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Anybody have installed dual transponders on their plane ?

If your transponder fails it becomes difficult to fly and to come back home. Is it worth to have a second transponder installed as a backup?


Depends on how much you fly in airspace that requires it. Most high end GA aircraft have two as they fly predominantly in class A or C airspace and losing a transponder means you are asked to leave.

Last Edited by JasonC at 05 Aug 23:04
EGTK Oxford

I had a transponder failure and flew airspace C in Germany under IFR without problems. ATC just asked me to confirm my altitude every few minutes. I don’t think it would be much a problem to deal with a transponder failure.

The transponder in one of our aircraft actually failed on the last leg back from Slovenia/Italy a few weeks ago. The encoder failed so it started to send out altitudes the were between 1000 to 4000 ft higher than the aircrafts position. That freaked ATC.

When they reached Sweden (with mod A) they were denied clearance in controlled airspace, so they had to take a more easterly heading while descending before getting back on track towards our airport.

What ATC does if they can not get a (good) transponder signal depends on the traffic situation and their workload. Obviously, they have no obligation to accept you inside transponder mandatory airspaces without a working transponder.
I have experienced both options over the years: Once, on an IFR training flight departing our homebase we were denied airspace access and had to proceed VFR. Since that transponder failure was the first sign of a total electrical failure it turned out that being forced to stay VFR right from the beginning was actually the best that could have happened to us…
While already enroute, we were allowed to continue to destination on two or three occasions with a failed transponder. But, IIRC, these were short/ish domestic flights.

EDDS - Stuttgart

Would it not be about time to start using GPS altitude? If it is precise enough to avoid flying into rocks on curved approaches, surely it ought to be good enough for en-route separation. Do away with (two) expensive altimeters, and with the failure prone altitude encoder in the transponder.

martin-esmi wrote:

When they reached Sweden (with mod A) they were denied clearance in controlled airspace

IFR or VFR? I would be very surprised if Swedish ATC would dump an IFR flight where mode C failed enroute. In fact, if they did that to me, I would file a formal complaint.

ESKC (Uppsala/Sundbro), Sweden

I cannot believe ATC would do that to IFR traffic, and anyway any pilot with an IQ over 2.5 would refuse (captain’s privilege) unless the conditions were CAVOK from SFC to outer space.

And even if they were CAVOK, I would just lie (“unable to comply due to icing conditions below”) because everybody knows the lower airspace is almost totally empty of traffic once away from the obvious terminal areas, so having a primary-only radar contact which actually tells them he is at FL150 is not a safety issue.

They have to believe the pilot’s reported altitude, because any aircraft could trivially fake its Mode C altitude simply by wiring up the transponder’s gray code input to a rotary encoder

Almost no light GA plane carries two transponders. I can think of only one plane which has/had that – an old TB20.

Shoreham EGKA, United Kingdom

Almost no light GA plane carries two transponders.

I wouldn’t say that. When Mode S became mandatory around here, many aircraft owners (and flying schools) simply left the old Mode C transponder in place, just in case. With an “Inop” placard next to it, so that there will be no complaints upon the Anzahl inspection.

EDDS - Stuttgart

With an “Inop” placard next to it, so that there will be no complaints upon the Anzahl inspection.

Can you still do that? When I was doing my PPL, 15 years ago, this was common, but I heard it isn’t allowed anymore (by EASA).

It’s a very interesting point though… anybody thinking about an avionics refit ought to consider this. You also need two antennae.

BUT IMHO you are more likely to get an encoder failure than a transponder failure. I saw FL520 the other day on mine, for a second or two… here we go again.

Shoreham EGKA, United Kingdom
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