There have been experimental setups – DA42 example – and I think we already have legal systems on some turboprops but they are limited to emergency use.
It seems that you would need
3 things for CAT III use in the airline world:
- certified crew
- certified aircraft
- certified airport
Which means, out of the ones you cited, only the “certified crew” bit is to be resolved. Certified as in trained and current. Not sure the annual IR check as done for SEPs nowadays is able to cover items as done in a sim, such as multiple failures, map drift, etc…
GA ALAND will probably happen. Being carried by autonomous drones will probably happen. Just not sure of the benefits for human kind 
Peter wrote:
avionics which can do it – autopilot, a navigator, and suitable sensors, probably including a RADALT like CAT3 jets have
Garmin has done that already. I suppose it’s “safe return” system would be up to whatever you need for that technologically and has demonstrated it can land the airplane following a precision approach. It “only” is not certified for that use and the way it is set up, only works in emergency.
Technologically I guess it would not be very difficult to couple that capability with regular GPS/ILS precision approaches, where the autoland sequence takes over at about 200 ft.
So autoland per se had been done. In order for it to be used as a regular certified system, that kind of certification would need to get done to use it. And there you will probably hit a wall of regulators waiting to get their teeth into it.
Dan wrote:
3 things for CAT III use in the airline world:
- certified crew
- certified aircraft
- certified airport
I agree with @Dan that No1, certified crew, will be the one regulators will get the biggest kick out of. In fact, all that would be needed is training and doing it as part of the IFR syllabus. Under part NCO I can imagine that to be quite an easy kind of thing, the question however will be if they will look at it as desirable or not. If the regulators will go the way saying “we don’t want GA to stuff the CAT III runways in fog” it won’t happen.
Part 2 and 3 are easier and largely done, in the case of 3 completely.
A few GA aircraft, e.g. AN-2, have an “autoland” feature in the form of full-stall landing: you just pull the yoke all the way and it lands undamaged. IIRC, Socata Rallye, a.k.a. “tin parachute”, can also do it.
Mooney_Driver wrote:
Garmin has done that already.
The Garmin “Autoland” does not more and not less than bringing the airplane down to a suitable runway with an impact at a vertical speed that is survivable for the passengers in the plane. That is good enough for an emergency landing, but not what Peter seems to have in mind.
I would expect that a system that can deal with gusts on the final 20ft of the approach at the typical (low) masses of GA aircraft is quite hard to implement if you have the requirement that the plane is airworthy for the next takeoff immediately after the auto land. This is one of the biggest challenges for autonomous drones as well but with vertical lift that have better options to correct for gusts.
To answer the title question see here
AC120_28Dappdx_pdf
It will never happen for day to day ops. Not enough benefit vs. effort. How many GA flights to actual airports (in lieu of grass fields etc..) are there all over Europe on a given day? Now compare that to AOC flights. Probably more AOC flights happen at any single one of the larger airports.
Even most business jets (incl. the proper ones such as Gulfstreams and Globals) are CAT1 only (whether by choice or by cert. limitations).
The biggest obstacle is the certification for the equipment I suppose. Airliners now mostly have triple redundant autopilots (and e.g. elevator control power source is split between left/right for autolandings, it’s pretty cool albeit complex system architecture from my uninformed pov).
What will be more common is Garmin‘s emergency autoland (Piper Halo, TBM Homesafe etc..). The usage of these will be extremely rare + their benefit provides an argument for less certification restrictions.
2 Major obstacles at present:
Redundancy: All systems, aircraft, airport, and crew must be redundant… as in 2 man crew…
Icing: The vast majority of Lovis (CAT III) approaches are conducted in icing conditions. The vast majority of SEP hasn’t got, save for a heated pitot, any anti- or de-ice capability…
Yes, and even the advanced personal GA planes (M600, TBM, C510, C525, EA50, PH10/30 etc..), while FIKI and quite capable, are still primitive compared to current Airbus/Boeing designs (CS 23 vs 25).
A simpler way for CAT III is HGS (HUD) based on INS. I remember when it was retrofit in a Dash 8 fleet because it bumps the operator up in priority in low vis slots and eg an improvement for only 3% of the flights vs. delays/canx warrants the investment (at ~1500 flights a week) for a hub and spoke carrier flying to lots of foggy airports (basically all airports, haha). It was several hundred thousand Euro per airframe for the equipment, then simulator training (a few thousand workdays gone) for the crew and fuel/flight time intensive trials of approaches at all airports (loooong vectors, slow stabilized approaches instead of 160 to 4nm).
Here is something to try in DA40/DA42 with G1000W, what if you keep GFC700 hooked to LPV autopilot bellow 200ft down to 50ft in good weather? “I think” they would fly nicely over threshold, then middle of the runway all the way to the other end without any “discontinuities in GPS/AP signal” (I said “I think”, as Peter pointed it’s illegal to keep AP ON bellow 200ft agl in normal operation but in emergencies at least you would know what will happen when the pax is pressing the button 
), now try that with ILS Cat1 signal at 200ft DH just as you cross threshold
I fully agree with this suggested by Malibuflyer, this is the only elephant in the room !
I would expect that a system that can deal with gusts on the final 20ft of the approach at the typical (low) masses of GA aircraft is quite hard to implement if you have the requirement that the plane is airworthy for the next takeoff immediately after the auto land
The real question do you need a 100k option with one “single button”? or a checklist of steps that produce the exact same result with same hardware if you can “hand throttling” bellow 50ft?
Maybe I misunderstand but 100k gets you a couple of nice Garmin boxes + screens but that stuff is still toy category compared to the big boys.
Again I believe certification will continue to be favorable towards autoland in small planes, especially in fadec equipped ones, but it will not be for low visibility CATII or worse conditions.
Cirrus sells many planes cause they have a chute. The newest seller is emergency autoland. „It’s fine honey, if I die just push this button.“ Looking at the age curve in GA it probably makes sense, lots of GA geezers are approaching TBO.
