Definition
Visible warning indicators that appear on the face of certain flight instruments — most commonly the attitude indicator and heading indicator — when the instrument is not receiving adequate power, vacuum, or input to display reliable information. When a flag appears, the instrument's indication should not be trusted.
Plain English
Small warning markers that pop into view on an instrument's face to tell the pilot, 'Don't believe what I'm showing you right now.'
Context Anchor
Seen during instrument scan and system-failure checks, especially when discussing vacuum or pressure problems in aircraft with air-powered instruments.
Derivation
The word 'flag' has long been used in instrumentation to mean a small visible marker that swings or slides into view when something is wrong — borrowed from the everyday idea of a flag being raised to signal attention.
Why Pilots Care
They alert the pilot to disregard unreliable attitude or heading information and switch to alternate references to avoid disorientation.
Intuition Check
Do not read “failed flags” as flags that have physically broken. Here, “flags” means warning indications, and “failed” means the instrument information is not reliable.
Example Sentence 1
Shortly after the vacuum pump failed, failed flags appeared on the attitude and heading indicators, so the pilot transitioned to partial panel flying.
Example Sentence 2
The pilot immediately cross-checked the turn coordinator after noticing the failed flags on the primary gyro instruments.