Menu Sign In Contact FAQ
Welcome to our forums

Anybody have installed dual transponders on their plane ?

I found this bit of text on one of the many Eurocontrol websites regarding ATC handling of transponder failures: “PANS-ATM provides some guidance but no specific procedures, these tend to be ANSP-specific. If primary radar is available, flight plan correlation should be maintained, however the availability of primary radar varies between States. In the absence of primary radar, steps taken can range from voice reporting by the pilot to enable procedural control by the ATCO, to military escort, or in extremis refusing the aircraft entry to the next sector/FIR or returning the aircraft to the airfield of departure/nearest suitable airfield. The aircraft should also be cleared out of RVSM airspace and be kept well clear of other aircraft.

Can you still do that?

Obvoiusly some people get away with it… It all depends how well you know the guy who signs the paperwork.

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.

That’s true. With modern solid state transponders, it is more often the encoder or it’s connection that fails than than the transponder itself. Therefore dual installations that share one common altitude source don’t make too much sense. But an altitude encoder for light aircraft use (up to 25.000ft) is not really expensive.

EDDS - Stuttgart

IFR or VFR? I would be very surprised if Swedish ATC would dump an IFR flight where mode C failed enroute.

I do remember the Norwegians did deny entry into their airspace to an aircraft without mode C return. I believe it was this one:

28 September 2013 outbound from Canada Transponder failure: LOT Polish Airlines Boeing 787-800, registration SP-LRA performing LOT42 from Toronto back to Warsaw climbing out of Toronto when crew halted their climb at FL270 due to transponder failure, then decided to continue towards Warsaw at FL270. About 130 nm west-southwest of Keflavik her crew diverted to Iceland.

But in this case we are talking about RVSM airspace.

EDIT: Correction: this was not even RVSM airspace. Hence stop climb.

Last Edited by Aviathor at 06 Aug 11:40

I can see being denied entry into CAS which would just be bad luck if the TXP packed up during that usually brief period. OCAS, the pilot has basically no rights, short of a life threatening emergency. His only option may be to remain OCAS and eventually land somewhere. In practice they are better but I can see ATC not letting you in.

But to be evicted from CAS while enroute is a different thing and the PIC could and should refuse.

Didn’t the 787 have two transponders? All airliners have two.

Shoreham EGKA, United Kingdom

Airborne_Again wrote:

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.

IIRC they flew VFR, so I understand ATCs decision. It wasn’t a big deal but for the pilot but still, an unexpected detour required.
But say I would fly IFR to an uncontrolled airport (Y-flightplan), they could radar vector me outside controlled airspace without dropping the IFR clearance?

Peter wrote:

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.

In this case we arrived from north/nw Germany and the southern Swedish airspace is not busy, but they do have a continous stream of flights dep/arr to Copenhagen and Sturup at peak times at lower altitudes.

Last Edited by martin-esmi at 06 Aug 14:37

Shorrick_Mk2 wrote:

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.

This comes up from time to time, but there can be a substantial difference between the altitude reported by a barometric altimeter and a GPS altitude. Although the GPS altitude is generally more accurate than the barometric altimeter, unless the entire fleet transitioned to GPS altitude, it would not be usable for vertical separation between aircraft. Also, the GPS altitude is not as accurate outside the service volume of SBAS and there are substantial periods of time, some of them relatively short in duration, say a 15 minute period, where the satellite geometry or magnetic storms from solar flares (longer duration) would render the system unusable. I seriously doubt this will happen in my lifetime. Differences from Baro and GPS at cruise altitudes under 10,000 can exceed 1000 feet on cold or warm days. I routinely see differences of 400 feet or more here in the US cruising at 8,000 MSL with WAAS GPS altitude and my transponder pressure altitude. Higher altitudes have larger differences. The differences are primarily a result of temperature and pressure variations in the barometric altimeter, but since a pressure altitude is referencing a constant pressure datum, all aircraft are affected to the same degree, IOW we fly at a constant pressure, not a true altitude. The GPS is much closer to a true altitude and is not affected by pressure or temperature.

KUZA, United States

The last 4 times I’ve flown over the Cairo city center (FL110-FL120 typically), all my GPS receivers including mobile phone went out. This was directly next to the main airport of Cairo. I presume it’s related to jamming to prevent GPS guided missiles from hitting specific government targets but that’s only speculation.

Not so easy to jam barometric altitude

achimha wrote:

Not so easy to jam barometric altitude

I beg to differ, weather does that all the time

LSZK, Switzerland

what_next wrote:

what_next 06-Aug-15 10:35 #09
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”

I dont know but I have an N reg plane with 2 transponders, one S and one C for the very reason stated earlier. And it s not placarded because it is used as a backup.


Digging up this old thread…

If one installed a second transponder, how would that interface to an existing TAS system? Or would it be just entirely separate? Presumably it would be turned off until the main one fails. But in an airliner it can’t be that simple, otherwise they would lose TCAS if they switched to the backup transponder.

Shoreham EGKA, United Kingdom

I had not seen this thread before.

I operate two aircraft. In both cases they had non-ES GTX 330s and in both cases they had avionics refits which included changing to a GTX33xES (this was before the 345 existed.)

So, in both cases, I had a spare GTX330 and little will to put it on eBay, so in both cases I kept it as a spare.

In one aircraft, I had it configured up for the hex code and reg and keep it behind the pilot’s seat wrapped in bubble wrap. The plan is that if the new transponder fails, I’ll just remove it and put the old one in the same rack space.

In the other aircraft, the new transponder was a remote GTX33ES, so I had the GTX330 wired up as a spare. If the main fails, I just have to change the configuration in the GTN (5 secs) and I have a working Mode S transponder again.

To answer @Peter’s point, I believe that there would be no effect on TAS at all. All that would happen is that I would stop squittering ES data, so would be invisible to ADS-B.

EGKB Biggin Hill
Sign in to add your message

Back to Top