Definition
An indicator on a navigation display (such as an HSI or CDI) that appears when the navigation signal being received is unreliable or unusable. When visible, the flag tells the pilot that the course and deviation information shown on that instrument cannot be trusted for navigation.
Plain English
A small flag that pops into view on the navigation instrument to warn you that the signal is bad and the needle is not telling you the truth. If you see it, do not use that instrument for navigation until it goes away.
Context Anchor
Seen on an HSI when using radio or electronic navigation guidance, such as tracking a course or setting up for an instrument approach.
Derivation
NAV is a shortened form of “navigation,” meaning finding and following a route. “Flag” comes from the idea of a visible marker that draws attention. Together, “NAV warning flag” means a visible marker warning that the navigation display is not dependable.
Why Pilots Care
It tells the pilot to disregard the displayed course and deviation information until a valid signal returns.
Intuition Check
Do not read the NAV warning flag as “you are off course.” Read it as “the navigation information itself cannot be trusted right now.”
Example Sentence 1
As they flew further from the VOR, the NAV warning flag appeared on the HSI, so the crew switched to a closer station.
Example Sentence 2
The NAV warning flag disappeared once the localizer signal was acquired on the ILS approach.