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Was flying last weekend on autopilot and pulled the heater lever rather abruptly which was followed by the airplane changing altitude 100+ feet. I was able to repeat this up and down by adjusting the heat lever which I could feel in my eardrums. The alternate static port was closed. Has anyone seen this before? I'm wondering if my static tube is disconnected, or possibly not re-connected from my last IFR Cert(?) If this sounds reasonable, (before I take a screwdriver to it) is this connection easily accessible behind the panel or elsewhere?

Take plane up and try opening and closing the alternate static. Check if altimeter changes.

I think your fault diagnosis is spot on.
It is most certainly a static pipe disconnected or badly split.

If they failed to reconnect, it might not be any connection in the cabin. Probably super-obvious once the instrument cover is off. This isn't a big deal to R&R. If you find it so disconnected, I would definitely let the shop know they screwed up.

Rich wrote: ↑Wed Feb 24, 2021 1:44 am If they failed to reconnect, it might not be any connection in the cabin.

If the issue is outside the cabin then I doubt it would be affected by the heater being turned on.
Almost certainly behind the instrument panel or under the pilot seat were the pitot/static water trap is.

Took it up again yesterday. The heater lever confirmed makes the airplane go up and down with KAP 140 A/P enabled. I didn't think to disengage the A/P and see how the altimeters behaved doing the same thing. With A/P set to capture 3500 feet, the airplane leveled off at 3300. They are usually within 20 feet of each other. I asked ATC how they read my altitude and they said 3300. Opening and closing the alternate static port had no affect at all. Back at the hangar, I removed the panel cover and traced the tubing which appeared to be in good shape and firmly attached everywhere. May be time to visit an avionics shop.

Hello,

I have noticed the same behavior on at least two different KAP140 equipped DA42.
I guess it has to do with the autopilot sensors as the G1000 indication is unaffected.
You would have the same behavior although more dramatic if you open the storm window while on autopilot

The KAP 140 has two altitude feeds to it: Static air pressure line common to the altimeter(s), Airspeed, etc. and input from the altitude encoder, which it shares with your transponder. The encoder is required only to support altitude preselect. I don't know if it has any role in VS mode or when holding altitude.

There is a possible variation to the static line to the AP, which is that it be fed from a dedicated line fed from rather large holes drilled in the aft cabin. This was retrofittable to earlier aircraft. The AMM is ambiguous here, but it appears it bypasses the alternate static port. Do you have this option perchance? It would be another potential place to check. There is also the issue of the o-ring in the back of the AP where it fits into the static line for either version.

Hi Rich -

That is interesting information - you are quite the encyclopedia. While tracing the static lines around the back of the panel, I'm pretty sure the KAP-140 was tied into the common static for the other instruments, but I will keep that in mind digging further. For now I think I need to dedicate another flight to this and see if the heater lever directly affects the altimeter display and backup. If there is no effect, then maybe this is one of those things that has always been there and I just haven't noticed. I felt better knowing my ADS-B output matches the cabin altimeters (ATC verified).

FWIW, my KAP140 doesn't react to the heater, vents, windows or anything other than the alternate static valve, which the altimeter also reacts to. But if I set the KAP Baro to be the same as the altimeter (as broadcast over AWOS, ATIS, etc.) my AP will fly 60 ft. too high. I have long been in the habit of setting the AP baro .06 in. lower than the altimeter setting and life is good. My recerts always show my mechanical altimeter within 0-20 ft.