Definition
A state of reduced mental and physical performance caused by inadequate rest, prolonged exertion, stress, or disrupted sleep patterns. In aviation, fatigue degrades alertness, reaction time, judgment, and decision-making, and is recognized by the FAA as a significant risk factor in flight safety. It is the 'F' in the IMSAFE personal-readiness checklist pilots use to self-assess fitness for flight.
Plain English
Being tired enough that your thinking, reactions, and judgment are not as sharp as they should be — even if you don't fully realize it.
Context Anchor
Seen in the IMSAFE self-check before flight, especially when considering sleep, long duty days, illness, stress, or repeated training flights.
Derivation
From the French fatigue, meaning 'weariness,' and the Latin fatigare, 'to tire out.' The aviation use keeps the everyday meaning but treats it as a measurable safety hazard rather than a vague feeling.
Why Pilots Care
Fatigue is a leading contributor to pilot error and is one of the six specific factors checked in the IMSAFE preflight self-evaluation.
Intuition Check
Do not assume fatigue only means falling asleep. In aviation, fatigue can also mean being awake but mentally dulled, slower to react, or less careful than normal.
Example Sentence 1
After a short night's sleep and an early showtime, the pilot ran through IMSAFE and grounded himself due to fatigue.
Example Sentence 2
After a demanding four-hour flight, the instructor advised resting to clear any remaining fatigue before the next lesson.