Definition
A warning indication on a cockpit instrument, typically a small mechanical or electronic flag that appears in the instrument's display window, signaling that the instrument or its associated signal is unreliable, has failed, or is not receiving valid data.
Plain English
A small visible warning that pops up on an instrument to tell the pilot that the instrument cannot be trusted right now.
Context Anchor
Seen on cockpit instruments and navigation displays during preflight checks, flight, or while using radio-based navigation.
Derivation
Called a 'flag' because the warning is literally a small flag-shaped marker that appears in a window on the instrument face, much like a flag being raised to get attention.
Why Pilots Care
It prevents a pilot from relying on false instrument readings that could lead to loss of control or navigation errors.
Grounding Statement
When a FLAG Alarm appears, treat that instrument’s reading as unreliable until the cause is found and corrected.
Intuition Check
Do not assume “alarm” means a loud sound here. A FLAG Alarm is usually a visible warning that tells you not to trust that instrument indication.
Example Sentence 1
When the glideslope flag alarm appeared on final approach, the pilot disregarded the glideslope and flew the localizer-only minimums.
Example Sentence 2
When the GPS signal dropped, the FLAG Alarm on the CDI prompted the pilot to select an alternate navigation source.