One can easily get overly religious with backups 
The only backup you need is a phone. Worst case scenario is Russian jamming of GPS (which is a real thing here, mostly problematic for airliners). Android devices (I have no clue about iPhone), still work without GPS. Not pinpoint, but more like thumb point, more than good enough to navigate after (I don’t think Skydemon will work with no GPS). You need a connection though, but can’t imagine that being a problem in densely populated Europe. It’s not a problem here in Norway with huge deserted areas. Even without no connection of any kind, there is still a map or two in there that can be used.
A good magnetic compass is a good thing though. I’m just sick and tired of non TSO’d compasses that swirl around and do no good. It seems like everyone can make a compass, and everyone does. 90% of those really shouldn’t A TSO’d compass is the next thing that get installed in the Savannah.
Another thing is that dead reckoning in unknown terrain requires practice. I learned that this summer with the competitions we had. You need a map of some kind, and you need a way to figure out your heading. With those two, the precision you can achieve is staggering. No need for rulers or watches when the exact arrival time is not of the essence. Even if your phone has lots of maps, do you remember where they are? are they downloaded? will they work with no GPS signal, will they show your position with no GPS signal and so on.
I also learned to fly before GPS. A simple VOR will solve most problems in theory at least. But when you haven’t used it for more than 20 years… I’m not sure how much it would help today Today these surveillance radars are a thing also. You know exactly where you are, if you only ask.
The biggest issue I see is this sort of thing i.e. consumer portable gear failing; in the case of tablets, especially Apple ones, shutting down in sunshine. But you can’t have a backup for that, because that will shut down also.
I have a yoke-mounted Aera 660 as the nav backup. It also backs up airport diagrams (Garmin safetaxi) when the Ipad has shut down
Then I have a Yaesu 750 in a bag on the back seat, but this would have only a short range.
The approach plates for a planned trip are printed out, so no battery to go flat. Then I have a backup via Foreflight and AIP plates but that assumes the Ipad has not failed.
There is now a backup alternator too which does 15A – enough to run more or less the whole plane.
Interesting thread!
As far as I am concerned, my backup is paper charts and printed out approach charts of the destination.
I usually navigate as much as I can using paper charts, navigation logs, etc. As a matter of fact, I like that old school method. However I have to admit that, when on shorter trips (30 mins or so), laziness gets the upper hand and I just follow the GPS (skydemon on ipad)…
When I started flying in very flat central Texas in the 70s, the backup was to get low enough to read water towers. The only real risk those days was radio towers – there were a lot and they were tall, hard to see, and had guy-wires that were impossible to see.
Today, I’ve got Skydemon on the ipad and iphone, along with FF and Air Nav Pro. Also, google maps works pretty well in the air too. And a compass. And an EFIS with Jeppesen nav data. Not sure which is primary, and which is backup, but flying around Switzerland, you only need your eyes to navigate – it’s impossible to get lost here!
Backup requirements depends a lot on mission. The main difference being VFR vs IFR. VFR is mostly about knowing where you are related to control zones, terminal airspace (so you don’t enter without clearance ) and visual reporting points. All this is given in a basic moving map GPS. In my RV8 (VFR only) I have a KMD150, it’s old but it does all you ever need VFR really. An iPad with a good app can do the same even better, although I would consider it more unreliable. Consumer tablets works poor in direct sunlight and they shut down in high temps. I use both for redundancy. Cloud base and visibility info for the route is critical VFR but that’s mostly a preflight thing.
IFR however is a different and much more critical game should something fail and one is unable to stick to procedures, control or situational awareness is lost, therefor redundancy is also more critical. While not knowing the correct approach procedure might get one in trouble VFR, it can turn fatal IFR in IMC.
In todays world backup has never been easier. Without it even a vacuum pump failure can be deadly.
