Definition
A visual indicator on a cockpit instrument that appears when the instrument is unreliable, has lost its signal, or is not receiving valid input. When the flag is in view, the information shown on that instrument should not be trusted for navigation.
Plain English
A small marker that pops into view on an instrument to tell you the instrument isn't working properly and you shouldn't rely on what it's showing.
Context Anchor
Seen on navigation instruments, including automatic direction finder (ADF) displays, when the instrument or its received signal may be unreliable.
Derivation
“Warning” means a notice of possible danger or error. “Flag” comes from the idea of a visible signal. In aircraft instruments, the “flag” is not usually a cloth flag; it is a small visual marker that signals the pilot not to trust the displayed information.
Why Pilots Care
Prevents use of inaccurate bearings that could cause navigation errors or deviations from intended course.
Intuition Check
Do not read “warning flag” as a general alert about the whole aircraft. Here it means a specific visual sign on an instrument that its information may not be usable.
Example Sentence 1
When the warning flag appeared on the ADF, the pilot stopped using its bearing information and switched to the VOR for navigation.
Example Sentence 2
With no warning flag visible the pilot accepted the ADF bearing as reliable for the approach.