transponder question

[corroded_camshaft]

Hey all,

My plane is going into annual soon. Currently I have a Cessna 300 transponder, one of these clunkers: http://www.bennettavionics.com/rt359a.html (not entirely sure this is the exact model but it looks the same). At last calibration test, I was told it was pretty iffy. The last time I flew in controlled ATC was reading me 200ft higher than actual. So Im willing to bet theres a good chance it needs to be repaired or replaced. So a few questions:

A) how much, if any, money is it worth putting into repairing this thing?
B) Im assuming the answer to 'A' above might be nil, I was looking at a replacement: http://www.aircraftspruce.com/catalog/a ... gtx327.php Suggestions?
C) Im assuming its also possible the transponder is fine, but wiring or encoder could be the issue. When you buy a new transponder such as the 327 linked above, "with harness", that does not including the antenna, cabling, or encoder correct?
D) If replacing the transponder, is it better to go all out and replace the wiring, antenna, and encoder also or if they seem to be working, its best to leave 'em.

Thanks for any insight

[robertsailor1]

Old stuff like that are not worth fixing but in this case its probably just an adjustment so I'd say get it adjusted because buying a new transponder is not a cheap experience. I just bought a new one and it cost me almost as much to have it installed and tested as it did to buy it.

[c170b53]

Well is it your altimeter or your transponder? Garmin products in my experience and opinion are worth the money and are backed up by the manufacturer. Solid state means reliability, once installed a new encoder and transponder becomes maintenance free. Your altimeter is an entirely different matter. Checked the price of a new altimeter lately?

[CID]

You altitude reporting problem is with your encoder. The transponder merely broadcasts the altitude it's given by the encoder.

Older Transponders can last a long time but the transmitter cavity wears out over time and has to be replace occasionally. It's pricey but a bargain compared with buying a new transponder. That is if a cavity is available for your particular model.

[robertsailor1]

Encoders are relatively cheap

[corroded_camshaft]

In talking with my AME he suggested that if last time I did corelation I had issues, it might just well be worth replacing the unit.

Question: Can I purchase a Garmin GTX 327 and it be a "drop in" replacement to the old clunker? RT-359A (Cessna 300) transponder. Ie: Can I use the same shelf, and wiring, antenna, connectors, etc. Same for the encoder?

I found a connector diagram for the 327 but couldnt find one on the Cessna 300.

[CID]

I think a short lesson in how these things work is in order.

When interogated (by ATC or other aircraft using TCAS or ADS-B) your mode A/C transponder broadcasts information in the way of pulses. The information includes your "4096" code set by the 4 dials and pressure altitude provided by your encoder.

The highest number you can dial into each code digit is "7". That's because the transponder reserves 3 bits per digit and "111" in binary translates to "7" in base 10.

The encoder provides pressure altitude (29.92 In Hg reference) in a modified "gray code" called "gilham code" directly to your transponder.

Gray code is a binary encoding method that never allows more than one bit at a time to change unlike normal binary. That way you don't encounter any indecision as the encoder transitions, which by the way is in 100 foot increments.

The transponder doesn't process or change the encoder output in any way. It spits out pressure altitude just like the encoder provides it. A fault in the internal timing of the transponder would result in fairly unpredictable random error, not a slight change in altitude.

In short, if you have "correlation" problem, it's with your encoder or altimeter, not your transponder.

[CID]

By the way, by all means, update your avionics when you can but I have a feeling you'll continue to have issues if you don't take a good look at your encoder and/or altimeter.

[photofly]

Why are you worrying about replacing the transponder when it's the encoder that's the problem? They're adjustable too - why not just get it adjusted?

In talking with my AME he suggested that if last time I did corelation I had issues, it might just well be worth replacing the unit.

Well he would, wouldn't he?

[Strega]

you need to re calibrate your encoder, its about a 10 min job. or overhaul your altimiter, about a $400 ticket. one of them is out of whack, and I guessing you dont have an expensive altimeter that has an integral encoder.

In a VFR aircraft,, the very last thing I would spend money on is a new transponder..... be cheap on your transponder, and spend money on a new Comm or GPS. (while your at the comm upgrade, make sure you change the coax (use RG-400) and antenna.

There are heaps of spares available for the old cessna units.

[cgzro]

The back of the encoder ( the square boxy thing that has a static line going to it and a ribbon cable usually going out to the transponder ) has a small adjustable dial usually marked + -. While your aircraft is having its correlations check done you can adjust up or down what the transponder responds compared to a known static pressure. So that is where you really should start.

The Garmin GTX transponder is a pretty good device although I've had problems with the cheap plastic face plate cracking .. then again mine gets abused rather badly. I did have the encoder die on me once when trying to adjust out a small error .. all of a sudden it just stopped working..expensive annoying problem probably caused by twisting the knob to far or perhaps pushing the knob too hard.

Usually a new Transponder/Encoder will require a new wiring harness and new slot etc. in the instrument panel, unless it is designed to fit in place of the old with no fiddling. The installation costs will run almost as much, possibly more than the actual transponder if you have to redo the wiring/slot etc.

[CID]

It's being implied that adjustments to the encoder can be done in the field or in-situ. CAR Standard 625 appendix C states:

13. Altimetry Devices
(amended 2007/12/30; previous version)

(a) Altimeters and other Altimetry devices installed in aircraft operating under Instrument Flight Rules, or under visual flight rules in Class B and C Airspace or Class C and D Airspace that is designated as "Transponder Airspace" shall be calibrated at intervals not exceeding 24 months, to the parameters and tolerances outlined in Appendix B of Standard 571, or to equivalent standards acceptable to the Minister.
(amended 2007/12/30; previous version)

(b) For the purpose of this section, the term "altimetry devices" includes any air data computer, or other barometric device, providing a flight crew station, or an auto pilot, or automatic pressure altitude reporting system, or altitude alerting system with altitude data derived from static pressure.

And, CAR 571 Schedule II defines such calibration as "specialized" maintenance that must be done by an approved shop. There are more calibration properties that must be considered than just accuracy at a single altitude.

This is quite a common problem. There are a great deal of light aircraft out there that often occupy designated "transponder" airspace whose encoder hasn't been calibrated for years. That's due to misinterpretation of the specialized maintenance rules. If you breeze through the paragraphs, you could come away with the impression that if you can find a manual with guidance to turn an adjustment screw, you can do it yourself without a specialized rating. Not true.

Many of the less expensive encoders like the ACK-30 have a limited amount of adjustment available and are quite often throw-aways when they hit the shop for calibration. It sometimes pays to spend a little more on equipment like that.

[Hedley]

less expensive encoders like the ACK-30 have a limited amount of adjustment available and are quite often throw-aways when they hit the shop for calibration

Exactly! Do you get your toaster repaired? Of course not. If an older encoder starts to give you trouble, just order a replacement:

http://aircraftspruce.com/catalog/avpag ... ncoder.php

[cgzro]

While your aircraft is having its correlations check done

I was implying that while your plane is connected to all the test equipement with the avionics tech there that knob can be adjusted. Did not mean to imply that you just tweak it yourself which was why I qualified with the above.

Sorry if it was not clear.

[c170b53]

I test my equipment every time I enter controlled airspace. Really the rules should reflect the reality