Need advise regarding KAP-140 malfunction on 172SP
The problem is that autopilot does not want to maintain heading and tends to turn left in ROL mode.
In heading and NAV(or GPS) mode it takes 15-20 degrees to the left of desired heading / track.
We have tryed to replace TC (took from another airplane). It gave no result. TC is okay.
I've directionally trimmed airplane. It also had no result.
I've tryed to level TC. Also nothing.
So it seems, that some offset apeared in KAP-140.
So, it would be great to understand, how to solve this issue and, probably, advise regarding some european company (close to Baltic states) who can repair it.
Best Regards, Sergey
So as it rolls to the left, I presume the heading bug wanders clockwise and gets further and further away from the lubber line? Or does it have a fixed 15-20 offset?
Does it also do that on the ground?
Have you tried swapping the DG/HSI? This is where it gets the offset between heading bug and present heading (lubber line).
Yes, it does the same on the ground. In roll mode it turns and turns and turn to the left untill control becomes fully inclined to the left. I can fly using AP but only in heading mode by setting heading bug approx 15 degrees to the right. And, ofcourse, due to gyro drift, I have to control the process continously.
In nav mode it's absolutely impossible to fly.
No, I did not try to swap DG yet. It looks like more complicated process. You thinks, it controls rolling offsets also?
The KAP140 Pilot's Guide and Installation Manual are not hard to find on the internet
I am not a specialist (I know more about the KFC225) but pages 6 and 7 of the Pilot's Guide show the main configurations.
The KAP140 gets roll data from the TC.
In ROL mode, it just keeps the wings level. The HDG bug (and AFAIK the whole of the DG/HSI) is irrelevant. I would therefore fix this first. However, is the roll angle error bad? If the autopilot flies in ROL mode with a 10-20 degree bank, that is seriously wrong. If it flies with a 1-2 degree bank, that should be trimmed out but won't be the cause of the large error in the HDG mode. And no autopilot will fly a constant heading in ROL mode (obviously).
Once the ROL mode is sorted (or if it isn't a problem) then the HDG error is the next thing. The HDG output from a DG/HSI is just a voltage which tells you how far away "straight ahead" is from the HDG bug setting. This is easy to debug even with just a voltmeter.
The Installation Manual will have all the details for sorting it out, but this is a job for an avionics shop with specific competence in Honeywell autopilots. Alternatively, a good electronics engineer should be able to sort it out. There are various nulls which can be adjusted in the unit, using an RS232-connected laptop running a VT100 terminal (Hyperterminal is usually used; Terraterm is a much better free terminal program).
Where in the Baltics are you? Honeywell have reps all over the world but unfortunately most of the "far out" ones are useless for anything specialised. I know of pilots in some of these places and they cannot get anything done. But there are good electronics engineers around... and this stuff is basically quite simple, for someone who can read the manuals and use an oscilloscope etc.
Peter, you are right, my location is the exact problem.
I'm from Latvia and, unfourtunetely, there's no avionics shop available here, and also in Estonia and Lithuania.
I've tried to contact Finland, but also no success. They have also recommended me to install new TC :)
In ROL mode the bank is no so big, just 2-3 degrees, not more. But before it was less.
I don't see any problem getting this fixed without leaving Latvia. All components involved (minus the servos but that's unlikely) are easy to remove and shippable via mail.
The better you can isolate the problem, the cheaper it will be to fix. If you can rule out the TC, that's one component less to be bench tested by a shop. Peter made some very good suggestions of what to test for and how to do it. It is actually quite simple with these analog systems, all you need is someone with basic knowledge in electronics and a voltmeter or scope. As with most these things, finding the problem costs 80% and fixing it 20%.
Regarding getting this fixed, I would send the DG/HSI and the KAP140 via mail to a competent shop that you can trust. There are a lot of incompetent and non trustworthy shops out there (guess how I know) so be very careful. In my experience, the only shop I can wholeheartedly recommend is Avionik Straubing in Bavaria. Just send in your stuff with a description and ask them to get back to you before repairing anything. Turnaround time is less than a week usually.
Wow, no avionics shop in the entire baltic states? Also, I find it hard to believe that there is no avionics shop at, say, Helsinki Malmi airport, which is the heart of finnish GA after all...
There are "avionics shops" everywhere, but most are not electronics engineers and unable to troubleshoot from first principles with e.g. a scope. Most just follow wiring diagrams, and have specialisations according to their company capabilities.
In this case I would look for a good electronics engineer. The former USSR is full of excellent hardware and software engineers. They had (have) an excellent scientific/technical education system - unlike the West where things have been watered down so nobody sees themselves as a failure The kind of 1970s "technology" involved in autopilots will be trivial to one of these.
Once the problem item has been identified, you can send it off to Straubing, or better still to a specialised firm in the USA.
The hardest job will be to find the wires. Ideally you need the wiring diagram for the aircraft. But if you can get installation manuals for e.g. the DG then you can check signals right on the back of that. What DG (and what aircraft) is it? Can you post some pictures?
It is possible that the problem may be fixable purely through the terminal (laptop) connection. But then I have to ask how did it start in the first place? If it is a "digitally" set parameter which is out, how did it get that way? Did somebody change the autopilot computer "quietly" without recalibrating it?
It sounds like there is no significant ROL error. Is the HDG error totally constant? The heading bug output is trivial to check with a voltmeter.
I know only Konekrohonen in Finland, but they are no more in Malmi and they can't help unfourtunetely.
So, I'll try to contact Avionik Straubing and also will try to find someone to do tests.
I would be happy to try to fix it using laptop. But I have no cable required for connnection :( It was started after electrical trim failure. I have restarted avionics in flight (probably, it was my fault?) and on the next flight it happened.