Transponder giving incorrect pressure alt readings

Dale Young

Dale Young

Line Up and Wait
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Hopkinsville, Ky
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Dale Young
I noticed yesterday, that my txpndr (narco 165) was showing a difference of 1500ft from my altimeter and my Electronics International Superclock ( which shows the alt accordiing to what atc sees). My altimeter and superclock were in agreement. What gives???
It MUST be the blind encoder going on the fritz,...Right?
BUT! ..... Doesnt the superclock just read what the blind encoder is transmitting?? If that were the case, SHOULD'NT the superclock agree with the txpdr??
 
Dean, Thats what I thought also, but the superclock situation confuses me. I always believed that the superclock just got info from the encoder. If that is the case, the superclock should have said the same as the txpdr.........weird!!
 
I had the same thing happening with mine and i was told it was normal to be off....forgot the reason why.
 
The encoder is permanently set to 29.92 barometric pressure and ATC computers adjust the readings for actual pressure. That can explain the difference with your altimeter. I don't know anything about a superclock so I can't help with that.

John


Sent from my iPad using Tapatalk HD
 
The "Altitutde Clock" (not the Super Clock) reads the encoder. It does not tell you what ATC sees. You have no way of knowing that. The best you can come is hope they gave you the same alitimeter setting that their computer is using to correct your Mode C.

The ASC-5 has two modes of operation depending on how it's powered up (or you can toggle it with the stop/start button). In one mode it is showing the raw encoder out, in the other it is correcting it with the "bar" pressure setting.
 
Your superclock should be connected to the same encoder lines as your transponder. It should show exactly what your transponder is transmitting. It should also read the same as your altimeter if you set the altimeter to 29.92. If it is not I would first suspect a wiring issue to the transponder. this could be as simple as dirty contacts on the back of the transponder tray. it could also be dirty connections n the transponder. You could also have a problem with the transponder. The most unlikely problem is a problem with weak outputs on your encoder or your superclock is pulling down the encoder output. This is a hard one to troubleshoot without a transponder test set. also technically if a mechanic pulls a transponder out, they are supposed to have it verified with a test set.
 
Ron,
This is a SC-5 superclock. It shows you what atc sees based on what your encoder is transmitting.
 

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Dale Young said:
Ron,
This is a SC-5 superclock. It shows you what atc sees based on what your encoder is transmitting.
Click to expand...
That should agree with your altimeter reading if and only if the altimeter setting is 29.92 inHg.
 
Not only does the "superclock" get the grey code data from the encoder, but your IFR GPS does as well. And you should be able to access that data from the setup/diagnotics menu on your IFR GPS.
 
you could also have a leak, or a plug, or a cracked case.
 
I had the same problem but it only showed up at certain altitudes. It was due to an oxiduzed pin at the transponder connector. I removed the transponder from its tray and sprayed contact cleaner on the transponser and tray contacts. Problem solved. I first noticed the problem on the Monroy Traffic Watch that also reads what altitude your transponder is actually transmitting out, very handy for this.

José
 
Dale Young said:
Ron,
This is a SC-5 superclock. It shows you what atc sees based on what your encoder is transmitting.
Click to expand...

No, that is an ASC-5A.

An SC-5 has LOCAL/ZULU UP/DOWN and ENGINE for indicators.

What it displays depends on what it is set for (you have the option of the raw encoder value OR the baro-corrected value).

NEITHER of which is what you said "What ATC seess."

ATC sees their computers' idea of what your mode C is corrected to the appropriate altimeter setting.
 
Ron,
Yep. You correct on all counts. I not only forgot the model of equiptment in my plane, I also forgot how it works! jeez....brainfart....
Anywhoo.....Its all working properly and I got it all figured out now!!

Thanks to all who helped along the way!!!
 
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