To me it's a mystery how electronics of that era ever worked at all, considering the pathetic tolerances of the individual components
If you look at the KN72 pic you will count
8 trimmers. That is completely ludicrous. With close tolerance caps, or with a digital processed solution, you would not need any.
On top of that are the 3 installer-accessible ones. They are probably needed to take out the tolerance on the avionics being driven - another piece of stupidity. A huge amount of avionics adjustments are needed only because the inter-equipment voltages are sloppily specified. For example I have a KI-256 horizon whose pitch and roll outputs have something like a 10-20% tolerance (I have the maint. manual somewhere) and consequently when you change this instrument, the autopilot needs a re-cal which is a load of hassle (unless the 256 is a Mod 11 version
).
If you look at a 1970s or 1980s state of the art piece of Tektronix or HP test equipment (I have a few next to me here) the thing is packed with trimmers. And it came with a massive maintenance manual full of instructions on how to set each one
In the age of 10% resistors, that's how it was done. All resistors are now 1% (0.1p) and I buy 0.1% ones for about 20p which is amazing; they were ~ £20 when I started.
and the appalling reliability of same. If a modern PC were built with the same underlying material it would execute about 50 instructions before crashing. Indeed, in the 80's, Hawker Siddley believed exactly that and arranged for the CPU in a certain missile to be hard reset on every 500 rpm rotation of the vehicle!
That's really clever actually. Just need to make sure the whole algorithm executes in less than 8.3ms
I was amazed to discover the KFC225 autopilot does not have a watchdog. If/when it crashes, it just crashes. No indication. Probably same for a Garmin x30 GPS.
Problem is, today we have unveriafiable software to fill the same evolutionary niche that once belonged to ghastly analogue and RTL electronics. Who can say that a GPS might not go off on a 'valid flag' adventure of it's own under the right circumstances?
Yes, see above. My KFC225 would sometimes load +2000fpm and then execute it. Of course the aircraft stalls soon enough, but -2000fpm would be different...
There's been no shortage of 'Innacurate GPS' anecdotes on here over the years and I bet none of those have been followed up with the agressive technical persistence that you've shown in this case. We'd need a technically literate CAA or AAIB for that.
I think a lack of accuracy is less of an issue than straight crashes, though these seem rare with the "old gen" IFR units. The later Avidyne stuff has had rather more freezing and self-reset issues though. A friend with an Aspen EFD1000 has just lost the whole instrument, in IMC, AHRS and all - after a history of duff sensors, which is a widespread issue but the mfg is denying everything.
Having seen what trouble some people
I know personally, face to face have had with "state of the art" avionics, I reckon the latest stuff is actually no more reliable than the "good quality" old 1980s/1990s stuff. A mech horizon might grind to a halt after 1000hrs but if an EFD1000 does 1000hrs (10 years?) you are doing extremely well. The big difference is that the individual instrument can be swapped locally; no need to visit an avionics shop in most cases.
PS if you have a spare moment to have a look at some of the perplexing avionics issues in my aerial Cortina, I'd love the help. It's got my £40/Hr engineers beat and every time I thunp the table it's another £40 times xs hours to not fix the problems that were probably designed in by the original equipment manufacturers in the first place.
I am no avionics specialist, unfortunately. But send me an email and we can have a cut of tea and I can have a look. A lot of GA issues are just old/dodgy wiring. Most avionics people (not wigglyamp here; he's extremely clever, even if - like most - he eventually does stop answering emails if you stop spending money
) are just wiremen who follow wiring diagrams in the back of the installation manuals