Primarily for general aviation discussion, but other aviation topics are also welcome.
By Mike Charlie
Reading through the Texel thread reminded me that when we flew Stockholm return via Ostende in 2018 air traffic asked if we were Mode S equipped. Only Mode C, so technically they shouldn't have given us clearance to transit their controlled airspace, but being a quiet period they did, it was much appreciated.

With other European destinations planned this year, planning to install Mode S, two questions,

1 - Is this an additional transponder or will a Mode S unit also have mode C so existing can be swapped out?
2 - Recommendations please on equipment brand and a competent avionics company to install.

Suffolk based, thanks in advance
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By alexbrett2
You only ever have one transponder on an aircraft*, so upgrading to a mode S capable unit will be a replacement of your existing transponder (and the new unit will emit modes A, C and S).

Probably the two most well known brands are Garmin and Trig, though you also see Bendix King (though their KT-74 is actually just a Trig TT31 with a different frontplate). Whatever you do, I would recommend getting a transponder that is ADS-B out capable, even if you don't have that set up straight away, so as to future proof you.

For my avionics I always use Bournemouth Avionics ( based at Lee-on-Solent, as Brian has always done good work for me etc.

* You can (or at least may be able to) have a mode S transponder and a separate ADS-B transmitter, but that's a different issue
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By Paul_Sengupta
1) Swapped out

I've recommended the Bendix King KT-74 on here before. It's a Trig unit with a decent push button interface. It's ADS-B "out" capable I believe.

I might sell my GTX-330 (not installed yet, sitting in my spare room!) if you're interested! :-)
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By irishc180
I am doing avionics upgrade at the moment. The ADSB-Out/In thing is far from clear. You could end up spending money on something that does not have diversity i.e. two antenna's and will get you no further. For my money;

- Cheap used GTX-330 Non ES is about $450 on Facebook, get it bench tested locally

Then Skyecho 2 with Ipad, and Zaon PCAS with Garmin Handheld for EC. Others will do different things. I have not been convinced by anyone else that I need to do more or will gain more operational capability by spending more.
By riverrock
A "Mode S" transponder also does Mode-A and Mode-C - so as above, its a swap out.
If you aren't looking to output your GPS position to other units, some models are literally swap in / out with their Mode-C versions (can use original rack, altitude encoder and connectors). If you say which model you have, others could advise.

If you are looking for additional functionality - such as ADS-B out via Extended Squitter (so Mode-S ES) - then you need to provide GPS location data to the transponder. ATC doesn't currently require this for anything smaller than airliners, but it does allow aircraft without TCAS to know your location if they buy extra equipment.
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By neilmurg
I think it would be short sighted not to get a Mode S-ES unit, if you can (and have a GPS source in your panel), have it connected up so you can emit ADS-B.
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By chevvron
I honestly don't know how many radar units have access to Mode S info rather than just A and C, but as Mode C shows altitude readout, I dare say some aren't too bothered if you don't have Mode S.
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By irishc180
neilmurg wrote:I think it would be short sighted not to get a Mode S-ES unit, if you can (and have a GPS source in your panel), have it connected up so you can emit ADS-B.

The reason I'm not doing it, call me short-sighted, is that the ADSB mandate applies to A/C over 5700kgs, and require ADS‑B 1090ES “Out” with diversity. Thus a Garmin GTX330D ES which is roughly 7 times the price for something I don't need right now. If it becomes mandatory I'll do it, but operationally I will get no benefit.
By condor17
MC , don't forget you'll probably be able to sell the mode C unit for good $ in the US .

rgds condor .
By Mike Charlie
G-JWTP wrote:MC,

What aircraft are you fitting it in?


Well two in fact, my 1965 Cessna 150 G-ATMC and 1943 Percival Proctor G-AKEX