erratic Mode C from encoder

alcladrv · #1 05-17-2007, 09:23 PM

In flying my -7A today, ATC reported that my Mode C output would jump randomly from 2800 to 3000 to 2600 to XXX(no altitude readout), etc. It wasn't very turbulent and I was able to maintain my altimeter within a 100 feet of 3000 during the whole time.

Has anyone experienced the same thing? Does anyone have any trouble-shooting ideas? Thanks,

Mike

brianwallis · #2 05-23-2007, 09:07 AM

My encoder on my AA-1A was jumping all over the place... I called Skip Moore at 678-468-8678, he ordered me one and replaced it... no problems so far!
best
Brian

johnp · #3 05-23-2007, 02:16 PM

sounds like it could be the encoder. if it were just a bad connection, you would expect it to be one or perhaps two connections bad, but to vary 2600, 2800, and 3000 would take 3 bad connections:

2600 000 100 011
2800 001 100 001
3000 001 100 010

(i think i got this right) -- so if it's bad connections it would be
a4,c2,and c4. it might be worth checking these before purchasing another encoder.

john

Ted Radclyffe · #4 05-23-2007, 09:08 PM

Here is my experience which probably has no relervance but gives food for thought.
I had my transponder connected to an encoding altimeter. The problem you described began so I bought an Ameriking encoder. The problem seemed to change but still persisted.

On contacting the manufacturer of the transponder they said that the ADSB Squitter messages which are now sent out from other aircraft (for TCAS) and also ground stations, can overloat the input filter and sent the software out of wack. The transponder then reboots and things settle down until the next overload.

Result: At a very minimal cost they are updating the hardware with an improved filter and putting in the latest software. I don't want to give the brand except to say that it is an Australian company and I am very pleased with their product and the service they provide.

Ted Radclyffe
"Rough Red" (RV-6A)

alcladrv · #5 05-23-2007, 10:36 PM

Just an update on the Ameri-King encoder that was jumping all over the place. It was observed by 2 different approach control facilities in northern Ohio.

I unplugged and reinserted the connectors to my Garmin 320A transponder and the problem subsided, but didn't completely go away.

One of the original complaining approach control facilities still saw some problems with it. However, over the last 5 days, I've flown my -7A into and out of the DC ADIZ and through the New York Class B airspace with no complaints or comments at all on my Mode C.

I don't know if it's fixed, but where the Mode C really counted, it seemed to work fine.

Mike