Apparatus

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sensors Mode-S transponder

A21656

ICAO 24-bit transponder address · allocated to United States

Currently assigned to N2332U · BOEING 777-300ER · operated by UNITED AIRLINES INC

Hex
A21656
Octal
50413126
Decimal
10,622,550
Country
US
Tail
N2332U
Type
B77W
No current signal · last broadcast 4h ago
Flights observed
211
Distinct callsigns
20
Country
US
Last broadcast
4h ago

Last known position

Most recent position reported by Mode-S A21656.

Country allocation

The ICAO 24-bit address A21656 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
UAL857 - 18 2026-06-30
UAL858 - 15 2026-07-01
UAL950 United Airlines 13 2026-05-27
UAL872 - 13 2026-06-26
UAL951 United Airlines 12 2026-05-28
UAL871 - 12 2026-06-25
UAL869 - 9 2026-07-10
UAL838 - 9 2026-07-07
UAL1175 - 9 2026-07-12
UAL43 United Airlines 8 2026-05-29
UAL42 United Airlines 8 2026-05-24
UAL989 United Airlines 7 2026-05-28
UAL988 United Airlines 7 2026-05-24
UAL862 - 7 2026-07-10
UAL837 United Airlines 7 2026-07-06
UAL878 - 6 2026-07-11
UAL1960 United Airlines 6 2026-06-07
UAL79 - 5 2026-07-02
UAL877 - 4 2026-07-09
UAL809 - 4 2026-06-14

