KAP 140: NAV and APPR Mode don't engage

The two buttons didn’t respond anymore.

Meaning the KAP140 display won’t show NAV or NAV ARM? Or displays NAV or NAV ARM but doesn’t fly the aircraft to follow the CDI?

If it won’t display NAV or NAV ARM on the KAP140 something might be wrong with the KAP-140 itself.

If it does display but doesn’t follow the CDI in any mode, it is likely there went something wrong with the setup / wiring.

If it doesn’t get in those modes it seems to be in the KAP140 itself. With unvalid inputs it should still at least go to ARM mode.

It could be faulty buttons Or more likely a faulty connector to the front panel PCB. It does happen… However, the autopilot should illuminate all those annunciators during its power-on startup sequence.

In flight, or on the ground if the right signals are in place and the main AI is not too far off straight-up, there are some signals which, if not present, prevent NAV (and probably also APPR then) being selected. I vaguely recall having this with my KFC225. The “heading valid” signal out of the KG102A heading gyro was left unconnected (to the KFC225) after some avionics work, and the autopilot would work only in ROL mode. HDG or any of the higher modes could not be selected.

So this could be a fault with whatever signal validates the NAV mode. You probably have a classic “NAV/GPS” switch

and that selects whether the NAV mode of the autopilot is fed from a (a) VOR/LOC, or (b) from the GPS. Both these lateral deviation signal sources have an accompanying “valid” signal. If you cannot engage the NAV mode in both NAV and GPS settings of that switch, then I would check the status of that signal entering the autopilot.

I have a huge collection of avionics manuals (on a private site) but unless you are electronically competent you will need a competent avionics installer to troubleshoot this. It may be trivial but often the wires are hard to get to, so a trivial job turns into days.

It depends on the implementation.

What might prevent AP engagement on the ground might be

• the pitch/roll source (KI256 AI, or a PFD that emulates the KI256) being more than (from memory) 15 deg off straight-up, on either roll or pitch (generally one needs to use a vac pump to erect a KI256 when doing autopilot tests on the ground, though if the AI is not too far off being erect one can often do an adequate engine-off ground test)

• pitch/roll invalid flag (not present on my KFC225, but there could be e.g. a vacuum not present switch, or an equivalent signal from an AHRS gyro that delivers pitch/roll data)

• a landing gear squat switch (also not present on my install, but preventing accidental AP engagement, and consequent trim servo activity, with weight on wheels would have prevented some accidents)

• the absence of nav signals (obviously) but these can be simulated with a signal generator e.g. an IFR4000

It could be faulty buttons

I wouldn’t suspect them in the first place, it would be vary rare to have both failing, while the others still work. It also isn’t a problem with the common contact of the switches, as the others are still operative.

Not sure how this A/P responds to NAV flag from my memory. I think it does NAV ARM with NAV flag active, not 100% sure though.
It is likely that it won’t do approach mode, as it doesn’t operate in NAV mode either, and approach mode is using NAV mode as well.

You would only have on signal output from the G1000 to the KAP-140, for NAV this is left/right signal and flag (valid). You will always use the same wiring. However the left/right shouldn’t matter, if it is to far out (to far left or right) the KAP-140 will go to ARM mode, and waits till you intercept the NAV signal using ROL or HDG mode.

I have a huge collection of avionics manuals (on a private site) but unless you are electronically competent you will need a competent avionics installer to troubleshoot this. It may be trivial but often the wires are hard to get to, so a trivial job turns into days.

I would recommend to you generate an as good a possible fault discription. Discussing it here is a good way to find out some more information. An avionics shop could then easily pull out the KAP-140, generate signals to test and measure if all signals will arrive at the edge connector, this wouldn’t take to long. I don’t agree it would take days. It would be easy to find out if the problem is inside the KAP-140, in the wiring, or the G1000. With a good fault discription you will save yourself money.

Yes; I wasn’t suggesting that one goes straight to an avionics shop. Just that one may be needed (or an electronically competent person if one is based where there are no good shops) to finally sort it.

I have some KAP140 stuff here because this has come up before.