Loss of nav system

The question isn't whether your navigation system will fail — over a flying career it will — but whether you'll notice in time to do something calm about it.

How to notice

The failures announce themselves if you're watching for the right signs:

• GPS — a RAIM or loss-of-integrity annunciation, a "DR" (dead-reckoning) mode flag, the moving map freezing or blanking, a position that suddenly disagrees with the world out the window, or a database-expired warning at startup.
• VOR — the OFF/NAV flag, a lost Morse ident, or a CDI that wanders and won't settle.

The common thread is an indication that has changed character: it froze, it flagged, or it stopped agreeing with reality. A pilot who only glances at the magenta line and never at the ground can miss all three — which is why the cross-check habit (VI.A.R1) is what makes the failure visible in the first place.

A realistic case

You planned a route that clips a GPS interference-testing NOTAM area — the military runs them regularly. Mid-flight the receiver flags integrity-lost and the map blanks. If you caught that NOTAM in planning and already have a VOR tuned, this is a non-event. If the GPS was your entire plan, you're now reorienting at the worst possible moment, low on options.

The reversion

In the moment, work it in order:

1. Aviate — hold heading and altitude; the airplane doesn't care that the screen went dark.
2. Confirm — is it a real failure or a momentary dropout? A flag that clears in seconds isn't an emergency.
3. Revert to the next-best source: GPS out → tune and identify a VOR; VOR out → pilotage and dead reckoning; everything out → fly a known heading toward a big, unmistakable landmark.
4. Communicate — if you're getting flight following, tell ATC; they can offer vectors or a position.

One more distinction worth holding: degradation is not the same as failure. A GPS that drops to lower accuracy, or a VOR fading at the ragged edge of its service volume, still gives you something — and a single degraded source is exactly the situation where over-trust does the most harm. Treat a questionable indication as a vote, not a verdict: weigh it against pilotage and the clock, and don't bet the flight on a source you've already caught lying once.

VI.A.R1 is the practice that keeps you ready; this element is the response when the screen actually goes dark. They're two halves of the same discipline — and if it deteriorates into truly not knowing where you are, that's the lost-procedures path in VI.D.