Hello everybody,
I have a problem with my KFC150 mounted on Socata TB21. I tried in flight and it passes the test. The trim works. The FD seems to work in roll but not in pitch. The roll servo is connected electrically but not mechanically, the previous owner had disassembled it because it was deemed not working. Is there any more precise trouble shooting operation that I can do to have more data to evaluate to identify the problem?
thank you
Many thanks for putting together all that information Peter – very useful and much appreciated !
Mark
Looking at the IM posted above, the servos are KS177 / KS178 for pitch and roll respectively. This is the page from the IM
Relevant signals:
This suggests the servos contain a tachometer (the “feedback” wires) but there is no “valid” status like the KFC225 servos return.
A bench test is certainly possible. Luckily I have the MM for the servos – here and sure enough it is exactly as I said
This is the circuit diagram
The MM is beautifully written, like all King stuff from that era. You need somebody who is into electronics to get stuck into this. This is no rocket science – despite King having done a lot of work on Apollo – and it just needs somebody who has a good brain and is willing to give you some of their time. This is unfortunately right at the edge of where most avionics installers are working; it is beyond simple wiring, which is why the best way is a private engagement of an electronics person local to you.
There are servos on US Ebay but they can’t be installed on an EASA-reg…. unless you can make use of this.
What’s happening is a bit like when there’s too much turbulence for the AP to cope with and it drops out. I guess when this happens there must be some sort of feedback from the servos to the KC191 to tell it and I wonder if this is what’s happening erroneously when trying to engage ? I don’t know what the KC191 checks during the self test but it must include the servos but maybe it doesn’t check whatever mechanism is used to disengage the AP during excess turbulence.
Is it likely to be one of the servos ? Is there a way to bench test them ?
Thanks…
Sorry for the delay – our starter bendix packed up & had to be replaced before we could start the engine & video the autopilot failing to engage ! Video is Here
As you can see, immediately on releasing the AP ENG button the AP light flashes with 5 beeps.
As a reminder the KC191 passes the self test on startup and was bench tested OK and the autopilot disconnect switch replaced so not likely to be those – any other suggestions how to troubleshoot gratefully received !
Mark
Did you check the trim switches on the yoke? I once had a sticky one and when one of this switches is actuated, the AP disengages/doesn’t engage. In my case the ground test worked great but directly after takeoff I usually have to trim a bit nose down. Quite frequently one of the trim switches sticked closed and the AP wouldn’t engage. You can easily check this by fiddling a bit with the switches and try the AP again.
On the King autopilots, pressing AP alone should set wings level (if roll is within 7 degrees or so) and capture current pitch (again if within certain limits).
If that alone doesn’t work, the issue is not related to the heading source (that is used in HDG mode) and is not related to the altitude source (that is used for altitude capture). It can be related to the attitude source. The KFC225 would also refuse to engage at this point if any of the three servos were returning an invalid status, but you got a loud beep and some warning lights.
I am not sure what external factor would prevent the basic AP mode engaging. I believe the KAP150 is analog but contains a micro for implementing the front panel scanning and stuff like that.
If pressing the AP button does literally nothing whatsoever, it could be a faulty switch! What is needed is a video. Can you take one and upload it to youtube or vimeo and drop the URL in here? A faulty switch is generally easy to replace.
There are two Pilot’s Guides for the KAP150. One is 30 pages and one is 120 pages. You should have the longer one – here.
Avionicare at Southend and Tatenhill Aviation are the remaining Bendix-King authorised dealers in the UK.
I had access to a test set until a couple of weeks ago but it’s been relocated as the business has exited GA.
@Wigglyamp – any idea who has a KTS break-out test set in the UK ?
@Peter – pressing AP ENG followed by ALT and HDG or NAV results in nothing happening. Previously it would sometimes engage and then drop out with the beep, beep you sometimes get with excessive turbulence. Now there’s just nothing. Looking like the only options are to take it to one of the few Bendix-King service centres in Europe – France (where we sent the KC191 for bench testing), Germany or Poland, but it’s a long way for a diagnosis…
