ACK A-30 Altitude Encoder Problems

I'm looking for service history experience with ACK A-30 altitude encoders. I've got an ACK A-30 (mod 3, I think, based on the serial number). The altitude reported by mine drifted out of tolerance last year - I had it recalibrated by a local avionics shop, and it was fine for awhile, but now it seems to have once again drifted down about 200 ft compared to my two altimeters. Just like last time it did this, I checked for a static leak, but the system is as tight as a drum.

I'm not sure if I have a bad encoder that tends to drift, or whether ACK-30s are not reliable. If ACK A-30s are generally reliable, the simplist thing is to buy a new one. If ACK A-30s are problematic, I'll change to something else. Or, is there perhaps something strange with my installation that could be causing the encoders to go out of tolerance?

Comments are appreciated.

I have found both the ACK and Ameriking can be prone to this after they get over about 5-6 years old, I tell customers I'll will recal the unit 1 time free of charge but if it drifts off again, it's time to toss it.

If you want something a little better, I recommend the Transcal or the Sandia units, higher quality, rarely drift out of tolerance and seem to last forever.

You get what you pay for

PS: for the folks that have a digital transponder where you can see the altitude, set your altimeter to 29.92 while flying and compare it to your transponder altitude, the tolerance is 125 ft but if you get to 100 ft diffenence its time to get it check/adjusted. The encoder is always referenced to 29.92 so that is the only time the two altitudes should match.

And don't forget, if you replace the altimeter or the encoder you must have a data correlation check performed (at least in the US, not sure about CA but probably the same) to ensure your primary altimeter matches your reported altitude.

I don't know about ACK but my old AK seemed to be very problematic. I switch to a Trans-Cal and that thing has been bullet proof so far.

Thanks for the benefit of your experience Walt. My ACK has about 4.5 years in-service, but it was bought several years before I started flying.

I had already started reading up on the Trans-Cal encoder. I like that they have an adapter plate for the ACK tray, and it is pin compatible, so that'll minimize the work to change over. I'll need to get another transponder correlation check done though to be legal. Drat.

I have one

Kevin, I have an ACK30 that's about 3 years old that I will sell you for $75.00 plus shipping.

I removed and replaced with new one but turned out my KT76A was dying.

Let me know if you're interested.

If you can get a trans cal. They are pretty well bullet proof. The others that you mention are flaky at the best of times.

Thanks for all the advice and comments guys.

I'll buy a Trans-Cal. It is slightly more expensive than another ACK, but I'll hopefully get a unit that will stay in calibration between the 2 year checks. The cost and hassle of having to have the ACK recalibrated regularly overweighs the small savings in purchase price if I went with a new or used ACK.