Mode S? Registration encoded into mode C?

Pilot_DAR wrote:

In southern Germany yesterday, and Austria and Italy today, I had controllers telling me that the transponder return was simply “N”, where the registration should be displayed to them.

Do you have an incorrectly configured mode S transponder or a mode C transpoder? The former should be a big no-no, the latter is only a problem if you are flying into airspace that requires mode S (e.g. in Germany IFR requires mode S, VFR requires mode S in class C or D, or anywhere above 5000 ft MSL/3500 ft AGL). I have recently went through all the available AIPs to check mode S requirements, so I can provide you with a table. I may have missed some requirements buried in obscure places, however.

Will a radar/information controller hold me up for it?

They may. I heard about a recent case of a plane being refused to enter Slovakian airspace with an incorrectly configured mode S transponder, even when it was flying to the avionics shop.

This plane will become LN registered within a month or two, so I suppose during that process, magic faeries will correctly configure the transponder output one misty night?

The new authority should select a mode S code for the aircraft during the registration process and that code should be programmed into the transponder. ATC will then see the registration of the plane no matter what the mode C code is. I can confirm that this works overnight.

@Pilot_DAR, there is no registration information whatsoever in mode C returns. In mode S, you have two relevant pieces of information. One is a unique 24-bit address, which can be used to identify the aircraft, but I doubt any ATC entity uses that. The other is a textual call sign, which can be programmed directly from the front panel. Just as with spoken call signs, it can be your registration number (the usual case for GA), the flight identifier (the usual case for airlines), or any other call sign routinely used by the operator (some airlines, state agencies, special flights, etc.). Maybe the installer forgot to program it. If you tell me what model your transponder is, I may be able to send you relevant pages from the manual to reprogram it.

Ultranomad wrote:

Maybe the installer forgot to program it. If you tell me what model your transponder is, I may be able to send you relevant pages from the manual to reprogram it.

Since the call sign can change from one flight to the next, it should be easily configurable by the pilot. On the G1000 that is done on the Timer/V-speeds page IIRC.

I don’t think there is a transponder in a GTN750. You must have some sort of remotely mounted one. @Jesse or @Wigglyamp might know.

I can get installation manuals for most things but need to know what it is.

A quick google suggests it may be a GTX32. I have emailed you the IM for that. However, on a quick look, it does not discuss the configuration, so for that you would need the GTN750 IM. I have that too but it is 30MB. I have put the STC IM on dropbox. Check your email… Predictably it is a massive manual.

Peter wrote:

I don’t think there is a transponder in a GTN750. You must have some sort of remotely mounted one.

Correct.

Peter wrote:

A quick google suggests it may be a GTX32.

This can not be correct, as GTX-32 is mode C only, and doesn’t broadcast Flight ID.

Pilot_DAR wrote:

I was responsible for the plane when the GTN750 was first installed, and I’m unaware that the registration was programmed in.

Did it have a mode S transponder at that time, or did you have another transponder? Per default the flight ID is on N only, so it seems the installer didn’t configure the unit correctly when installing this transponder.

On the GTN go the transponder page. There it will show an Flight ID box. Change this box to your actual registration, without – sign if applicable.

If this box is not available, it should be done in the configuration menu.
Have this sorted out, and don’t fly around another two months like that. Also be sure that the 24 bit ICAO address is ok,.

Aviathor wrote:

Since the call sign can change from one flight to the next, it should be easily configurable by the pilot. On the G1000 that is done on the Timer/V-speeds page IIRC.

This depends on the kind of operation. On normal private GA the flight ID and ICAO address never change other then when the aircraft is registered in another country. The installer has therefore the ability to remove this option from the menu for use by the pilot. For private GA I always set this setting as configuration mode only, e.g. a pilot would not be able to change this.

It is quite common to found aircraft moving from country to country with this option available to the pilot, to find that the flight ID has been changed for the new registration, and the ICAO address is still on the old address, which is not good.
Offtopic, on ELT’s I sometimes see an ELT where the registration marking on the placard is changed, without the registration marking inside changed. This is also dangerous.

Aircraft on the N registry have a one to one algorithmic relationship between the 24 bit ICAO ID and the N number. Many transponders will automatically enter the ICAO ID from the entry of the N number or visa versa. Other countries the registration number is assigned and not related to the ICAO ID, in which both values must be entered. Flight in the US will not be prohibited with an incorrect or invalid ICAO ID/registration, but you are very likely to get a letter from the FAA to get the issue resolved prior to further flight. ATC can see if there is a mismatch, as the target gets tagged.

Aircraft on the N registry have a one to one algorithmic relationship between the 24 bit ICAO ID and the N number

We have done this before (e.g. search on “24 bit”, with the quotes) but there is not AFAIK any evidence that any European ATC has the capability to use the 24 bit code to look up the aircraft reg. The N-reg conversion is easy while most others need a database.

AFAIK the lookup has been implemented by Avidyne in some glass cockpits (maybe @avijake might confirm?) and I saw it in a Cessna (Lancair) 400. It showed the tail numbers of N-reg planes, only.

So I think what Pilot_DAR got was a mismatched transmission of the tail number itself, not anything to do with the 24 bit code. That should be easy to configure, once you know how to get inside the GTN750 config.