Definition
Authorized departures from the rules, regulations, or clearances normally required during flight operations. Deviations are permitted when the pilot in command determines that strict compliance is not possible or safe under the circumstances, such as during an in-flight emergency, in response to weather, or when instructed by air traffic control. Many deviations require the pilot to notify ATC as soon as practical and, in some cases, to file a written report.
Plain English
A deviation is when a pilot is allowed to step outside the normal rules or instructions because the situation requires it. The pilot does what is needed to keep the flight safe and then lets ATC know.
Context Anchor
Pilots encounter this word in ATC instructions, instrument indications, flight training, procedure discussions, and reports about not following an assigned route, altitude, or clearance exactly.
Derivation
From the Latin deviare, meaning 'to turn off the road' (de- 'off' + via 'way'). In aviation it carries that same sense -- temporarily leaving the expected path, rule, or clearance.
Why Pilots Care
Uncorrected deviations can trigger ATC alerts, safety reports, or loss of separation from other traffic.
Intuition Check
Do not assume a deviation always means someone did something wrong. In aviation, it often just means there is a difference from the assigned, planned, or expected value; it may be corrected, authorized, or necessary for safety.
Example Sentence 1
The pilot requested a deviation twenty degrees left of course to avoid a line of thunderstorms.
Example Sentence 2
The pilot corrected the heading deviations after the autopilot was re-engaged.