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KI256 problems, replacement options, etc

I have seen this twice so far.

One was years ago.

Another was today, FL100, in cruise i.e. a ~ 2.5 deg UP pitch. I suddenly saw the autopilot take a dive, losing a few hundred feet in maybe 10-20 secs. So I did the usual debugging, flying by hand and engaging the FD-only mode and see if the FD is asking for the dive i.e. could it be the pitch servo. It turned out to be quite simply the KI256 indicating about 10 degrees UP when it should be indicating a “few” degrees UP (the KI256 is not good at showing these small angles but my Sandel system is very much more precise).

The KFC225 will compensate for small deviations e.g. 1-2 degrees but it can’t cope with the pitch returned by the KI256 going up by best part of 10 degrees in such a short time. That causes a de facto altitude hold disconnect.

So I hand flew for a bit, and watched the KI256 pitch indication settle back down to the more normal 4-5 degrees (the little plane indicator on it may not be in the right place ). Then re-engaging the AP normally was OK.

The whole episode lasted about 20 mins.

How is this possible? There are only the bearings in there…

The vacuum was always perfect.

I have a spare KI256 on the shelf, overhauled by a now-defunct UK firm but should be working fine. But the old one is working fine too…

Administrator
Shoreham EGKA, United Kingdom

Looks like a dirty gyro. I had the same kind of problem in the roll channel of my AI several months ago (on my IR check ride, no less). An avionics repair company here in Prague cleaned it for some 125 euros.

LKBU (near Prague), Czech Republic

Peter ………I am going to wind you up on purpose here!……perhaps a bit of crumbly rubber from your rubber vacuum hose jammed in your gyro……………remember my post a few weeks ago?

Perhaps you need one of those cheap non TSO’s Dynon systems driving your Auto Pilot – I hear they don’t skip a beat!

Thanks for starting a great forum BTW – I have learnt a lot!

Best regards – Archer 181 (with the 30 year old…crumbly hose….Vacuum system!)

United Kingdom

Dammit – just spotted a typo in the thread title… fixed it.

The hoses should be spotless. They were renewed about 3 years ago with really high grade aerospace ones and proper screw fittings. The vac filters are changed every Annual.

The bearings in the KI256 cannot be lubed legally. They have to be replaced…

But yet it can only be dirt, I think, or maybe debris, but had to be something that dislodges?

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

I have seen this twice so far.

I’ve experienced the same phenomenon with a KG258 in roll.

At some point during a flight, the KG258 indicates a 5° degrees bank (left or right, I don’t remember) while the wings are leveled.
Then, the AP (KAP 100) keeps the wing level but begin to fly off route (left or right side). About 1/4 of CDI deviation : it flies a parallel track.

After 20 mins or so, operations resume normally. This failure may have happened three times to me.

Last Edited by Guillaume at 27 Nov 22:51

I’m not trying to be funny here, is this a MIF? A bit like the Mike Busch examples of people who clean out their injectors and then dislodge debris and cause issues.

When was your Annual Peter and ditto with the previous failure & that Annual – just an idea?

United Kingdom

Just “the other day” we got a thread about abbreviations and you bring out this gem. So, for people scratching their heads, MIF = maintenance induced failure.

Martin wrote:

for people scratching their heads, MIF = maintenance induced failure.

Thanks Martin !

ELLX (Luxembourg), Luxembourg

Martin wrote:

So, for people scratching their heads, MIF = maintenance induced failure.

As long as we’re not discussing Maintenance Induced Localizer Failures here, it would be safe to have that abbreviation-explaining popup someone asked for on the other thread.

I got this from the top US instrument shop which used to make the KI256 originally:

Administrator
Shoreham EGKA, United Kingdom
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