Definition
A warning indicator displayed on a cockpit instrument to alert the pilot that the instrument or its associated signal is unreliable, has failed, or is not receiving valid information. When a flag appears, the pilot must disregard the affected indication.
Plain English
A small warning marker that pops into view on an instrument to tell the pilot, 'Don't trust what this is showing you right now.'
Context Anchor
Seen on cockpit instruments and navigation displays during checks, taxi, flight, or instrument approaches.
Derivation
From the everyday sense of a flag as a signal raised to draw attention. On instruments, the warning indicator often physically resembled a small coloured flag that would drop into view when the instrument lost a valid signal.
Why Pilots Care
Flags prevent pilots from relying on faulty instrument data that could lead to loss of situational awareness or control.
Analogy
A flag is like a warning light in a car: it does not fix the problem, but it tells you not to ignore it.
Intuition Check
A flag here is not a cloth signal or airport marker. It is an instrument warning that means the displayed information may be unusable or unreliable.
Example Sentence 1
When the GS flag appeared on the glideslope indicator, the pilot disregarded the vertical guidance and continued the approach using the localizer only.
Example Sentence 2
When the VOR flag appeared during approach, the pilot discontinued the procedure and requested vectors.