Recent flights last 50

DateFlightRouteTakeoffLandingDuration
2026-07-14 UAL200 - Jul 14, 03:55 UTCJul 14, 04:15 UTC20m
2026-07-13 UAL200 GUM → HNL Jul 13, 21:18 UTCJul 14, 04:21 UTC7h 3m
2026-07-13 UAL201 - Jul 13, 00:50 UTCJul 13, 01:01 UTC10m
2026-07-13 UAL201 HNL → GUM Jul 13, 00:49 UTCJul 13, 07:49 UTC7h 0m
2026-07-12 UAL1175 - Jul 12, 16:35 UTCJul 12, 21:10 UTC4h 35m
2026-07-12 UAL1175 SFO → HNL Jul 12, 16:43 UTCJul 12, 21:09 UTC4h 26m
2026-07-11 UAL878 - Jul 11, 15:45 UTCJul 12, 03:50 UTC12h 5m
2026-07-11 UAL878 - Jul 11, 14:50 UTCJul 11, 15:05 UTC15m
2026-07-10 UAL869 SFO → HKG Jul 10, 21:42 UTCJul 11, 11:49 UTC14h 7m
2026-07-10 UAL869 - Jul 10, 21:50 UTCJul 10, 22:20 UTC30m
2026-07-10 UAL862 HKG → SFO Jul 10, 04:11 UTCJul 10, 16:15 UTC12h 4m
2026-07-10 UAL862 - Jul 10, 04:10 UTCJul 10, 08:00 UTC3h 50m
2026-07-09 UAL877 SFO → HKG Jul 9, 07:41 UTCJul 9, 20:51 UTC13h 10m
2026-07-09 UAL877 SFO → HKG Jul 9, 07:35 UTCJul 9, 21:00 UTC13h 25m
2026-07-08 UAL878 - Jul 8, 14:55 UTCJul 9, 03:00 UTC12h 5m
2026-07-07 UAL869 - Jul 7, 21:25 UTCJul 8, 10:45 UTC13h 20m
2026-07-07 UAL838 - Jul 7, 08:25 UTCJul 7, 17:20 UTC8h 55m
2026-07-06 UAL837 SFO → NRT Jul 6, 19:45 UTCJul 7, 06:00 UTC10h 15m
2026-07-06 UAL862 HKG → SFO Jul 6, 04:11 UTCJul 6, 16:23 UTC12h 12m
2026-07-06 UAL862 - Jul 6, 04:00 UTCJul 6, 16:25 UTC12h 25m
2026-07-05 UAL877 - Jul 5, 07:20 UTCJul 5, 20:30 UTC13h 10m
2026-07-04 UAL878 HKG → SFO Jul 4, 15:00 UTCJul 5, 03:05 UTC12h 5m
2026-07-04 UAL878 HKG → SFO Jul 4, 14:51 UTCJul 5, 03:05 UTC12h 14m
2026-07-04 UAL869 - Jul 4, 10:40 UTCJul 4, 10:50 UTC10m
2026-07-03 UAL869 SFO → HKG Jul 3, 21:12 UTCJul 4, 10:20 UTC13h 8m
2026-07-03 UAL869 - Jul 3, 21:05 UTCJul 3, 21:30 UTC25m
2026-07-03 UAL838 - Jul 3, 08:45 UTCJul 3, 17:45 UTC9h 0m
2026-07-03 UAL838 NRT → SFO Jul 3, 08:31 UTCJul 3, 17:40 UTC9h 9m
2026-07-02 UAL79 EWR → NRT Jul 2, 16:02 UTCJul 3, 04:54 UTC12h 52m
2026-07-02 UAL79 - Jul 2, 16:00 UTCJul 2, 23:00 UTC7h 0m
2026-07-02 UAL2416 SFO → EWR Jul 2, 08:06 UTCJul 2, 12:52 UTC4h 46m
2026-07-02 UAL2416 - Jul 2, 07:50 UTCJul 2, 12:35 UTC4h 45m
2026-07-01 UAL858 - Jul 1, 05:25 UTCJul 1, 15:15 UTC9h 50m
2026-06-30 UAL858 - Jun 30, 07:35 UTCJul 1, 07:00 UTC23h 25m
2026-06-30 UAL857 - Jun 30, 06:20 UTCJul 1, 05:30 UTC23h 10m
2026-06-29 UAL857 SFO → PVG Jun 29, 20:57 UTCJun 30, 08:59 UTC12h 2m
2026-06-29 UAL857 - Jun 29, 20:44 UTCJun 30, 08:05 UTC11h 21m
2026-06-29 UAL838 - Jun 29, 08:39 UTCJun 29, 17:19 UTC8h 40m
2026-06-29 UAL838 NRT → SFO Jun 29, 08:46 UTCJun 29, 17:10 UTC8h 24m
2026-06-28 UAL837 SFO → NRT Jun 28, 19:59 UTCJun 29, 06:24 UTC10h 25m
2026-06-26 UAL872 - Jun 26, 03:45 UTCJun 26, 13:10 UTC9h 25m
2026-06-26 UAL872 - Jun 26, 02:20 UTCJun 26, 02:30 UTC10m
2026-06-25 UAL871 - Jun 25, 10:45 UTCJun 25, 11:10 UTC25m
2026-06-24 UAL871 - Jun 24, 22:39 UTCJun 24, 23:10 UTC30m
2026-06-24 UAL838 - Jun 24, 09:09 UTCJun 24, 17:59 UTC8h 50m
2026-06-24 UAL838 NRT → SFO Jun 24, 09:11 UTCJun 24, 17:58 UTC8h 47m
2026-06-23 UAL837 SFO → NRT Jun 23, 19:52 UTCJun 24, 05:49 UTC9h 57m
2026-06-23 UAL837 SFO → NRT Jun 23, 19:55 UTCJun 24, 05:59 UTC10h 5m
2026-06-23 UAL872 - Jun 23, 02:29 UTCJun 23, 13:09 UTC10h 40m
2026-06-23 UAL872 TPE → SFO Jun 23, 02:23 UTCJun 23, 13:04 UTC10h 41m

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 N2332U page for operator, model, and routing details .

Mode-S A21656 frequently asked

What is Mode-S code A21656?
A21656 is an ICAO 24-bit transponder address. Allocated to United States. Assigned to N2332U.
Where is Mode-S A21656 registered?
Mode-S code A21656 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 A21656 broadcasting right now?
No active broadcast right now. Last signal was 4h ago.
Hex, octal, and decimal Mode-S codes: what's the difference?
The same 24-bit identifier expressed in different bases: hex A21656, octal 50413126, decimal 10,622,550.
What flights does A21656 operate?
Mode-S A21656 has broadcast 20 distinct callsigns, including UAL857, UAL858, UAL950, UAL872, UAL951.