Question on missing Mode-C

What can make my xponder only give out mode-a and only mode-c intermittenly.

Thanx in advance.

Ray

The usual case is the encoder will not output the mode C altitude
until it warms up which takes about 10 minutes on most blind encoders.
The pressure sensor is temperature stabilized.

Sometimes the switch on the transponder needs to be turned back and
forth a few times to clean the contacts.

On Mon, 23 Feb 2004 00:02:52 GMT, "Ray Bengen" wrote:

What can make my xponder only give out mode-a and only mode-c intermittenly.

Thanx in advance.

Ray

Ray Bengen wrote:

What can make my xponder only give out mode-a and only mode-c intermittenly.

First thing to do is check the cable that runs between the encoder and the
transponder. Make sure that it is securely seated at both ends. If it is, get
it to a shop with test gear. In my limited experience, you will probably find
the problem is in the encoder.

George Patterson
A diplomat is a person who can tell you to go to hell in such a way that
you look forward to the trip.

Ray Bengen wrote:
: What can make my xponder only give out mode-a and only mode-c intermittenly.

: Thanx in advance.

: Ray

Mode-A all the time and Mode-C intermittently? Almost certainly the encoder
or wiring. There can be some diode voodoo required in the harness for some
encoder/transponder combinations with other equipment. Is there anything else
connected to the encoder (e.g. IFR GPS, etc)?

Could be the usual bad wiring/connection corrosion, etc. Slightly possible of
flakey encoder too.

-Cory


--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

We had the same problem on our AT-150. What we finally found was that there
was a wire chafed from the altitude encoder that had been remotely mounted.
Over the years the wire chafed due to vibration and was grounding out,
causing a loss of signal to the transponder. A wire reroute and
re-insulating solved the problem.

Good luck


"Ray Bengen" wrote in message
...
What can make my xponder only give out mode-a and only mode-c
intermittenly.

Thanx in advance.

Ray

We had that same problem. It was the encoder.


"Ray Bengen" wrote in message
...
What can make my xponder only give out mode-a and only mode-c
intermittenly.

Thanx in advance.

Ray

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?

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.

Eddie


"Ray Bengen" wrote in message
...
What can make my xponder only give out mode-a and only mode-c
intermittenly.

Thanx in advance.

Ray

"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.

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

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

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.]

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.

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

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

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

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.

True, but I do NOT see this being tested by avionics shops. Their antenna
is placed wherever they can, to get a signal. Sometimes that's several
feet away, and sometimes it's within a couple of inches of the transponder
antenna. I've NEVER seen an on-aircraft certification fail a transponder
that was still working for power. [I have seen bench tests fail one for
power, but that's virtually always because it was so weak that they
couldn't get it to work at all with the standard tarmac test set.