Definition
A visual warning indicator on the Horizontal Situation Indicator (HSI) that appears when the heading information from the compass system is unreliable or has failed. When visible, it tells the pilot that the heading shown on the HSI cannot be trusted.
Plain English
A small flag that pops into view on the HSI to tell you the heading reading is broken or unreliable. If you see it, do not trust the heading the instrument is showing.
Context Anchor
Seen on an HSI, which is the cockpit instrument that combines aircraft heading with navigation information.
Derivation
The word 'flag' comes from old Germanic roots meaning something that flutters or signals. In instrument design, a flag is a small marker that drops or appears into view to signal a warning -- the same idea as raising a flag to get attention.
Why Pilots Care
It tells the pilot to stop using the HSI heading and switch to the magnetic compass or other navigation references to maintain accurate directional control.
Analogy
It is like a warning light on a car dashboard: the display may still be visible, but the warning tells you not to rely on it until the problem is addressed.
Intuition Check
Do not read this as a warning from the aircraft’s standby magnetic compass, and do not picture a cloth flag. It is a small warning indication inside the instrument that means the displayed heading may not be trustworthy.
Example Sentence 1
During the run-up, the pilot noticed the compass warning flag in view on the HSI and delayed departure until the heading system was checked.
Example Sentence 2
The instructor pointed out that the compass warning flag will stay in view until power is restored to the heading system.