Btw the xponder had no discrepancies in recent 91.411 & 413 checks
We’re dealing with an aircraft → flight plan association problem of certain ATC units.
There are multiple ways to associate a transponder return with a flight plan:
1. Mode S flight ID (the standard in Europe)
2. Individual octal transponder code (squawk 1234)
3. Transponder hex code (Mode C and up) when mentioned under CODE/ in the flight plan
4. Ident and marking on the radar screen with semi-manual following (tedious)
The ATC unit could make use of both 1 and 3 and use 3 as a fallback. It is possible that a transponder has a problem and the Mode S response cannot be read by certain installations in all cases. The analog circuit tends to shift frequency (very common with Funkwerk/Filser). There could be a hardware problem or a software problem that requires a firmware update.
PS: I have passed 3 annual transponder checks with a transponder that was clearly not airworthy. These checks highly depend on who is doing them. It was when I did the test with Jesse from JP Avionics (unfortunately no longer permitted on this forum) when I learned that it was totally off and got it fixed on the spot. So these checks might not necessarily be trustworthy.
JasonC wrote:
Unless you are N-reg and squitting ADS-B then it is also required.
In the US, it is not required, but is requested. Not many aircraft comply.
Michael wrote:
How can something be wrong with the xponder if all is ok with a discreet squawk ?
It might not correctly transmit the aircraft callsign. That would not show up with a discrete squawk as the ATC system will have an association between the discrete squawk and the callsign.
It might not correctly transmit the aircraft callsign.
That would not show up with a discrete squawk as the ATC system will have an association between the discrete squawk and the callsign
Uh, sorry, but I fail to see how both could be true
Michael wrote:
Uh, sorry, but I fail to see how both could be true
If a discrete squawk is assigned the flight plan can be associated to the flight without reading the Mode S flight ID (basically falling back to operation as a Mode A/C transponder). If there is no discrete squawk, only Mode S specific ways could be used to identify the aircraft:
- the flight ID (callsign)
- hex code
As it was stated may times, the latter is not the usual way to identify mode S radiating aircraft by European ATC, but if for some reason the flight ID is not correctly set / radiated, they could fall back to using the second. This fallback could be automatic (and it probably is in case of the units you listed).
Ok then that means the ATC controller is manually linking the reg to the FP rather than picking up the xponder codes ?
Michael wrote:
Ok then that means the ATC controller is manually linking the reg to the FP rather than picking up the xponder codes ?
a) In case of the Mode A octal code (0000 – 7777), which coexists with Mode S, the system assigns a code to the flight plan, the code is communicated to the pilot (usually via VHF voice), the pilot sets it and the system shows the flight plan info next to the target.
b) With Mode S, there is the flight ID (in case of small GA, same as REG), which is transmitted by the transponder to the ATC. In case of a nondiscrete squawk, this is the primary method of identifying the target and assigning it to the filed flight plan. This associating is usually done automatically, without controller intervention.
c) With Mode S, there is a second option, the hex code. This is always specific to the aircraft, does not change between flights, but for most countries it is not possible to get the call sign / registration from this code without inquiring the registration authority. Therefore ATC computers can only use this option if the flight plan has the CODE/ section specifying the hex code belonging to the aircraft.
Since you were not having problems when a discrete squawk was assigned, we can assume that the octal code was transmitted properly. OTOH, you were having problems when on a nondiscrete code and not having the CODE field in your flight plan. This suggests that the underlying problem was/is that method b) could not be used, i.e. your flight ID was not set or radiated properly.
The CODE field is never used as a primary means of identification and is only used as a secondary means of identification for Mode S transponding aircraft by some ATC sectors / systems.
Since you were not having problems when a discrete squawk was assigned, we can assume that the octal code was transmitted properly. OTOH, you were having problems when on a nondiscrete code and not having the CODE field in your flight plan. This suggests that the underlying problem was/is that method b) could not be used, i.e. your flight ID was not set or radiated properly.
Thank you for the very concise explanation.
In our case the hex code & tail number (reg) are both in the xponder parameters, but flight ID is not used .
unfortunately no longer permitted on this forum
There was a very good reason for that, in case somebody wonders, or got some “info” behind the scenes. We do not allow offensive posts, against anyone (even a mod/admin).
