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Altitude Encoder Issue

bill.hutchison

Well Known Member
According to my local avionics shop, my ACK30 was working intermittently, so they replaced it with another used one.

This one may also be a problem, because it's encoding 500-600 feet higher than I'm flying, which is a real problem when you operate a lot under a bravo shelf.

I'm going to take it back to the shop so they can troubleshoot further, but I'm thinking I might just replace that encoder altogether with a new one; those ACK30s have a terrible reputation, apparently.

There are several Trans Cals out there that look promising, but I can't exactly which one I'd need, or if there are some other models you guys would suggest?

I have a GTX327 transponder, so I need a gray-code encoder according to the shop.

What would you recommend?
 
Easy swap if you use the ACK A-30 which in my experience is a pretty decent unit although the Transcal SSD20 would be my preference for a new install.

The encoder they installed 'may' be fine, however part of the required install procedure is to make sure the altimeter and encoder match within 125', sounds like they dropped the ball there. You have to run the encoder and alt on a P-S test box to verify (don't know which one is off until you check both).

PS: the 327 will accept 232 data from a A-30 mod 9 unit if rewiring is required it's a much easier install.
 
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Bill, would you like to swap the ack for a known good one? I have a known good unit I just pulled if you’re interested.

Yeah - I'll send you a PM.

Easy swap if you use the ACK A-30 which in my experience is a pretty decent unit although the Transcal SSD20 would be my preference but not much different.

The encoder they installed 'may' be fine, however part of the required install procedure is to make sure the altimeter and encoder match within 125', sounds like they dropped the ball there. You have to run the encoder and alt on a P-S test box to verify (don't know which one is off until you check both).

PS: the 327 will accept 232 data from a A-30 mod 9 unit if rewiring is required it's a much easier install.

I asked about 232 and the shop told me that would require re-wiring.

This is a temporary solution; my new panel should be coming in a couple of weeks and the Dynon will handle this function for me when it gets here. I just need something operable so I can fly until it gets here.

Walt - I looked up the SSD-120 and it specifies RS-232 on some websites, and others it says both. Would that work for my purposes? Does it support gray code AND RS232?
 
The encoder they installed 'may' be fine, however part of the required install procedure is to make sure the altimeter and encoder match within 125', sounds like they dropped the ball there.

Beyond this, the ACK can be adjusted. The procedure is online at the ACK website.
 
Yeah - I'll send you a PM.



I asked about 232 and the shop told me that would require re-wiring.

This is a temporary solution; my new panel should be coming in a couple of weeks and the Dynon will handle this function for me when it gets here. I just need something operable so I can fly until it gets here.

Walt - I looked up the SSD-120 and it specifies RS-232 on some websites, and others it says both. Would that work for my purposes? Does it support gray code AND RS232?

Yes you would have to run a 232 wire to the Xpdr if you wanted to use the serial output, the unit supports both formats. If it’s temporary I’d just get the adjusted like it should have been in the first place. You can’t legally replace an encoder or altimeter without doing a data correspondence check.
https://www.aircraftspruce.com/catalog/avpages/encoder11-15366.php
 
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Glass?

Is your "primary" altimeter analog or an EFIS?

If your EFIS has an altimeter serial output (RS232) use that as input for your transponder. Then you won't have a discrepancy between primary altimeter and transponder.

Finn
 
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Before you do anything....

How do you know you are 500-600ft off? Is it what ATC is telling you or are you using some "internet" flight tracking data to establish this? Remember, your altitude encoder sends out uncompensated altitude not actual altitude. So if your information is from an "internet" flight tracking web page they may show you 500-600ft off from what you are actually flying since they do not compensate for local barometric pressure. ATC displays takes this into account and compensates for local barometric pressure.

Next time you are flying, with the correct local altimeter setting, ask ATC what altitude they show you at. If they report you within about 100ft then your encoder is working fine. If they report a 500-600ft error, then suspect an altitude encoder problem.

:cool:
 
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Before you do anything....

How do you know you are 500-600ft off? Is it what ATC is telling you or are you using some "internet" flight tracking data to establish this? Remember, your altitude encoder sends out uncompensated altitude not actual altitude. So if your information is from an "internet" flight tracking web page they may show you 500-600ft off from what you are actually flying since they do not compensate for local barometric pressure. ATC displays takes this into account and compensates for local barometric pressure.

Next time you are flying, with the correct local altimeter setting, ask ATC what altitude they show you at. If they report you within about 100ft then your encoder is working fine. If they report a 500-600ft error, then suspect an altitude encoder problem.

