Apparatus

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sensors Mode-S transponder

A8C121

ICAO 24-bit transponder address · allocated to United States

Currently assigned to N663UA · BOEING 767-300 · operated by UNITED AIRLINES INC

Hex
A8C121
Octal
52140441
Decimal
11,059,489
Country
US
Tail
N663UA
Type
B763
No current signal · last broadcast 1h ago
Flights observed
223
Distinct callsigns
20
Country
US
Last broadcast
1h ago

Last known position

Most recent position reported by Mode-S A8C121.

Country allocation

The ICAO 24-bit address A8C121 falls within the block assigned to United States (US) by ICAO Annex 10, Volume III. Every aircraft registered in United States receives a permanent address from this block at the time of registration; the address stays with the airframe even if the aircraft is re-registered to a different operator within the country.

Callsign history 20 distinct

Callsign Airline Flights Last seen
UAL928 United Airlines 12 2026-06-06
UAL920 United Airlines 11 2026-06-11
UAL941 - 10 2026-06-29
UAL958 - 9 2026-06-27
UAL956 - 8 2026-06-24
UAL934 United Airlines 8 2026-06-18
UAL929 - 8 2026-06-28
UAL134 United Airlines 8 2026-05-12
UAL959 United Airlines 7 2026-06-10
UAL957 United Airlines 7 2026-06-25
UAL938 United Airlines 7 2026-06-07
UAL904 - 7 2026-07-13
UAL17 - 7 2026-07-13
UAL16 - 7 2026-06-26
UAL146 United Airlines 7 2026-06-06
UAL964 - 6 2026-07-13
UAL3 United Airlines 6 2026-06-16
UAL273 United Airlines 6 2026-05-23
UAL15 United Airlines 6 2026-06-07
UAL135 - 6 2026-06-17

