Transponder giving the wrong altitude

[Challenged]

I went to fly home yesterday and while I was waiting to depart, the controller had me recycle my transponder, as it was showing me at 200 ft (sea level airport); recycling it didn't fix the problem. I took off and noticed that my Monroy traffic device was reporting my transponder altitude at 300 feet less than the altitude that my altimeter and GPS were indicating. The transponder is a Garmin GTX320 I believe, although the pictures I've seen online of other GTX320's indicate that it has a "Test" mode, that I don't believe mine has. Do I need a new blind encoder, or is there an adjustment for this?

 

[roncachamp]

I went to fly home yesterday and while I was waiting to depart, the controller had me recycle my transponder, as it was showing me at 200 ft (sea level airport); recycling it didn't fix the problem. I took off and noticed that my Monroy traffic device was reporting my transponder altitude at 300 feet less than the altitude that my altimeter and GPS were indicating. The transponder is a Garmin GTX320 I believe, although the pictures I've seen online of other GTX320's indicate that it has a "Test" mode, that I don't believe mine has. Do I need a new blind encoder, or is there an adjustment for this?

A Mode C report 200 feet different than what your altimeter indicates is within limits for ATC purposes, 300 feet or more off is no good.

 

[Old Geek]

You know that your blind encoder always reports altitude at standard pressure, right? Set you altimeter to 29.92 and see how close it agrees with the transponder altitude reported by the Monroy.

It could also be slight miscalibration plus cold weather. AK-350 encoders and most others do have multiple adjustments, but it's not something you want to mess with unless you have the equipment and manuals to know what you're doing.

 

[poadeleted20]

If it happens only once with only one ATC sector, it's probably them. If it happens consistently with many ATC facilities and the same sort of error, it's probably you. If it's them, don't worry about it. If it's you, take it to the avionics shop for troubleshooting and repair.

 

[Dan Thomas]

Clean the antenna. A dirty or oily antenna can mess up the Mode C.

Dan

 

[Capt. Geoffrey Thorpe]

I went to fly home yesterday and while I was waiting to depart, the controller had me recycle my transponder, as it was showing me at 200 ft (sea level airport); recycling it didn't fix the problem. I took off and noticed that my Monroy traffic device was reporting my transponder altitude at 300 feet less than the altitude that my altimeter and GPS were indicating. The transponder is a Garmin GTX320 I believe, although the pictures I've seen online of other GTX320's indicate that it has a "Test" mode, that I don't believe mine has. Do I need a new blind encoder, or is there an adjustment for this?

How long was it on? It can take time for the encoder to warm up and indicate correctly.

 

[Challenged]

It was on for a while, probably 10 minutes or so before the controller mentioned it to me. I'll check the antenna the next time I'm at the hangar..thanks for that suggestion.

 

[Ghery]

I had ATC remind me that I was cleared to an altitude significantly higher than my mode C was squawking. An altitude my altimeter claimed I had reached some time previously. No biggie, killed mode C and flew on home reporting altitudes for ATC. Had club maintenance get it fixed. Get yours checked out and all will be well.

 

[Geico266]

Story Time.

I picked up an RV in Oregon and fly it to NE. Turned the transponder to "Alt" from "ON". Long story short it was reporting 31,000'

Needless to say ATC wanted to talk to me when I landed. I called center and they asked what the hell are you flying that goes to 31,000' and only flies 180 MPH? No I don't do flight following when the weather is good.

We got it fixed.

 

[gismo]

Clean the antenna. A dirty or oily antenna can mess up the Mode C.

Dan

An antenna problem could result in the lack of mode C reception by ATC but it shouldn't cause an error in the reported altitude.

 

[gismo]

You know that your blind encoder always reports altitude at standard pressure, right? Set you altimeter to 29.92 and see how close it agrees with the transponder altitude reported by the Monroy.

It could also be slight miscalibration plus cold weather. AK-350 encoders and most others do have multiple adjustments, but it's not something you want to mess with unless you have the equipment and manuals to know what you're doing.

Some of the passive traffic detectors have an internal pressure sensor that adjusts the reported transponder altitude readout. Obviously that's only using cabin pressure which could be 50+ feet off in the air but not 300 ft off on the ground. I think some of the later Monroy units have this feature.

 

[DavidWhite]

Story Time.

I picked up an RV in Oregon and fly it to NE. Turned the transponder to "Alt" from "ON". Long story short it was reporting 31,000'

Needless to say ATC wanted to talk to me when I landed. I called center and they asked what the hell are you flying that goes to 31,000' and only flies 180 MPH? No I don't do flight following when the weather is good.

We got it fixed.

Had that happen to me when I was renting in a 172 in the practice area. The 330 said I was at 34,000'. I had like 15 hours and just kept on flying around.