Ah, sorry. Thought it was a 252.
Has anyone installed a GI275 to replace a KI256, with any King autopilot, or specifically with the KFC225?
Someone was asking me today about this.
I think there have been plenty of these installs. See e.g.
Can anyone shed any light on this? I am getting into IFR flying in my own aircraft and was flying an ILS last week with the KFC 150 to see how it performs. All good in heading mode but when it captured in APP mode it started behaving weird and the KI 256 started indicating a bank in straight and level flight (test was done in VMC). At that point I hand flew the remainder.
Today I went out to diagnose. In the video below I’m tuning in a VOR and picking a nearby radial to see if the KFC will pick it up and track it. Same behaviour again where when I hit NAV on the KFC (bottom right) it doesn’t really do it and then the KI 256 is indicating a false left bank! I turned off the A/P and within a couple of mins the 256 is behaving correctly, continues to work fine in HDG mode back to base and perfect during the landing circuit. I certainly won’t be engaging anything other than HDG mode until I can figure this one out. The only other relevant information is that I have a new vac pump following overhaul and the 256 takes a couple of mins to erect on startup. Not sure if it always did this as I’m only really concentrating on it now.
This is really weird.
The KI256 is a gyro, with pitch and roll pickoff coils (basically an LVDT bent into a circle; one for pitch and one for roll) which get an excitation signal, nominally 400Hz, from the autopilot.
There should be no way for the autopilot to alter the pitch or roll which the KI256 is indicating. So I think it is a dud KI256.
OTOH, it may be possible for a really defective autopilot to send a powerful AC voltage to the excitation coil(s) and force the KI256 to move. I very much doubt that though, and most likely the malfunction coinciding with NAV selection is a coincidence.
But maybe you are looking at the FD (flight director) bars? Those are driven from the autopilot computer.
Looking at the video it looks like the KI256 is just about to seize up.
I’m inclined to go with the 2nd option. The fact that a) it is reproducible and b) it slowly rights itself after disengaging NAV/APP suggests some signal is screwing with the KI 256 and maybe is some capacitive effect that decays away after removing the signal?
The other weird thing is why it just decides to return to level flight when this is happening. It seems like it’s telling the pilot one thing, but telling the KFC150 something else. It’s possible that it is indeed tracking the radial (although there seems to be about 1-2 degrees of deviation), and is obeying a KFC wings level command, but just telling me something different.
A couple of autopilot experts on Mooneyspace also suggest that the KI 256 gyro is the culprit. What are the repair options and prices for this in Europe?
I don’t see how a KI256 can show a false horizon (i.e. the gyro toppling) as a result of an autopilot computer failure.
It is like saying that an encoding altimeter can show the wrong altitude as a result of a transponder failure.
Is this really possible? I would really like to know the physics behind that!
I wonder if @wigglyamp has ever seen that.
As regards getting a KI256 repaired, your best bet is to buy a spare on US Ebay. They come up fairly cheaply now. And send it off to a decent shop to get it overhauled so you are starting with a known quantity. No decent shops in the UK AFAIK; the last one I used kept it for 6 months, “waiting for bearings”, and then told me that the 145 outfit they had an “arrangement” with for printing off the Form 1 no longer lets them borrow their laser printer (not quite how they put it) but I didn’t need a Form 1 anyway. I use Castleberry Instruments in Texas for gyro overhauls.
I had a look on that Mooney forum and the posters say “Looks like the rotor speed is slow in the KI256” which is probably exactly right 
It is nothing to do with the autopilot mode or the autopilot at all. It is just the gyro rotor about to seize up. The funny thing about these horizons is that they do that for a bit and then they run ok for a bit. My KI256 has done it numerous times – see e.g. here. I don’t know why; it could be a bit of dirt getting into the bearings, which later dislodges.
I have the KI256 MM. Too big to post here – a scanned copy and about 256MB
Thanks for that info. What does an overhaul cost? Are we at “well I might as well just get a GI-275 then” territory?
I suggest contacting Castleberry. I don’t remember how much I last paid; it was a few years ago. These things last a few years.
It will not be anywhere remotely near what a GI275 is going to cost you, in the real world, by the time it is installed and wired up to do something useful.
The by far lowest hanging fruit in this game is to buy a spare KI256. Then you get practically zero downtime. You can swap it yourself, off the books. I am sorry to be banging this drum so much but so many people suffer so much downtime in GA, and do little flying as a result, because they are depending on maintenance facilities which, in this part of the world, just don’t deliver due to deep structural issues in the industry.
And while you are at it, get some decent fittings (from e.g. LAS Aero) which will make the job much faster next time.
