Question on missing Mode-C

Thanks for the info. Our relationship with the avionics shop is relatively
new but they have been outstanding so far and came highly recommended both
by fellow aviators and Garmin and King manufacturer reps My take on this
subject is that I need to take it back and specifically have them bench test
the xponder. (hmm, could it be time for that mode-s?)

Thanks again,
Eddie

"dave" wrote in message
news:lUn%b.126184$jk2.539089@attbi_s53...


James M. Knox wrote:

"eddie" wrote in
om:


We just had the pitot-static, encoder / transponder certification
completed on 2/21. On 2/23 ATC repeatedly failed to receive the
mode-c. Recycling didn't seem to help. What exactly did the shop
"certify" when they certified the xponder/encoder? Was it the encoder
calibration?


They did three things. One - they checked the static system for
integrity (i.e. it doesn't leak). Two - they checked the altimeter for
accuracy. [They used to do this at the same time, but now they have to
pull the altimeter and test it separately, as I understand the latest
changes).

And three - they put a small "ATC simulator" out by your aircraft and
test the transponder. Basically this applies a small vacuum to your
static system while they "ping" your transponder and listen for the
replies. This tests the transponder system, including the encoder.


BTW this has been a problem in the past too, so whatever they did
didn't cause the problem but it didn't fix it or find it either.


Now... what the transponder tests DON'T catch.



1. A weak transponder transmitter section. It works, but doesn't put
out the full rated power.

There are minimum requirements for passing the transponder power tests
one is power. This is rated on the class of the transponder.



2. Bad cable or corroded connectors between the transponder and antenna.


Quite often a corroded antenna connector can be caught during the
transponder test as the transponder is quite often shifted off
frequency. It can't test for intermitten connections.
Again, power will also verify this problem.


3. Bad antenna, including bad connection to ground plane.

See above


4. Poor positioning of the antenna, including areas subject to shadowing
or too near other antennas. [If the system has NEVER seemed to work
well, then this is a possibility.]

Correct, but usually not a big problem.


And lastly, of course, don't rule out ATC problems. They too have poor
receivers, cables, antennas from time to time, and sometimes it can take
years to get them to actually test/repair the problem. The usual
symptom here is a) multiple people getting reports that their Mode-C
isn't being received, and b) in other parts of the country you don't
seem to have any problems.


This is 100% correct!!


Also, don't rule out that the person who tested the unit didn't do his
job.

The transponder test also do not catch problems which are caused by
vibration or electronic parts of the transponder which degrade over time
with heat.

-----------------------------------------------
James M. Knox
TriSoft ph 512-385-0316
1109-A Shady Lane fax 512-366-4331
Austin, Tx 78721
-----------------------------------------------


Dave
www.craigmileaviation.com