Definition
A warning flag on a Horizontal Situation Indicator (HSI) that appears when the heading information being displayed is unreliable or has failed. When visible, the HSI's heading display should not be trusted for navigation or course tracking.
Plain English
A small flag on the HSI that pops into view to tell the pilot the heading shown by the instrument cannot be trusted right now.
Context Anchor
Seen on an HSI or other cockpit heading display when the heading source or signal has failed or is not usable.
Derivation
HDG is the standard cockpit shorthand for heading. A 'flag' in instrument terminology is a small visible indicator that drops into view to warn the pilot of a problem — the same idea as a red flag being raised to signal a fault.
Why Pilots Care
It prevents the pilot from relying on erroneous heading data that could lead to navigation errors or loss of situational awareness.
Analogy
It is like a warning light on a car dashboard: the display may still be visible, but the warning tells you something behind that display is not working correctly.
Intuition Check
A flag here is not a physical flag outside the airplane. It is a cockpit warning marker. HDG means heading, so the warning is about the heading information being unreliable.
Example Sentence 1
During the instrument check, the pilot noted the HDG flag was in view and selected the standby compass as the primary heading reference.
Example Sentence 2
After aligning the heading system, the HDG flag retracted and the course display became usable again.