| Time Zone Crossings | TimeZones × 3 (capped at 15) | 15 | Jet lag disrupts sleep homeostasis. |
| Sleep Debt (4-day) | (32 - TotalSleep) × 1.5 (capped at 20) | 20 | Chronic sleep deprivation accumulates. |
| WOCL Encroachments | WOCLCount × 5 | 20 | Sleep during Window of Circadian Low (WOCL, 02:00–06:00) is less restorative. |
| Eastbound Flight | +10 if flying eastbound | 10 | Eastbound travel is more disruptive to circadian rhythm. |
B. Crew Adjustments
| Crew Size | Multiplier | Rest Mitigation |
|---|---|---|
| 2 (No relief) | ×1.15 | None |
| 3 (1 relief) | ×0.95 | −1.5 per rest hour |
| 4 (2 relief) | ×0.85 | −2 per rest hour |
Final Fatigue Score:
FatigueScore = (BaseFactors × CrewMultiplier) - (RestHours × RestMitigation)
Capped at 100 (maximum impairment).
3. BAC Equivalent Algorithm
Fatigue-induced impairment is modelled as a BAC-like percentage (0.000%–0.100%) using:
A. Wakefulness Factor
| Hours Awake | BAC Increase | Formula |
|---|---|---|
| <16h | Minimal | 0.000% |
| 16–20h | Linear | 0.02% + (HoursAwake - 16) × 0.005% |
| 20–24h | Accelerated | 0.05% + (HoursAwake - 20) × 0.0075% |
| >24h | Severe | 0.10% + (HoursAwake - 24) × 0.003% (capped at 0.10%) |
B. Additional Penalties
| Factor | Penalty |
|---|---|
| Duty >12h | +0.002% per extra hour (max +0.03%) |
| Circadian Low (02:00–05:00) | +0.02% if departure is in this window |
C. Rest Mitigation
- Each hour of in-flight rest reduces BAC by 0.005%.
Final BAC Equivalent:
BAC = (WakefulnessBAC + DutyPenalty + CircadianPenalty) - (RestHours × 0.005%)
Capped at 0.10% (equivalent to severe impairment).
4. Risk Assessment & Recommendations
| Fatigue Score | BAC Equivalent | Risk Level | Recommendations |
|---|---|---|---|
| <40 | <0.02% | 🟢 Low | Normal operations. |
| 40–59 | 0.02–0.049% | 🟡 Moderate | Caution advised; consider breaks. |
| 60–79 | 0.05–0.079% | 🟠 High | Mitigation needed; 12h rest before next flight. |
| ≥80 | ≥0.08% | 🔴 Critical | Rest required; 24h recovery recommended. |
5. Scientific Basis
- 17h awake ≈ 0.05% BAC (Dawson & Reid, 1997)
- 24h awake ≈ 0.10% BAC (Williamson & Feyer, 2000)
- Circadian low (02:00–05:00) adds ~0.02% BAC impairment (CAA, FAA studies)
- Eastbound flights are more disruptive (due to phase advance vs. delay)
6. Limitations
- Does not account for individual sleep quality or medical conditions.
- Rest mitigation assumes high-quality sleep, which may not always be true in-flight.
- BAC equivalent is an estimate—actual impairment varies by individual.
7. Conclusion
This algorithm provides a standardised, evidence-based approach to fatigue risk assessment in aviation. Future improvements could include:
- Personalised sleep efficiency factors
- Real-time biometric integration (e.g., wearables)
- Machine learning adjustments based on crew feedback
For operational use, always comply with regulatory fatigue management systems.
References:
ICAO Fatigue Management Guidelines
Dawson & Reid (1997), Nature
FAA Advisory Circular 117-3 (Fatigue Risk Management)
Prepared by: Safety Matters Foundation
Date: May 2025
Version: 1.0
