Steven P. McNicoll wrote:

Primary navigation (the VFR GPS) is no longer reliable, the aircraft is
IMC, and the pilot is unaware that the unit is no longer reliable. Those
are not properties of a problem?

I don't think so. If the aircraft drifts off course the controller will
nudge it back and the pilot will then be aware that the unit is no longer
reliable. No problem.

The controller's attention might be elsewhere (have you never been sent
right through a localizer?). That said, my Garmin 196 does warn me
when it loses reliable reception, though it's not proper RAIM.


All the best,


David