An aircraft has three orthogonal axes: roll, pitch and yaw….a basic single axis controls roll only…..this can mean basic wing level, or heading or tracking a VOR….then there is two axis which adds pitch control…this can mean just holding a pitch attitude or holding an altitude…..a three axis would add a servo for the rudder to damp yaw oscillations….
EDIT: I just saw MooneyDriver’s post which says basically says all the same things…sorry
But to answer the original question, I would say a full digital engine monitor is one of the best add-ons if you care about your engine…..and I am still to add one!….looking at the CGR30-P….
There are no pitch-only APs
S-TEC 60 PSS (Pitch Stabilization System) is one I know off.
Altough I have always seen it combined with older roll A/P’s, such as Century II
A good old thread.
If I was to rank them from the cheapest first, it would be the ADL150 first. It does a great job, a very desirable function for IFR flying, and is not expensive.
Next is a good autopilot. A dramatic drop in cockpit workload.
The best add-on however is the full TKS system.
Very happy with the ADL150, but also with the GTS800 for traffic!
If I was to rank them in order of utility for IFR enroute:
The best value (less than 10 AMU) must be 31” Bushwheels – carry a runway with you so you can land wherever you want to be, instead of wherever someone built an airport.
After that I’ll cheat a little and say GPSS, because that encompasses an autopilot and GTNx50 etc. with “Telligence” and “Flightstream”, of course.
Edited to add, for a bushplane, PRACTICE.
@Jacko , the discussion seems to drift about IFR and avionics, you are re-centring it on VFR and raw materials 
On VFR: two folding bikes 8kg each fits easily inside a C172 boot, does not make my en-route easier but on arrival it is always nice
JasonC wrote:
traffic and weather
Isn’t the whole point of IFR that somebody else worries about separation? Or are class G ops really that much of a concern?
Weather is obviously quite useful.
Yes; TAS/TCAS is not generally useful under IFR in CAS. It is very useful OCAS though, and in Europe, light GA flies OCAS (in “bandit country” ) a lot.
It is quite an expensive add-on though.
In fact most things mentioned are pretty expensive. The ADL satellite wx receiver is the only relatively cheap item.
