Definition
A warning indicator on a navigation or flight instrument that appears when the instrument is not receiving a usable signal or is otherwise unreliable. When the OFF flag is in view, the indications on that instrument should not be trusted for navigation or flight guidance.
Plain English
A small flag inside the instrument that pops into view to tell you the instrument is not working properly or is not receiving a good signal, so you should not rely on what it is showing.
Context Anchor
Seen on cockpit navigation displays during instrument flying, especially when using radio-based navigation guidance.
Derivation
Called an OFF flag because it signals that the instrument is effectively 'off' duty -- not providing valid information -- even though the needles or pointers may still be moving.
Why Pilots Care
It alerts the pilot to disregard the instrument immediately, preventing reliance on erroneous attitude, heading, or course information that could lead to spatial disorientation or navigation error.
Intuition Check
Do not assume an OFF flag only means the equipment is turned off. In this context, it means the information being displayed is not valid enough to use.
Example Sentence 1
During the ILS approach, the pilot saw an OFF flag on the glideslope indicator and immediately initiated a missed approach.
Example Sentence 2
During the approach, the ADF needle froze and the OFF flag popped into view, confirming loss of signal.