:cool:

Thanks for the tips.

Altimeter was set correctly for the conditions at the time, and ATC was reporting that I was off, not an internet flight tracker. Pressure altitude displayed on the GTX was the same as what ATC showed....so, while my altimeter was set correctly and I was at 3,000, ATC (and my transponder) said I was at 3,500.

I verified correct altimeter, did a couple of climbs and descents to determine if the problem was constant or temporary (in the time I had) and the results were a consistent 500-600 discrepancy between where I actually was and what was being reported.
 
Is your "primary" altimeter analog or an EFIS?

If your EFIS has an altimeter serial output (RS232) use that as input for your transponder. Then you won't have a discrepancy between altimeter and transponder.

Finn

Primary altimeter is analog and has been working consistently correctly up until this issue. I do not have an EFIS with an output yet.
 
Is your "primary" altimeter analog or an EFIS?

If your EFIS has an altimeter serial output (RS232) use that as input for your transponder. Then you won't have a discrepancy between primary altimeter and transponder.

Finn

Just to be clear….use a ‘pressure altitude’ serial output to your transponder. Except when the local altimeter setting is 29.92, there will be and should be a difference between what the transponder sends and the altimeter reads.
 
Primary altimeter is analog and has been working consistently correctly up until this issue. I do not have an EFIS with an output yet.

OK, so that's not an option for you.

I mentioned it because I suspect several or perhaps many are flying with an unneeded alt encoder -- extra weight and possible source of static leak, as well as something that has to be adjusted/verified against the EFIS altimeter.

The RV-3B I bought did, in spite of it having a Dynon D-10A.

Finn
 
Thanks for the tips.

Pressure altitude displayed on the GTX was the same as what ATC showed....so, while my altimeter was set correctly and I was at 3,000, ATC (and my transponder) said I was at 3,500.

.

Strictly speaking the GTX should not agree with ATC (except near-standard conditions) because the GTX displays PA, but the ATC computer uses local altimeter settings to change PA back to IA.
There’s a very simple test: Sitting on the ground, adjust your altimeter to local setting. Does it agree with field elevation? Now set altimeter to 29.92. Does GTX readout agree with altimeter?
Now, all that said, 500’ is a lot, I’d sure guess it’s the encoder.
 
Altimeter was set correctly for the conditions at the time, and ATC was reporting that I was off, not an internet flight tracker. Pressure altitude displayed on the GTX was the same as what ATC showed....so, while my altimeter was set correctly and I was at 3,000, ATC (and my transponder) said I was at 3,500.

I verified correct altimeter, did a couple of climbs and descents to determine if the problem was constant or temporary (in the time I had) and the results were a consistent 500-600 discrepancy between where I actually was and what was being reported.
Good so it seems to indicate a problem with the encoder.

The encoder they installed 'may' be fine, however part of the required install procedure is to make sure the altimeter and encoder match within 125', sounds like they dropped the ball there
Take the airplane back to the shop. A reputable Avionics shop should be able to correct the problem.

:cool:
 
Good so it seems to indicate a problem with the encoder.


Take the airplane back to the shop. A reputable Avionics shop should be able to correct the problem.

:cool:

Yup. I spoke to the shop and they're going to take care of it as soon as I can get the airplane back to them, which will likely be this week sometime.
 
quick update

Decided to go up today just to make sure I wasn't crazy or had completely missed an altimeter setting.

Departed VKX, altimeter was 30.04.

Climbed to 1000 MSL indicated, called approach. Transponder observed, on course. Checked altimeter that ATC was using, which was 30.14.

I asked them to give me an altitude reading.

"Charlie Whiskey, showing you at 2,000."

Grrrr. Decided that was enough. Asked for a return to the airport (This is in the DC SFRA and the FRZ, so you don't go anywhere without telling them) they told me no problem, vectored me around an inbound (who turned out to be a friend that I followed in) and advised me to stop squawking alt. Cool.

"Charlie Whiskey, let us know when you've got VKX in sight, and...uh....the local security services have been notified of your situation and everything's okay - you're fine with what you're doing."

Gulp. They don't mess around in the FRZ.

Landed uneventfully.

My friend who was the inbound told me that he was seeing me on ADSB at 900'.

Getting it into the shop on Thursday or Friday.
 
Encoder replaced, all good.

Got the airplane back today. It was definitely the encoder.

The shop shrugged and apologized for giving me a "Serviceable" encoder that wasn't. Everything tested and checked out, ATC was happy.

Kyle, check your PMs...

:)
 
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