But fuel gauges and GPs systems can go wrong. It is then that you require a manual back up.
Sure you want a backup but with any backup system (not just in aviation) you have to ask yourself what the value of it is, relative to the primary system.
To take a silly example but one which illustrates the point: would you drive a Vauxhall Viva, and tow a Toyota behind it in case the Viva breaks down?
In flying, lots of things are inter-related and end up being only as good as the thing they are depending on.
Take the whole concept of MSA for example. This is utterly dependent upon accurate navigation. If you are unsure about position, MSA is out of the window. As many people know; most are dead, but some were very lucky.
Fuel reserves are dependent upon various things, all of which
can be accurately known if you have the right equipment, but in the typical GA context they are not:
The fuel reserves firstly depend on how much you started off with. You had a chance to physically check that, so that's
(well not always but there is one born every minute and some of them are bound to get a PPL
).
Once you get airborne, the fuel reserves depend on various things. Ground speed (not airspeed, which you should know well) is the key. Lateral nav is of course important if you want to get to the destination
But along the route it is not too vital so long as you are going roughly in the right direction.
If you cannot find the destination then you have a potentially major problem. It could be anywhere.
Most cases of a pilot doing dead reckoning getting lost are ones where he "positively" identified a ground feature but got the wrong one, so he then flew quite a bit further before realising the c0ckup. Or he forgot to start (or stop) the stopwatch. The outcome of such an error is usually highly embarrassing...
This is why a cunning choice of visually unambiguous waypoints is vital.
Once airborne, most GA pilots have no idea of the
actual fuel state. The fuel gauges are invariably useless. The fuel burn is only taken from the POH, which was prepared about 30 years previously. The pilot has never been taught what the red lever does, and that its correct use can yield an extra 20-30% more MPG.
In GA, the way this is properly addressed is with a GPS, which gives you precise navigation both laterally and along the track (ground speed, time and distance to run to target) and an accurate fuel flowmeter which is initially loaded with the FOB (fuel on board) and which gives both the instantaneous flow rate and the fuel remaining. This data is fed to the GPS (this is panel mounted kit, not handheld) which is programmed with the route and thus knows the distance to run, the ground speed, and it will constantly recalculate the projected fuel at destination based on the current GS (which assumes that the wind etc doesn't change, but that is a reasonable assumption, and pilots doing long trips should be aware of the overall weather pattern anyway).
Here is an article which describes the typical system. It really is far removed from the stuff taught in the PPL, but is not expensive. It is probably also far removed from 1960s-1980s RAF practices, if a
I have just read is anything to go by. I know this will draw ridicule from RAF people here, and I don't suppose the Russians had anything better at any given time, but I think it is a huge piece of good luck that the RAF rarely had to do this stuff for real. For example, IIRC, after the Falklands bombing it was realised that the fuel burn of the Vulcan, under the particular conditions of that flight, was several tens of percent worse than had been assumed for many years beforehand.