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GTX330 FL display showing FL100 or FL515

Recently, flying for a few hours at FL100, I noticed that the FL display on the GTX330 would very occasionally show FL515, for about 1 second.

ATC said nothing. Just as well, since RVSM cert would be expensive

I assume this is explainable by just a single Gray code bit being intermittent. Is that correct?

The altimeter is the KEA130A which is quite pricey - $8k for a new one, $1750 for an exchange refurb one. The altimeter has for some time been showing a "wobble" on the pointer, during climbs or descents only, but more recently it has started doing it in level flight too, and if the wobble exceeds about 50ft (peak deviation) the KC225 autopilot computer warns of an invalid altitude. The wobble is strongly panel vibration related and holding the LH instrument cluster with one's hands usually stops it. But it's not there 99% of the time anyway, which is curious...

Clearly I need to get this unit swapped ASAP, but there should not be any connection between the wobble, and the GTX330 showing FL515. So maybe the altimeter has a clapped out encoder also.

Administrator
Shoreham EGKA, United Kingdom

Looks like D4 is faulty.

ALTITUDE A1 A2 A4 B1 B2 B4 C1 C2 C4 D1 D2 D4

10000 0 1 1 1 0 1 0 1 0 0 0 0

51500 0 1 1 1 0 1 0 1 0 0 0 1

http://www.airsport-corp.com/modecascii.txt

(This post is best viewed in a monospace font 8-))

LSZK, Switzerland

the Garmin doesn't have its own encoding altimeter builtin?

No. It takes in the 12-bit Gray code from an external encoder.

It may also (without looking at the IM) take ARINC429 pressure altitude but then you have to get that from somewhere...

And then the KFC225 would need to be similarly sorted and I don't know if that is even possible because, even if it takes in ARINC429 altitude, the TB20 STC specific IM is not going to show that option in use. One would need an ARINC429 altitude preselect box... I did look into installing one of the very nice single-pointer "digital" altimeters but I don't see any remotely simple legal way to do it.

And you don't want one pressure altitude encoder for the transponder and a different one for the autopilot

Administrator
Shoreham EGKA, United Kingdom

And you don't want one pressure altitude encoder for the transponder and a different one for the autopilot

Why? Most modern avionics have their own encoding altimeters and connect to the static system directly. Garrecht, Funkwerk/Filser and Trig transponders, S-TEC autopilots, Avidyne DFC90 autopilot, Aspen PFD etc.

The only place where I use that crappy Gray interface is for the GNS430W. I wonder if it could use ARINC429 from the Aspen...

Do you have your D4 really connected? Most GA encoders don't have a D4.

EGBE - Coventry

The GTX330 IM (URL above) shows its D4 is used but D1 and D2 are not. So it uses just 10 wires, which I think is fairly normal in GA avionics.

I have ordered a new (overhauled) KEA130A altimeter from Castleberry Instruments in Texas. A really good firm, which I have used for various "mechanical" instrument overhauls and all have been fine. $3200 outright, so I will get the original one fixed and it can sit on the shelf, which is always a good idea.

Interestingly, the FAA requires the full altimetry test to be carried out after any opening of the static pipe system. This needs to be done by an FAA 145 Repair Station. Evidently, very few avionics installers in the UK do this... luckily I am hangared in one so it's OK.

Administrator
Shoreham EGKA, United Kingdom

Why don't you just remove the D4 wire? It's pointless anyway in a GA aircraft.

Do you have your D4 really connected? Most GA encoders don't have a D4.

Yes but then D4 should have been grounded, shouldn't it?

LSZK, Switzerland

It could be a bad d4 from the altimeter, or an internal fault inside the transponder, or a bad wire between the two.

The altimeter needs changing anyway so I will start there.

Administrator
Shoreham EGKA, United Kingdom
21 Posts
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