KAP-140

[pshz11]

Hi,

I'm very new here, but need some advise from more experienced pilots :)

I have problem with my C172S KAP-140. Today I had two flights. On a first flight there was electrical trim malfunction. I have restarted avionics and trim worked fine. Autopilot also worked fine except an issue with NAV mode.

I few hours I've made one more flight and on this flight autopilot did not work properly. When I set heading on DI, airplane starts deviating by 10-15 degrees to the left of desired heading. The same happens in NAV mode with desired track. Crosswind exists, but it is not a reason for sure. When it is heading select mode used, it maintain strictly selected heading.

I've done some tests with right and left turns. Left turns are steep, right turns are very shallow. So, it seems, that autopilot tends to make left turn (thinking it flies straight in).

All self test are okay. Probably there's some way to calibrate device? Here in Latvia there's no maintenance organization, who can repair bendix avionics :(

I would appreciate any advice on this issue.

Best Regards,
Sergey

[wigglyamp]

The bank angles are a function of rate of turn derived from the turn coordinator. First job is to level the aircraft and check that the slip ball is centred so that the TC doesn't give erroneous turn data (the TC has an inclined spin axis so will give a turn output if a bank exists even if you're not actually turning). If the slip ball is not centred, rotate the turn coordinator in the slotted holes on the panel to get the slip ball in the middle. That will hopefully sort all of the problems. To test further, you have to plug in a laptop (connector is underneath the panel by the park brake lever) and then interrogate the maintenance pages to check sensor inputs.

[pshz11]

Thank you so much for response!

So, first of all I'm going to check TC on the ground.

Regarding KAP-140, is there any specific cable and software? I have one for KLN, but I think, they are not same.

[achimha]

Remove your TC and have it bench tested by an avionics shop. That doesn't cost much (I paid around €60 last year). The AP works by measuring the voltage from the TC output.

[wigglyamp]

The test cable for the laptop has a 9D connector on one end and an US NATO plug on the other. You need to run Hyperterm (RS232 emulator) to talk to the autopilot and will need to set this up according to the KAP140 install or flight line maintenance manual. Unless you are familiar with this I would suggest going to a competent avionic shop. You have to be careful as there is a configuration module which contains airframe-specific programme data and if you inadvertently delete the files you'll lose the autopilot until a shop can reprogramme this from a floppy disk.

[pshz11]

I think, I will not try to hack autopilot myself at this time:) But anyway I'll try doing something with TC. I think, closesest avionics service station for me will be outside Baltic states :(

[peterh337]

These notes (http://www.peter2000.co.uk/aviation/kfc225/index.html) are on the KFC225 but they have the cable details.

I also have the KAP140 install manual.

[pshz11]

I've done more tests today.

It seems, that it's everything okay with TC. And airplane also rolls with right bank angles (probably first impression was impacted by turbulence and cross wind). But problem still exists: airplane flies 20 degrees left of track or heading selected.

I.e., if I set heading 180, it flies 160 and so on.

peterh337, could you please send it to me by some way?

Also, probably someone knows maintenance organization, who is able to solve it (if I would not my self) in Finland or east of Sweden?

[peterh337]

If you email me I can email it to you.

To be honest I think you need to find at least somebody who knows electronics well. If you can find an avionics shop who is familiar with Honeywell autopilots that would be better, but it is possible that options in your area are less than great.

To me it looks like the heading offset parameter has been corrupted, but 20 degrees is a strange amount. I wonder if perhaps a ground wire has come off and you are getting a ground loop generated offset.

[wigglyamp]

Do you have a directional gyro with heading bug or an HSI with both course and heading bugs?

[pshz11]

Direction gyro with heading only.

So, at least I should ask technicians to check cables?

[huv]

This happened to me on a flight with a KAP140-equipped DA40. As a flight instructor, I had a student converting to the DA40, and he was new to the KAP140. When trying to select lower altitude from FL90 on the KAP140, he must have hit the BARO and turned the QNH setting way off. He then managed to set correct target altitude, toggled the VS key, which made the A/P enter VS mode, and selected 500 fpm down. The A/P obeyed, IAS was around 125 K and all was well.

Then my student checked the BARO setting. It was around 1070, and QNH was around 995, a huge difference. He turned the knob (a lot), setting 995 as the proper BARO setting for the descent. Then I noticed the trim wheel moving steadily forward, the nose lowered and in very few seconds the airspeed approached Vne. I pressed the copilot's A/P disconnect botton, steadied the aeroplane and retrimmed, then re-engaged the autopilot, which now worked fine again. We were IMC, and perhaps due to c/s propeller, light turbulence and a generally low noise level the student had not noticed the dive in time to react.

On the next flight with the same aeroplane I managed to re-create the upsetting KAP140 behaviour. During descent on A/P in VS mode, I changed the BARO setting. Moderate (realistic) changes produced no reaction, but after turning to an absurdly high setting like 1070 and then turning down to the actual setting of around 1000, the same thing happened - a dramatic dive towards redline.

Apparently there is no warning in the manual against changing BARO setting during descent - which would indeed be a nuisance, especially IFR, since descend clearences to a low flight level are often extended, while descending, to an altitude below transition altitude, requiring the pilot to set QNH.

The autopilot works flawlessly in every other respect as far as I can tell. I would like to hear from someone with similar experience or perhaps an explanation.