My problem with a twin is the sheer scale of the thing – there’s a beautiful short nose, swept tail 310 for sale near me for peanuts and it’d be great to play with. But too much hardware and too big a hangar needed… even if in partnership with the A&P IA with whom I discussed it (he needs something to make his rounds).
Small airplane AP’s I have only worked with the KFC150/200 as well as for a while the Altimatic III in a Seneca I. They did what they were advertized to do, the KFC’s more than the Altimatic thing. Never have used a Cessna FD/AP but one of my friends flies a C310 with one installed and likes it.
But I will get to know the S-TEC 55x quite well in the near future, as well as the Aspen.
I agree that in an update today, this would be the course of action (and that is why I do it) but had my plane had a working Autopilot which does what I want it to do, I’d not touch it. AP installations are by far the most expensive avionic upgrades. Yet, an Aspen for instance can do quite a lot for a legacy AP as it provides GPSS via the Heading Channel which even the old Brittain AP’s can work with. Reckon that is one reason why they are so popular.
The autopilot of that Cessna 421 is quite good actually, an ARC/Navomatic/Cessna 400B. That is an attitude based autopilot and as good as an analog autopilot can get. It can still be serviced without problems. It is far better than anything from S-TEC available today. Digital autopilots are very rare, only the GFC700 which is not retrofittable and the DFC90 for which you can count the STC on one hand (with some fingers missing).
Just put in one GNS430W and you’ll be legal again (8.33kHz + BRNAV) and equipped OK for a lot of flying. Glass cockpit etc is all nice but does not add any major capabilities.
The only way to burn more AVGAS faster than in a Cessna 421 is to set an AVGAS delivery truck on fire…
@Mooney Driver, sorry to hear you are getting to know the STEC55X. It is sure to disappoint.
Haha! 
I am glad I don’t have the S-TEC55X, and all pilots who switched from it to the DFC90 will agree for sure. It was the first item i chenaged, even before I flew the new for the first time. If I had the choice (and if there was an STC, i’d much rather have the DFC90 than a glass cockpit). While the digital A/P adds a lot of safety the glass cockpit is more “nice to have”
Worth a mention that the other way to interface an old analog autopilot is via a Sandel SN3500 EHSI, which can convert ARINC429 roll steering data into analog deviations. It does both the lateral (which comes via ARINC429 – RS232 is not supported for this purpose) and vertical (ILS GS and LPV GS are both passed through transparently to the autopilot).
When I install a modern GPS that is what I will do, and then the AP will be doing exactly what the EHSI is indicating, rather than having a separate roll steering converter which fakes a heading bug.
Let’s not be unfair, very often S-TEC is the only option on the market. In the GDR, people were looking forward to receiving their Trabant many years after placing the order
When S-TEC got all those STCs, regulations were much less strict. Only that way they were able to assemble that long list. What would need to happen is to develop an all new digital autopilot which on paper is just a new variant of the S-TEC 55X and can use the existing approvals. JPI used that trick for the EDM830/930 which is a variant of the EDM700 although it has absolutely nothing in common with it and about 10x the features. I was able to install the 830 under an STC granted by LBA before EASA times. Shows how nonsensical our regulations are.
the other way to interface an old analog autopilot is via a Sandel SN3500 EHSI, which can convert ARINC429 roll steering data into analog deviations.
That’s what the Aspen EFD does as well and I think Sandel is pretty dead product nowadays with its limited feature set and exquisite price although at its time it surely was a very good product.
Yes, you are right. I just got spoiled so quickly by the DFC90 …
I do have an S-TEC30+ in the Warrior, with GPSS run by a GNS430, and in a VFR airplane like that the S-TEC is a great A/P.
I think the MTBF of a Sandel is about 100x longer than the Aspen 
They sell a lot into the military and especially the military helicopter market. The SN3500 is about $9k, discounted.