Recent flights last 50

DateFlightRouteTakeoffLandingDuration
2026-07-14 UAL965 - Jul 14, 08:05 UTCJul 14, 10:15 UTC2h 10m
2026-07-13 UAL964 - Jul 13, 21:00 UTCJul 14, 05:00 UTC8h 0m
2026-07-13 UAL17 LHR → EWR Jul 13, 10:25 UTCJul 13, 17:35 UTC7h 10m
2026-07-13 UAL17 LHR → EWR Jul 13, 09:40 UTCJul 13, 17:34 UTC7h 54m
2026-07-13 UAL904 - Jul 13, 00:20 UTCJul 13, 07:05 UTC6h 45m
2026-07-12 UAL965 - Jul 12, 08:20 UTCJul 12, 17:30 UTC9h 10m
2026-07-11 UAL964 - Jul 11, 20:40 UTCJul 12, 04:25 UTC7h 45m
2026-07-11 UAL381 BRI → EWR Jul 11, 07:53 UTCJul 11, 17:45 UTC9h 52m
2026-07-11 UAL381 - Jul 11, 08:05 UTCJul 11, 17:55 UTC9h 50m
2026-07-10 UAL380 - Jul 10, 21:05 UTCJul 11, 05:15 UTC8h 10m
2026-07-10 UAL380 EWR → EWR Jul 10, 20:35 UTCJul 10, 20:40 UTC5m
2026-07-10 UAL306 - Jul 10, 08:05 UTCJul 10, 17:20 UTC9h 15m
2026-07-10 UAL306 SPU → EWR Jul 10, 08:05 UTCJul 10, 17:26 UTC9h 21m
2026-07-09 UAL305 - Jul 9, 21:05 UTCJul 10, 04:45 UTC7h 40m
2026-07-09 UAL381 BRI → EWR Jul 9, 08:23 UTCJul 9, 17:36 UTC9h 13m
2026-07-09 UAL381 BRI → EWR Jul 9, 08:23 UTCJul 9, 17:32 UTC9h 9m
2026-07-09 UAL381 - Jul 9, 08:05 UTCJul 9, 10:15 UTC2h 10m
2026-07-08 UAL380 - Jul 8, 20:45 UTCJul 9, 04:55 UTC8h 10m
2026-07-08 UAL3920 - Jul 8, 02:45 UTCJul 8, 05:55 UTC3h 10m
2026-07-08 UAL3920 IAH → EWR Jul 8, 00:48 UTCJul 8, 06:06 UTC5h 18m
2026-07-06 UAL3918 - Jul 6, 10:10 UTCJul 6, 13:10 UTC3h 0m
2026-07-05 UAL967 - Jul 5, 12:50 UTCJul 5, 14:15 UTC1h 25m
2026-07-05 UAL967 NAP → EWR Jul 5, 12:54 UTCJul 5, 22:11 UTC9h 17m
2026-07-05 UAL966 - Jul 5, 01:25 UTCJul 5, 09:35 UTC8h 10m
2026-07-04 UAL967 - Jul 4, 12:50 UTCJul 4, 22:10 UTC9h 20m
2026-07-04 UAL966 EWR → NAP Jul 4, 03:06 UTCJul 4, 10:50 UTC7h 44m
2026-07-04 UAL966 - Jul 4, 03:00 UTCJul 4, 08:15 UTC5h 15m
2026-06-30 UAL17 - Jun 30, 09:55 UTCJun 30, 17:20 UTC7h 25m
2026-06-30 UAL904 - Jun 30, 01:25 UTCJun 30, 07:40 UTC6h 16m
2026-06-29 UAL941 - Jun 29, 15:30 UTCJun 29, 16:15 UTC45m
2026-06-29 UAL941 LHR → EWR Jun 29, 15:24 UTCJun 29, 22:46 UTC7h 22m
2026-06-29 UAL941 LHR → EWR Jun 29, 15:24 UTCJun 29, 22:40 UTC7h 16m
2026-06-28 UAL929 ORD → LHR Jun 28, 22:03 UTCJun 29, 05:35 UTC7h 32m
2026-06-28 UAL929 ORD → LHR Jun 28, 21:59 UTCJun 29, 05:40 UTC7h 40m
2026-06-27 UAL958 - Jun 27, 07:39 UTCJun 27, 08:14 UTC35m
2026-06-27 UAL958 LHR → ORD Jun 27, 07:39 UTCJun 27, 15:35 UTC7h 56m
2026-06-26 UAL16 EWR → LHR Jun 26, 21:58 UTCJun 27, 05:26 UTC7h 28m
2026-06-26 UAL16 - Jun 26, 23:00 UTCJun 27, 05:44 UTC6h 45m
2026-06-25 UAL957 GVA → EWR Jun 25, 07:30 UTCJun 25, 08:14 UTC45m
2026-06-25 UAL957 GVA → EWR Jun 25, 07:22 UTCJun 25, 15:51 UTC8h 29m
2026-06-24 UAL956 EWR → GVA Jun 24, 22:25 UTCJun 25, 05:20 UTC6h 55m
2026-06-22 UAL937 NCE → EWR Jun 22, 13:41 UTCJun 22, 22:20 UTC8h 39m
2026-06-22 UAL937 NCE → EWR Jun 22, 13:35 UTCJun 22, 22:19 UTC8h 45m
2026-06-22 UAL936 - Jun 22, 03:40 UTCJun 22, 11:04 UTC7h 25m
2026-06-21 UAL921 - Jun 21, 17:29 UTCJun 22, 00:40 UTC7h 10m
2026-06-20 UAL16 EWR → LHR Jun 20, 23:03 UTCJun 21, 05:31 UTC6h 28m
2026-06-20 UAL16 - Jun 20, 22:54 UTCJun 21, 04:14 UTC5h 20m
2026-06-20 UAL17 - Jun 20, 09:49 UTCJun 20, 17:10 UTC7h 20m
2026-06-19 UAL929 - Jun 19, 21:54 UTCJun 20, 05:39 UTC7h 45m
2026-06-19 UAL958 - Jun 19, 07:09 UTCJun 19, 15:14 UTC8h 5m

About Mode-S

A Mode-S transponder code (also called an "ICAO 24-bit address" or "hex code") is a 6-character hexadecimal identifier permanently assigned to an aircraft when it's first registered. Unlike callsigns (which change with each flight) or registrations (which can change if an aircraft is sold internationally), the Mode-S address stays with the airframe for the life of the aircraft in that country.

Every ADS-B transmission from an aircraft includes its Mode-S address, making it the most reliable way to identify a specific airframe. Flight-tracking services like Apparatus use the Mode-S address as the primary key for tying position broadcasts, callsigns, and ownership records together.

Prefer a tail-number view? See the N663UA page for operator, model, and routing details .

Mode-S A8C121 frequently asked

What is Mode-S code A8C121?
A8C121 is an ICAO 24-bit transponder address. Allocated to United States. Assigned to N663UA.
Where is Mode-S A8C121 registered?
Mode-S code A8C121 is allocated to United States (US). All aircraft registered there receive a Mode-S address from a fixed block defined by ICAO Annex 10.
Is A8C121 broadcasting right now?
No active broadcast right now. Last signal was 1h ago.
Hex, octal, and decimal Mode-S codes: what's the difference?
The same 24-bit identifier expressed in different bases: hex A8C121, octal 52140441, decimal 11,059,489.
What flights does A8C121 operate?
Mode-S A8C121 has broadcast 20 distinct callsigns, including UAL928, UAL920, UAL941, UAL958, UAL956.