Hello everyone, with some latency a question to which I couldn’t find an answer about in the older threads:
I’m in the process of buying a used KFC 225 AP, but I finally had concerns because most of the replacement devices on offer are equipped with the Yaw Damper additional option. @Peter once mentioned in an older thread that a yaw damper in a TB probably has little to no effect, since the aircraft design has little yaw effects noticeable anyway. I can confirm this myself, although I rarely fly with MTOW, where this might be more likely to occur.
Currently, in my opinion, the only option would be a KFC 225 without a Yaw Damper – the problem I suspect is that with a KFC 225 from a Beech or Cessna with a different servo configuration, it would be necessary to re-introduce the certification files issued by HBK into the KFC 225 in the case of previous operation with other servos. I was offered a certification file only for $100 via Dropbox, but I know that on the original 3.5" floppy disk there are several files for different “gain” settings. Can someone tell me something specific about this? Or is the required certification file read back from the KCM 100 config module during installation – similar to KLN 94?
I just removed a KFC-225 (factory) from my TB20 GT if your looking for a used unit. It did not have the yaw damper and the system worked well when removed.
I will say the GFC-500 that replaced my KFC-225 required the yaw damper to be installed via STC (or so I was told by the shop selling it to me ;-). So, while it may not do much, I don’t think you hurting yourself if the only option you can find is to have the yaw damper.
@ MG: Thank you for your reply. I am interested in your unit without yaw damper and it would be a nice exchange solution but the only problem coming up in case of exchange the KFC 225 would be to know which s/n your roll servo has. If it ends on -500 I would need the certification files for the KFC 225 which I don‘t have yet. From my POV I would have to der re run this slight certification software reinstall when changing the KFC (at least if not having the same servo – or I would need to exchange the servo, too.)
From my writeup I have
-0500 roll servo – a marginal fix?
It has been claimed that an alternative roll servo that can be used on the TB20, the 065-00179-0500 – KS271C, burns out less often, but has been known to suffer from occassional roll axis oscillation. This is not a straight replacement for the -0200 however; the KC225 autopilot computer requires a different configuration uploaded into it. This process uses what Honeywell call a “certification file”; these files are uploaded into the KC225 during a config mode which is done using a laptop acting as a dumb terminal. I do not have these config files and have not been able to find them. In the UK, only about 1 avionics shop has these and can do significant KFC225 work. I think any difference in the -0500 burn-out rate is illusory and just an artefact of the smaller numbers installed.
At the time, about 2010, only Lees Avionics in the UK, later GAMA, were able to do this. @wigglyamp will know.
Not being able to get the certification files is a real PITA. The dealers all have them but then will not give them to anybody even if they cannot any longer do the work themselves. This has prevented a lot of autopilot fixing and caused huge costs to owners, some of whom have had to go to the GFC500, only to report problems with that. I would happily pay $100 for the KFC225 certification file, to add it to my collection! AFAIK there is only one, but you may be offered a 3.5" floppy with multiple files, which is even better.
The config is stored in a separate little module, yes. It contains a 93C46 EEPROM which is about $0.10 
I would not fit a YD to a TB20 KFC225. Not because of the extra complexity in operation but because it is another servo which can fail. And I would think the YD servo gets a lot of exercise, unlike the pitch trim servo which hardly ever moves (and hardly ever fails).
Yes, my servos have the -500 so you would probably want all 3 of my servos so they are matching set.
I have an EEPROM reader, it should be possible to extract the config files from these units with a little trial and error. Though its helpful if you ave a couple of them to compare where the differences are.
Reading a 93c46 is easy but we don’t know what the data represents. I doubt the entire content comes from some data on the diskette. Quite possibly it contains the KFC225 S/N and if there is a check… but probably it doesn’t because the whole point of the config module is that you can slide out one KC225 and slide in another, which I know works. So if you had a config module for say -500 servos and copied the 93c46 from that, that data could be used for another config module with -500 servos – for the same aircraft type of course.
Exactly. And I have been surprised the number of times I dumbed an eeprom and could easily determine they were using simple list of strings that I could reverse engineer. This is why I think if I had two, I could quickly identify what’s the same and what’s different, and where the -500 config is getting stored.
MG wrote:
And I have been surprised the number of times I dumbed an eeprom and could easily determine they were using simple list of strings that I could reverse engineer.
Those were the days when hardly anybody was too much concerned on data tampering with and main controls were checksums in order to prevent data transfer failures.
There is likely to be a checksum but it tends to be the last byte, so fairly obvious.
However I would not want to try to read out my own config module in case I buggered it up. I’d rather find one on Ebay…
I don’t think anybody in avionics is worried about data tampering (other than to stop hacks to enable charged-for extra features, or to protect Jeppesen data). Stuff seems to be done mainly to prevent competitors making plug-compatible add-ons. In modern kit I would expect Garmin to use crypto to secure their CAN data. The great thing about the old ARINC429 is that almost nobody was playing those tricks.
Also I would expect some of the config module content to come from the diskette and some from the RS232-laptop-accessible KC225 config. It is easy enough to reverse engineer e.g. how some of the RS232-access config changes some bytes in the config module, but very time-consuming, and if you make a mistake you end up with a grounded aircraft because almost nobody in Europe will help you.
@wigglyamp probably knows about this from his “previous life”.
@MG: First of all, thank you very much for the offer. I’ll come back to it when I actually get (at the moment it doesn´t look like that) the original floppy disk. Well, when I replied to the potential US seller to pay 100 US$ for one file that I can´t verify in a Dropbox is a bit much I just got the stroppy answer “I´m sorry then I´m not your guy” The big suppliers charge US$575 for the floppy – that’s outrageous and as @Peter rightly said, the relevant avionics workshops turn a deaf ear, among other things with excuses like “the floppy disks are no longer readable” or similar. It is clear here that anyone who cannot get their avionics problem under control with spare parts will sooner or later have to deal with new avionics that are more or less well implemented, whether they like it or not, since this path is always a one-way street in our time. In the end; the question is who will have the longest breath or go bankrupt first
