"Your transponder is showing the wrong squawk"

gil_mor

gil_mor

Pre-takeoff checklist
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gil_mor
Happened to me this morning on VFR XC.
A Beautiful one if I may say so!

Truth be told, I don't usually use get FF on my VFR flights, that's the 2nd time I tried.

ATC: "Squawk 0724"
Me: "0724"
(I set 0724)
ATC: "Squawk Z E R O 724"
Me: "Um, I'm squawking 0724"
ATC: "Are you sure? I'm reading 2724"
Me: "Um, I'm seeing 0724"
ATC: "Oh well, its probably my computer"
Me: "Would you like me to turn it "2" back?"
ATC: "Na, its ok"

I wonder if the problem is on the plane or at ATC

Sure enough, 10 minutes later - "Lost you on the radar, too low, squawk VFR"

Tried it again on the way home, they couldn't see me, and I was at 10,500.
Too bad, I was kinda excited to sharpen my ATC skills.

Any ideas what happened there?
 
That can happen if you've got a bad connection inside the transponder..or to the the altitude encoder.

In my case my encoder was showing me in the flight levels, which was real exciting when I flew down the lake front under Chicago's Class B. ATC only knew it was me because I talked to Meigs tower. Not no more.

The error will be a power of 2 - as in your case a 2 for a 0 (the 2nd bit on) - because the data links and the encoding are BCD - 4 words of 3 bits each to go from 0-7.
 
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What kind of transponder? If it's the older one with rotary switches you may have a shorted contact or wire holding the 2 bit in the on position.
 
mikea said:
The error will be a power of 2 - as in your case a 2 for a 0 (the 2nd bit on) - because the data links and the encoding are BCD - 4 words of 3 bits each to go from 0-7.
Click to expand...

Looks more like binary coded octal to me.
 
There are only 10 kinds of people in the world.
Those who understand binary, and those who dont.
 
Very interesting, thanks.
Its the rotary type, its not my plane but I'll mention that to the owner, who uses FF.

Don't talk to me about bits, I work as a C programer.
I'm surrounded by bits.
Worlds are colliding, you are killing independent George :)
 
mikea said:
That can happen if you've got a bad connection inside the transponder..or to the the altitude encoder.

In my case my encoder was showing me in the flight levels, which was real exciting when I flew down the lake front under Chicago's Class B. ATC only knew it was me because I talked to Meigs tower. Not no more.

The error will be a power of 2 - as in your case a 2 for a 0 (the 2nd bit on) - because the data links and the encoding are BCD - 4 words of 3 bits each to go from 0-7.
Click to expand...
This problem has to be internal to the transponder and the encoder isn't involved. An encoder problem might generate an improper altitude response (Mode C) but cannot affect the Mode A code.

And that problem is either the switch or the wiring/circuitry fed by the switch. First thing I would have tried is spinning the left most switch around a couple turns each direction and then back to zero. If the switch is the problem that might fix it, at least temporarily.
 
Had exactly same scenario last week flying a friend's Bo from MN to KY. No issues for 2.5 hrs. Then had Chicago center ask me if I saw another plane near me. I didn't. Finally told me he was seeing a squawk code one digit off my assigned code. Ultimately recycled transponder and problem went way.
 
TMetzinger said:
There are only 10 kinds of people in the world.
Those who understand binary, and those who dont.
Click to expand...


attachment.php


Gotta get a little Trig in ! :D
 

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I've had a similar thing happen. I was flying around COS one day and the transponder was or-ing 0021 in with whatever I had set. I figured this out with an indulgent controller who would read back my code for various inputs.
 
I bought a Heisenberg sedan. When I looked at the speedometer, I got lost.
 
gismo said:
This problem has to be internal to the transponder and the encoder isn't involved. An encoder problem might generate an improper altitude response (Mode C) but cannot affect the Mode A code.
Click to expand...

Yeah. I knew that, Perfessor. I just didn't make it clear.

I got the lesson from the great avionics guy I found that showed me the test box that read out the output from my faulty altitude encoder with rows of LEDs. I saw immediately how the link worked.

gismo said:
And that problem is either the switch or the wiring/circuitry fed by the switch. First thing I would have tried is spinning the left most switch around a couple turns each direction and then back to zero. If the switch is the problem that might fix it, at least temporarily.
Click to expand...

We might suggest spraying in some (tuner) contact cleaner and working the switch, but we will assume no risk for such action.
 
ScottM said:
A neutron walks into a bar and asks how much for a drink. The bartender replies "for you, no charge"
Click to expand...

Nah, nah, nah, since the new research it goes like this.


"We don't serve your kind in here."

A neutrino walks into a bar.
 
I've seen this problem before, and generally speaking, it will only get worse with time (I'm guessing it's one of the older mechanical transponders like a Narco AT-50 rather than a new digital one like a Garmin GTX-327). Like they said, it's probably a stuck "bit" (I've also seen dropped bits). Sooner rather than later, that transponder is either going to the shop or getting replaced.
 
jsstevens said:
I bought a Heisenberg sedan. When I looked at the speedometer, I got lost.
Click to expand...

Mork: "I got a ticket for going the speed of light in a speed of sound zone. I would have pulled over earlier but I didn't see the lights or hear the siren."
 
gil_mor said:
Happened to me this morning on VFR XC.
A Beautiful one if I may say so!

Truth be told, I don't usually use get FF on my VFR flights, that's the 2nd time I tried.

ATC: "Squawk 0724"
Me: "0724"
(I set 0724)
ATC: "Squawk Z E R O 724"
Me: "Um, I'm squawking 0724"
ATC: "Are you sure? I'm reading 2724"
Me: "Um, I'm seeing 0724"
ATC: "Oh well, its probably my computer"
Me: "Would you like me to turn it "2" back?"
ATC: "Na, its ok"

I wonder if the problem is on the plane or at ATC

Sure enough, 10 minutes later - "Lost you on the radar, too low, squawk VFR"

Tried it again on the way home, they couldn't see me, and I was at 10,500.
Too bad, I was kinda excited to sharpen my ATC skills.

Any ideas what happened there?
Click to expand...

Just as a "what else you could do.."

You could have cycled the transponder or scrolled all the way over back to zero, just as shots in the dark at what might help.
 
You guys are funny.

By the way (I'm sure its been asked before)...

How many Programers you need to change a light bulb?
 
Skylane81E said:
That's a hardware problem
Click to expand...
There you go!

Which might explain why I didn't mess with the transponder to begin with.

Although muscle memory from my experience with PC's should have kicked it and I should have power-cycled it.
 
Power cycling would probably not have fixed it, if it's set with rotary or thumbwheel switches. Rolling the offending switch a few times might have helped, though. You can get a little oxidization or schmutz on the contacts, which may be at least temporarily made less of a problem by the wiping action of the switch contacts.

Wafer and thumbwheel switches are expensive to manufacture and prone to maintenance issues, which is why you don't see them much on stuff made in the past 15-20 years.
 
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