Agreed. Replacing the fin probe with a triple probe is probably the last resort, and replacing tubing not far behind it.

Does the probe itself work? The small holes can clog. Some models have joints; is there a leak there?

Is the probe or socket clogged? Pretty common with spiders, water, dirt, etc. I might go here first. No or minimal instrument indications would suggest a closed system. Having the TE line open to static pressure from a big leak near the probe would make the varios behave like they're connected to static and uncompensated, I think.

Then go looking for the leak as Karl describes. If not in one of the instruments, start closing or clamping off sections of tubing and then retesting.

You can convert the tail pitot to a TE socket, tape it each time you insert the probe, and move the pitot to the nose. What source does your handbook spec for the ASI? Tail pitot was added originally, in part, to reduce errors from the signals from different sources arriving at sensitive instruments at different times (e.g., nose pitot, aft statics) but if that's a problem, you can adjust the timing of the pitot signal with restrictors. I know the instrument experts will cringe when they see this! Moving the TE probe to the fin is desirable; the fuselage-mounted ones are said to not work as well and be more prone to errors because they're not in relatively undisturbed flow.

You mentioned static/TE input. Is this a model with two signals: static and TE? If so, there's likely an o-ring in there that can go bad. I've never replaced one but some guys have done so on the fin mounts: frustrating but possible without cutting anything.

If there truly is a leak in the aft tubing, make sure it's not somewhere within reach. If it is, just replace the offending section. If there's a leak where the tubing fits over the barbed end of the socket (process of elimination), you might be able to get to that to slip on a few wraps of safety wire or cable tie or whatever to tighten it up.

This was just stream of consciousness stuff and some of it may not be on the mark. My point is that there are lots of things to try and test before cutting holes...unless you just want to install a triple probe. They're standard equipment now but that hasn't always been the case and you can do just fine without one.

Chip Bearden
ASW "JB"