Transponder

[33alfa]

Here is what happened to a buddy of mine. ( VFR Aircraft) Lands and ground loops. Faa gets involved and checks logs. No 24 month transponder test noted in logs. Fined for that. So if nothing happens you may be ok However! Anyway as a guy who does this for a living heres my take on it. You can not do a proper VFR or IFR transponder test without actually runing the aircraft static system up to 14,800 feet in order to activate the A1 bit.This makes sure that all the bit codes are working. To sign off a transponder test you have to remove it t verify S/N and plugging it back in without a test will not assure you that everything is working correctly or working at all. As to if you need it done or not. Remember some day you may need it and woudnt it be nice to know its right. Roger in Alaska.

[GAHorn]

Excellent ponts, 33alfa. (What's your name anyway?)
Another consideration on avionics repairs is that removing your radio and shipping it off might work most of the time....but, it does not address other issues that might be affecting the avionics such as aircraft voltage and installation problems such as bad connectors, corroded antenna connections, poor/degraded antenna, etc. You'd be surprised how often the radio is not the actual problem...or that the radio's failure was caused by an airframe issue. (and how often the problem is antenna related, even tho' it's "been working for years!") Doh! Old installation antennas, wires, connectors ARE common problems. That's why "conversion" radios aren't always a good answer to outdated avionics.

Several shops specialize in receiving shipped radios for repair. Eastern Avionics, Pacific Coast Avionics, American Avionics, Avionics West, are the ones that "pop" into mind. (Sorry for the pun.)

AND..... if you have Narco....don't forget....the factory does repair maintenance, and they do a very nice job with good turn-around and loaner/rental units.

[33alfa]

A good old ATC 600A box should tell you for the most part if the power, framing, reply and ect, are good but i always tell people the the best way to make sure things are set up correctly is to remove the altimeter, encoder if installed and have them matched on the bench. same with the transponder. By doing so when reinstalled the integrated systems test should be a breeze.
After talking with my local FAA rep, what i talked about was confirmed.
When performing a VFR cert on the aircraft it still needs to be run to 14,800 feet. It would then be tested per FAR 91.413 Appendix F for use in the ATC system. Hope this helps. Roger Swanstrom. Aircraft Instrument Repair. Anch Ak. AKA 33alfa.

[GAHorn]

Yep, good words 33alfa. Lots of folks don't realize that simply installing an encoder without bench-test matching with their txdr and altimeter is sloppy (and probably violates the TSO and therefore, illegal.)

[jrenwick]

If this discussion arouses the interest of any computer geeks out there then here's a helpful reference: http://www.airsport-corp.com/modec.htm.

Your transponder responds to Mode C interrogation with the same 12-bit logic used by your squawk code, but it encodes the altitude instead (in the upper 9 bits). The 3 bits of the first digit of your squawk code encode altitudes beginning at 2800 feet, 6800 feet, and 14,800 feet.

I've had a "VFR" transponder check in one of my aircraft, that only verified that the altitude was correct at field elevation. This checked all twelve bits partially, but any of them could have been "stuck at" the correct value for where I was. I'm actually confident of all but the A1 and A2 bits because I often talk to ATC at altitudes below 6800 feet, and I've had confirmation that my Mode C is correct at various altitiudes in that range. When I occasionally get flight following between 6800 and 14700 feet, I'm checking all but the A1 bit -- the one that I don't expect to turn on unless I get deep into a thunderstorm. If that one ever turns on, I might not live through the event.

[33alfa]

John you make a good point on the transponder portion. Unless you have ever looked at the inside of some of the altitude digitizers this would be hard to know but let me shed some light on the situation. The newer encoders for the most part are solid state. The older ones including the united 5035 series use a glass disc that for some reason after a while becomes bent causing an alignment issue, also causing at various altitudes an incorrect altitude display to the tower. this would never be caught by just varifying the transponder bit codes are working. also there is a certain serial number range of the trans cal encoders we have found to not display the 10 ft increments for pressure altitude correctly, (ie to the GPS) this also can not be varified without a bench test.
Now that i have made you tired just let me say that this debate has been going on for a long time and i would suspect it will continue. The solid state units are making life a little better but nothing is without its glitches. If i have to come out to your plane to do a test im going to make sure whatever situation your in your going to have the resources to remove yourself from it. Be Safe and remember If its there and its working....Use It Roger. A.I.R

[jrenwick]

It's good to learn something about how these things work. Takes me back to stuff I learned in the Navy 35 years ago! (Gray codes and all that.)

John

[GAHorn]

Roger, don't forget about the joy of Aerosonic Encoding Altimeters.