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KI256 problems, replacement options, etc

Three similar threads merged.

It is probably a bearing problem. I have used various European repair shops for these but none were satisfactory. The last one, in the UK, took 6 months and the unit came back without any paperwork (they had a “deal” with another company to use their “part 145 paper printer” but by the time they finished the job the arrangement has ended) I prefer to send mine to Castleberry in Texas, US.

You can pick these up fairly cheaply on US Ebay now (because they are being replaced by various “glass” options) but that won’t be any good for an EASA-reg plane.

This thread may also be of interest.

Best thing is to buy a spare KI256 and keep it on the shelf. Then you can swap them and sort out the old one at your leisure.

Administrator
Shoreham EGKA, United Kingdom

The Garmin GI275 may become relevant as a KI256 replacement.

Administrator
Shoreham EGKA, United Kingdom

The AI can be the root cause for that “Bonanza” effect, i.e. the constant up and down when on Autopilot. There’s a very slow up and down, of +/- 50-200ft, which usually is a worn out trim servo. The shorter up and down, high frequency, can have many reasons.
In our case, it turns out it was the Attitude Indicator. The KFC150’s pitch is connect to it, and if it wears down, it apparently gives changing signals.

Just for info.

Safe landings !
EDLN, Germany

It was the AI allright. Worn bearings.

EBST, Belgium

The pitch oscillation issue is complex. Fundamentally the cause is either a dead band (hysteresis) in the control loop, or excessive proportional gain.

The causes can be

  • an under-tensioned servo capstan cable (this is nearly always given as the reason but actually rarely is)
  • a broken / worn out tachometer – see my KFC225 failures article
  • some damage or excessive wear in the KI256
  • oscillation of the KI256 instrument in the panel; the cause is complex and related to the crap way the KI256 signals are demodulated in the KC225 (Socata & Av. Straubing spent some time on this and produced a largely bogus fix comprising of rubber grommets for mounting the LH panel). This effect also helps to wear out the servo motors faster
  • encoding altimeter gray code generating some crap codes (the King autopilots reject obvious huge errors only)
  • static system leaks (cracked pipes, water in them, etc)
  • shagged servo motor (worn out commutator)

Some of the above happen only in certain altitude bands, and only in certain w&b configurations relative to the envelope.

Administrator
Shoreham EGKA, United Kingdom

Having been watching some old Sandel videos I think the issue with building an electronic KI256 replacement – as mentioned e.g. above – is that while the FAA has certified a purely mechanical solution to a primary attitude instrument (KI256, or any other vacuum or electric AI) and no doubt this is because they have been in widespread use for perhaps 100 years they are unwilling to accept any AHRS solution unless it has a background erection mechanism (as discussed e.g. around here) and that requires either airdata or GPS to be introduced.

The KI256 can be confused it by flying round and round for a long time but once certified nobody will un-certify it

So you would be looking at a fairly complex device. Aspen (EFDxxx+EA100) or Garmin (GAD43) solved this but their boxes already contain all the required extras.

That would give you certified primary attitude. Then you have the next challenge: certified primary attitude reference for an autopilot. Again, the KI256 gets in “under the wire” because of ancient history. I suspect it is harder because some flight testing is required, with each autopilot which is to be on the STC, and around the loading and speed envelope (because the dynamic response of the AI affects the pitch/roll control loops in the autopilot).

Administrator
Shoreham EGKA, United Kingdom

The Garmin GI275 is a fully certified replacement for the KI256 and STC’d on multiple aircraft so it can be done successfully without lots of external interface boxes. But as suggested, it does require a GPS input and has internal air data computation.

Avionics geek.
Fairoaks. EGTF

I suggested to Sandel almost 10 years ago that they develop exactly such a product. They had the technology in-house, more or less. They could have sold many thousands, at say 5k-10k $ each (a new KI256 lists/listed at some 10k). The then owner of the company – a really nice guy btw – said it is almost impossible to certify.

How much would a GI275 cost, for a TB20 with a KFC225? Does the GPS have to be a WAAS one and what’s the cheapest way to get that? Could the GPS in a GTX335 be used?

Administrator
Shoreham EGKA, United Kingdom

Do it right with 2x GI275 and a proper navigator and you will have significantly enhanced functionality, clean up most legacy round gauges and lose the vacuum system.

Alternatively, a G3X + G5, which also opens the upgrade path to the G500.

Last Edited by T28 at 26 Oct 08:49
T28
Switzerland

which needs a GPS to fly an ILS!

No thanks – even if you paid for it

It is also nonexistent. So now I have to spend my time linking to other threads explaining this, in case somebody reads this and thinks they can put a GFC500 into a TB20.

A number of people have already installed a G5 with a King autopilot and discovered only afterwards… one-liners are cheap and easy.

The Garmin GI275 is a fully certified replacement for the KI256 and STC’d on multiple aircraft so it can be done successfully without lots of external interface boxes. But as suggested, it does require a GPS input and has internal air data computation.

How much would a GI275 cost, for a TB20 with a KFC225? Does the GPS have to be a WAAS one and what’s the cheapest way to get that? Could the GPS in a GTX335 be used?

Administrator
Shoreham EGKA, United Kingdom
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