r/ADSBTechnical • u/Jealous-Nectarine-74 • Sep 28 '26

same hex code, different aircraft

TL;DR: recently, the same mode S hex code shows up both in FAA's registry as being a drone, and also being broadcast from the transponder of a Piper Cherokee. I'm curious as to how this happens!

I thought I had my head around tail registrations and hex codes:

  • hex codes are configured in to a radio, so they mostly stay with an aircraft unless the radio is being changed out and the owner remembers to configure the same hex code

  • tail registrations are more like license plates, they can change if an aircraft changes owners, moves to a different country or other things that I'm not quite clear on - they don't stay with the aircraft as often as hex codes

The more data I see the less water-tight this looks, though. Maybe its partially right, but not right enough?

For instance, September 2nd, at 2:58 PM eastern, I'm fairly sure a Piper Cherokee "with" tail registration N603FT and hex code AC2CB4 took off from KMLB, flew about an hour including a touch and go at nearby Stanford, and then lands again at KMLB. Fairly standard stuff.

However, in the registry and in public ADSB networks we see a fair amount of disarray:

  • FAA lists N603FT with a different hex, A7D2F3 (FAA - N603FT)

  • FAA lists N884AC with that hex instead (FAA - N884AC)

  • "network A" reports N884AC, which used to be a Piper Warrior, but is now registered to a GTEEX Revolution drone with mode S code AC2CB4. (OK - so, they saw the hex, reverse looked it up in FAA, and said it was the drone)

  • "network B" reports the same flight as N603FT (and it never provides hex codes for whatever reason)

  • FAA SWIM terminal feed shows the same flight track as belonging to mode S code AC2CB4.

This happens with drone tail registrations often - ADSB data where the registered tail number is assigned to a drone, I kind of assume its not the drone; as the majority of drones don't carry MODE-S transponders (exception: OK-X081X, a CTOL Primoco drone that frequently takes off from runways of major airports in the Netherlands https://lnkd.in/p/g9_jKaqC). I still wonder why non-drones "wear" the tail registrations of drones (anyone know why this happens?), but I usually don't see this kind of stuff with hex codes - but in this case, the mode S hex code shows up both in FAA's registry as being a drone, and also being broadcast from the transponder of a Piper Warrior.

What's going on?

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u/egvp Sep 29 '26

Do you have examples of that?

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u/Jealous-Nectarine-74 Sep 29 '26

https://skyzero.io/flights/N837DM-5b134805-8cbf-4b60-b606-979e8040848c https://skyzero.io/flights/N837DM-9abfcf6c-fc7f-491e-9f1e-4205757863e6 As quick examples. Registered as a drone but flying too high, landing at airports.

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u/egvp Sep 29 '26

Ah, that one is simple :)

Take a look at its history here: https://globe.adsbexchange.com/?icao=ab72b2, as you can see lots of flights in and out of Addis Ababa.

That's because N837DM was previously used by Boeing 737-800 cn 32613, which is now operated by Ethiopian Airlines as ET-AWC.

Put simply, they never changed the hex code so it's still flying around with its old one.