Definition
An indicator on a VOR display that warns the pilot the received signal is too weak or otherwise unsuitable for navigation. When the flag is showing, the course and bearing information presented on the instrument cannot be trusted. Depending on the equipment, this warning may appear as a red OFF flag, a NAV flag, a barber-pole striped indicator, or the disappearance of a TO/FROM indication.
Plain English
A warning shown on the VOR instrument that tells the pilot the signal is not strong or clean enough to use. If the flag is up, ignore what the needle is showing.
Context Anchor
Seen on cockpit VOR indicators during instrument flying, especially when tuning, identifying, or using a ground-based navigation station.
Derivation
“Flag” here comes from the small warning marker used in older mechanical instruments. It is not a cloth flag; it is a visible indicator that pops into view to get the pilot’s attention.
Why Pilots Care
It alerts the pilot that the displayed course information may be inaccurate, preventing reliance on faulty navigation data.
Intuition Check
Do not assume the course needle is still mostly usable when the unreliable signal flag appears. In this context, the flag means the displayed navigation information should not be relied on.
Example Sentence 1
As they flew farther from the station, the unreliable signal flag appeared, so the pilot stopped using that VOR for course guidance.
Example Sentence 2
Before using the radial for course guidance, the pilot confirmed that the unreliable signal flag was not showing.