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Fwd: X-ponder squawk -

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cjpilot710(at)aol.com
Guest

Posted: Sat May 22, 2010 6:46 am Post subject: Fwd: X-ponder squawk -

 	  Quote: 	
		  	


         Received from pilot friend--(general aviation)

     	  Quote: 	
		                   

 

     Transponder: Garmin 320

 

         Encoder:       Ameri King 350

   I thought I would post this for others flying small airplanes. I spent a week this month chasing a frustrating problem in my PA-12. It all started about two weeks ago as I was approaching Bowman field (LOU). As I approached the pattern the tower asked me to "say altitude". I told them 1500 feet. The tower replied that I was showing 1900 feet. A moment later they showed 1200 feet. Basically on every radar sweep they showed a different altitude. After flying around a bit it was clear that my Transponder or encoder was all over the place. Ironically my biannual transponder check was due and I had my radio man coming to the hanger the next day. I called him to inform him that I now also had a squawk with the system. After explaining the problem he suggested I re-rack the encoder as well as the transponder. He figured there was a good chance something was loose. So I climbed under the dash and re-racked both instruments. For good measure I blew compressed air over all the connections. I then took the airplane around the pattern and asked the tower to tell me what they saw. There was no change, my altitude was all over the place. 

   I called my radio man back and told him the results. He suggested that I should replace the encoder. So I grabbed the Aircraft Spruce book and spent some money. I waited 3 days for the new encoder and when it arrived I swapped out the old unit for the new. My flight around the pattern revealed that the problem still existed. Basically the altitude being transmitted from my airplane was hopping up and down as much as 700 feet.

   I called my radio man again and told him the news. It was clear the encoder was not the issue. He suggested swapping out the transponder with a similar unit. I was able to locate an airplane with a Garmin 320 like mine. To get the replacement transponder I flew to a grass strip just west of Lexington, KY. The gentleman who owned the replacement transponder was very generous. I landed by his hanger and within ten minutes was back in the air with the replacement transponder. I climbed up to 3000 feet, called Louisville Approach and asked them what they saw. Unfortunately they saw me all over the sky. I descended back to the grass runway, swapped out the transponders and headed back to Bowman. It was clearly not a transponder problem.

 

 I called my radio man and he decided the next thing to check would be the wire harness running from the encoder to the transponder. I contorted myself back under the dash and removed the harness. We tested each of the 9 wires and could not find a problem, but just to be safe we replaced the harness with a new one. At that point I had to leave for work and my radio man stayed behind. His plan was to hook the encoder up to the transponder and power everything up on the work bench to see if it worked. With his test equipment he pumped up the system to watch it climb. The encoder seemed to be working just fine, and then while it was climbing his cell phone rang. When it rang the encoder altitude jumped a few hundred feet. After he took the call he continued to watch the encoder. Each time he spoke the encoder jumped.

 

 You probably know where this is going. I have an I-phone and yes I always have it with me in the airplane. I usually do not use it but I stow it in a map pocket and it stays on.

 

 We put the airplane back together. I flew around in the pattern for fifteen minutes (Iphone off) while the tower watched my altitude. It was rock solid right on the money. I landed and then turned on my Iphone. I took off again and immediately the tower saw my altitude jumping all over the place. While talking with the tower I turned the phone off and immediately things returned to normal.

 

 My radio man has never encountered this anomaly but is convinced the phone is the culprit. I am as well. By the way the encoder is made by Ameri King. I have an old Ack unit that we put in after discovering this problem and the Iphone does not effect the Ack unit.

 

 Hopefully this information will prevent someone else from chasing their tale.

 

 

 

 

 

 

     

 

 

 

 	


    

 

 

 

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mark.bitterlich(at)navy.m
Guest

Posted: Sun May 23, 2010 6:21 am Post subject: Fwd: X-ponder squawk -

A really interesting story!    



There are two types of encoder interfaces these days, the new serial

method, which should not be impacted by this problem, and the old Grey

Code method using multiple wires that very easily could suffer from EMI

like this. 



Most interfaces using grey code (my own aircraft included) do not use

shielded wire.  This little story is a good reason to rethink that and

start using shielded wire!  It is highly likely that this story was

caused by the individual wires from the encoder to the tranponder having

RF ride on them and get into the transponder where they were

interpretted as a level change.  The RF also might have gotten into the

encoder and caused this to happen too, but less likely.  



Shielded wiring... Including power... Is the very clear answer.  Darn...

Now we understand why the airlines want those cell phones off!   



Mark

 



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