Definition
In aviation, a descriptor applied to an instrument, navigation signal, system, or information source that cannot be trusted to provide consistently accurate or dependable readings or service, and therefore must not be used as the primary basis for flight decisions.
Plain English
Something that may give wrong or inconsistent information, so a pilot cannot count on it being right.
Context Anchor
Used when discussing instrument readings, weather information, radio reports, aircraft systems, and performance planning.
Derivation
From the prefix 'un-' (not) and 'reliable,' which comes from the Latin 'religare,' meaning 'to bind back' or 'fasten.' Something reliable can be 'tied to' or counted on; unreliable means the opposite — it cannot be counted on.
Why Pilots Care
Relying on unreliable data can cause loss of control or incorrect flight decisions.
Intuition Check
Unreliable does not mean “always wrong.” It means “not trustworthy enough to use by itself.” If an instrument or report might be wrong, delayed, or inconsistent, treat it as unreliable until confirmed another way.
Example Sentence 1
After takeoff, the crew noted disagreement between the two airspeed indicators and ran the airspeed unreliable checklist.
Example Sentence 2
Icing conditions can make the pitot-static system unreliable and require immediate use of alternate procedures.