Transponder Problem

[150 Driver]

This is a slightly academic post as I'm shortly going to be upgrading my Narco AT150 Transponder to a Mode S model.

But the following is puzzling. I have flown over quite a large area of the UK in the last couple of years, always squawking when asked and never having a problem. But when in the vicinity of Southend I always have a transponder issue - occasionally it squawks an incorrect code, more likely it gives a negative altitude !! I always try recycling which more often than not cures the code issue but not the altitude.

My maintenance organisation has looked into it, tested the transponder and can find nothing wrong with it. Has anyone come across anything like it ?

[Mach Jump]

I used to fly a Cessna 150 with a similar problem, only with Humberside.

At the time, Humberside was the only radar in the area that derived it's SSR from a remote head, from where it was transmitted by phone line to LATCC where it was processed and sent back to Humberside, again by phone line.

Never really got to the bottom of it, but we put it down to the combination of an old transponder producing data incompatible with the transmission and processing involved in the new system.


MJ:ok:

[150 Driver]

MJ, thanks, maybe I should fly to Humberside to see if I get the same problem!

The explanation makes sense.

[9 lives]

Decades ago, Toronto radar would tell me that I had altitude or code errors. Several times I took the transponder, then the whole plane to the avionics shop at Toronto, and each time it was pronounced perfect. It turned out that the problem was entirely within the ground based system, but the controllers were not really honouring up on that. It was a rather expensive learning exercise for me, three times to the avionics shop.

So, I got it fixed, checked on the plane for accuracy, had the certification, and if ATC told me that there was an error, I told them there was not, and I had the certification to prove it. Never a problem after that - same txp, for 26 years since, not a blip....

[thing]

FWIW I was flying back from mainland Europe the other year and Southend passed me onto Manston who thought I was at 7,500 on the transponder when I was at 3,000. Nothing wrong with the transponder.

[flybymike]

We currently have an infuriating intermittent problem with our Trig TT31.

On take off it usually fails to show any squawk at all, but then after about 15 minutes flight, operates normally and completely accurately for mode C readout etc. Then maybe after switching a code it goes u/s again , only to operate normally again after a short while.

We've had two shops look at it only to report "no fault found on test."

Short of renewing the entire installation and unit I've given up hope.

Any bright ideas much appreciated!

[Shoestring Flyer]

flybymike

I would send the unit back to Trig. They are only in Edinburgh and are very helpful. When I had a problem with my TT31 I got it back in 72hours.

[Whiskey Kilo Wanderer]

Yesterday while wandering around the south of England I heard Southend having fun with someone’s transponder.

SND: “Please can you set your transponder to Standby? It’s showing an altitude of over 50,000 feet.”
Aircraft: “Wow, that’s good for a Cessna 150….”

[flybymike]

Thanks for that shoestring. May well send it back to them if the latest attempt doesn't solve the problem

[Rod1]

flybymike- Assuming you continued to use your old encoder, that may be the issue. The very old units take 10 - 15 min to literally warm up and as they age it gets more like 13 - 17min. Before this the encoder could be sending garbage and upsetting the trig. Does the altitude appear on the trig screen as normal?

150 Driver

That Narco had a mandatory fix which can cause it to fail to work with some of the new radar systems - what transponder are you planing on fitting to replace it? I would strongly recommend one that supports Extended Squitter out of the box.

Rod1

[flybymike]

Thanks for that Rod.

Alas Ive No idea whether it's the old encoder or a new one that was fitted with the box (about four years ago). It's operation is completely unpredictable and it still stops squawking, even after it has been operating normally for quite some time. Frequently it seems to go on the blink with a change if squawk code, but sometimes it just goes on and off in a random fashion. All indications on the unit itself are entirely normal inc flight level readout. When it is working properly cross checking with ATC shows correct altitude and normal returns!

[patowalker]

I had an intermittent problem with my transponder and it was solved by cleaning the antenna connections. Easy to do the TNC on the box, but a bit more difficult to access the BNC on the stub antenna.

[flybymike]

Yes thanks, we are going to do the antena connections next.

[150 Driver]

Whiskey Kilo Wanderer, I was airborne yesterday in the Southend area (about 15.00 local) but the transcript you've quoted wasn't me, I was minus 400 feet and showing the wrong code.

I know some would say that flying brings forward the risk of being 6 feet under but 400 !!

Odd (or perhaps not) that it happened twice in the same day with the same aircraft type.

Rod 1, I haven't decided yet, other than that it is going to get changed for something later this year.

[Helicopterdriverguy]

I have a Narco remote encoder and a TT31, and i've never had any of the issues described, i must be one of the lucky ones :ok: