The OP isn't talking about identing the VOR, they are talking about
testing it's serviceability by varying the course, and seeing the course bar behave appropriately.
The VOR works by running a phase comparison between the reference signal and a variable signal. The warning flag
usually appears when no signal is received (either reference or variable signal), or when the one received is too weak, and also monitors the receiver itself and will appear if the receiver or indicator is malfunctioning.
There are a couple of rare failure modes which can result in no flag, and an apparently valid signal, however:
http://www.pprune.org/private-flying...lure-mode.html
Originally Posted by IO540
The point is that the circuits driving the bar, the t/f flags, and the Invalid flags, are all substantially separate, and can thus fail separately. This is how it has been since day 1.
If this is a certification issue then VORs and NDBs and ILSs will be shut down too
I wrote it as a heads-up (as the Americans call it) because the flight training system maintains that if the Invalid flag is not showing, the instrument can be trusted, which is bollox.
http://www.pprune.org/private-flying...ml#post6006783