The autopilot (if engaged) should drop out of NAV mode and into ROL mode, so the plane will fly wings-level but not holding any particular heading.
It should hold altitude (if it was in ALT mode) because that’s nothing to do with GPS.
Are there any systems where the autopilot will revert to HDG mode, instead of ROL mode? That would be pretty weird because the heading bug could be set way off, while you were flying in NAV mode, so a loss of GPS signal would send you in some totally wrong direction.
Also I heard there is a “dead reckoning mode” in the Garmin boxes. What exactly does this do? They don’t contain any accelerometers, and without an airdata computer they don’t know the wind, so flying a heading (heading probably is available to a modern GPS, though I am not sure what it would be used for) won’t work either.
Depends what the setup is.
If you are on GPSS steering and that one is routed via the HDG mode, it is up to the GPSS computer to determine if there is a valid steering signal or not. What it will do if not is up to the computers specification. The reaction I would expect from the pilot is to hit the GPSS/HDG selector and continue on HDG for the time being.
If the GPS is traditionally linked to the NAV mode which follows CDI deviation, I would think that it it would stop correcting as the CDI will get a nav flag and revert to zero deviation. Again, the pilot action should be to synchronize the heading bug (which is good practica anyway) and then sort out with which mode to continue.
Also I heard there is a “dead reckoning mode” in the Garmin boxes. What exactly does this do?
It simply computes new positions from the last known one using the last ground speed and track. Unless you change anything that should be fairly accurate for a couple of minutes, until either the signal is regained or you had time to look for a different way to navigate. The autopilot will stay in (L)NAV mode all the time. This is why scanning the instrument panel is so important, even during the most boring parts of the cruise flight, otherwise you won’t notice the little amber “MSG” annunciator. In our FMS installation it is similar, only that it will fall back to DME/DME first and to DR next, again only telling you by flashing a tiny little yellow LED. But the DR mode is better, because it gets air data values and compass heading, so it only has to make assumptions about the wind.
Around 2011 Garmin shipped a large batch of defective WAAS antennae. I had one installed with my GNS430W. I have a lot of experience flying IFR in “dead reckoning” mode. As so often in this business, Garmin first refused to acknowledge that there is more than “customer specific issue, never heard of before” but eventually they paid for a new antenna and the complete labor cost to replace it. I know quite a few who also have this “customer specific issue” and got the same treatment.
So yes, this dead reckoning mode works well and at the time, the AP would continue flying GPSS (S-TEC fed by Aspen GPSS). Back in the days I got a lot of respect for German ATC’s service attitude. I had to do several IFR flights with IMC and the GPS issue and they always gave me permission. At one point even the transponder would not work reliably (ADS-B GPS feed from GNS430 would make it lock up). ATC were OK with that, too. They only asked “what is your current heading” about every 5 minutes.
