Apparatus

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sensors Mode-S transponder

A21DC4

ICAO 24-bit transponder address · allocated to United States

Currently assigned to N2352U · BOEING 777-300ER · operated by UNITED AIRLINES INC

Hex
A21DC4
Octal
50416704
Decimal
10,624,452
Country
US
Tail
N2352U
Type
B77W
No current signal · last broadcast 2h ago
Flights observed
195
Distinct callsigns
20
Country
US
Last broadcast
2h ago

Last known position

Most recent position reported by Mode-S A21DC4.

Country allocation

The ICAO 24-bit address A21DC4 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 - 19 2026-06-28
UAL858 - 14 2026-06-29
UAL878 - 13 2026-07-02
UAL877 - 13 2026-06-18
UAL869 - 12 2026-07-01
UAL862 - 11 2026-06-19
UAL872 - 8 2026-07-01
UAL838 - 8 2026-06-23
UAL837 United Airlines 8 2026-06-22
UAL189 - 8 2026-07-09
UAL809 - 7 2026-06-16
UAL871 - 6 2026-07-13
UAL79 - 6 2026-06-20
UAL78 United Airlines 6 2026-06-21
UAL190 - 5 2026-07-10
UAL810 - 4 2026-06-17
UAL41 United Airlines 4 2026-06-08
UAL2416 - 4 2026-06-20
UAL1960 - 4 2026-07-13
UAL1844 - 4 2026-07-08

Recent flights last 50

DateFlightRouteTakeoffLandingDuration
2026-07-13 UAL871 - Jul 13, 22:15 UTCJul 14, 08:45 UTC10h 30m
2026-07-13 UAL1960 - Jul 13, 11:30 UTCJul 13, 16:20 UTC4h 50m
2026-07-11 UAL435 SFO → EWR Jul 11, 15:30 UTCJul 11, 21:10 UTC5h 40m
2026-07-10 UAL190 MNL → SFO Jul 10, 01:01 UTCJul 10, 13:25 UTC12h 24m
2026-07-10 UAL190 - Jul 10, 01:00 UTCJul 10, 03:00 UTC2h 0m
2026-07-09 UAL189 SFO → MNL Jul 9, 08:54 UTCJul 9, 22:00 UTC13h 6m
2026-07-09 UAL189 SFO → MNL Jul 9, 08:45 UTCJul 9, 22:05 UTC13h 20m
2026-07-08 UAL1844 - Jul 8, 22:30 UTCJul 9, 03:55 UTC5h 25m
2026-07-08 UAL43 - Jul 8, 09:45 UTCJul 8, 17:30 UTC7h 45m
2026-07-07 UAL989 IAD → FRA Jul 7, 22:40 UTCJul 8, 06:00 UTC7h 20m
2026-07-07 UAL988 - Jul 7, 11:30 UTCJul 7, 19:35 UTC8h 5m
2026-07-07 UAL42 - Jul 7, 00:50 UTCJul 7, 02:25 UTC1h 35m
2026-07-07 UAL42 EWR → FRA Jul 7, 00:00 UTCJul 7, 07:41 UTC7h 41m
2026-07-06 UAL435 - Jul 6, 16:05 UTCJul 6, 20:50 UTC4h 45m
2026-07-06 UAL1346 - Jul 6, 08:25 UTCJul 6, 12:45 UTC4h 20m
2026-07-04 UAL1175 - Jul 4, 21:15 UTCJul 4, 21:35 UTC20m
2026-07-04 UAL1175 - Jul 4, 16:45 UTCJul 4, 17:15 UTC30m
2026-07-04 UAL190 - Jul 4, 00:50 UTCJul 4, 13:00 UTC12h 10m
2026-07-03 UAL189 SFO → MNL Jul 3, 08:19 UTCJul 3, 20:35 UTC12h 16m
2026-07-03 UAL189 - Jul 3, 08:20 UTCJul 3, 20:40 UTC12h 20m
2026-07-02 UAL878 - Jul 2, 14:50 UTCJul 3, 02:40 UTC11h 50m
2026-07-01 UAL869 - Jul 1, 21:20 UTCJul 2, 10:40 UTC13h 20m
2026-07-01 UAL872 - Jul 1, 03:30 UTCJul 1, 12:55 UTC9h 25m
2026-07-01 UAL872 TPE → SFO Jul 1, 02:23 UTCJul 1, 12:53 UTC10h 30m
2026-07-01 UAL872 - Jul 1, 02:05 UTCJul 1, 02:15 UTC10m
2026-06-29 UAL871 - Jun 29, 22:24 UTCJun 30, 10:45 UTC12h 21m
2026-06-29 UAL858 - Jun 29, 05:20 UTCJun 29, 15:14 UTC9h 55m
2026-06-28 UAL857 - Jun 28, 07:05 UTCJun 29, 06:50 UTC23h 45m
2026-06-27 UAL857 SFO → PVG Jun 27, 21:12 UTCJun 28, 09:38 UTC12h 26m
2026-06-27 UAL857 - Jun 27, 21:25 UTCJun 28, 08:59 UTC11h 35m
2026-06-27 UAL858 - Jun 27, 15:30 UTCJun 27, 16:04 UTC35m
2026-06-25 UAL857 SFO → PVG Jun 25, 21:12 UTCJun 26, 09:20 UTC12h 8m
2026-06-25 UAL857 - Jun 25, 21:10 UTCJun 26, 08:30 UTC11h 20m
2026-06-25 UAL872 - Jun 25, 12:50 UTCJun 25, 13:35 UTC45m
2026-06-25 UAL872 - Jun 25, 02:14 UTCJun 25, 02:30 UTC15m
2026-06-25 UAL872 TPE → SFO Jun 25, 02:23 UTCJun 25, 13:30 UTC11h 7m
2026-06-23 UAL871 - Jun 23, 22:05 UTCJun 24, 10:39 UTC12h 35m
2026-06-23 UAL838 - Jun 23, 08:25 UTCJun 23, 17:25 UTC9h 0m
2026-06-22 UAL837 SFO → NRT Jun 22, 20:14 UTCJun 23, 06:04 UTC9h 50m
2026-06-22 UAL1960 - Jun 22, 12:00 UTCJun 22, 17:20 UTC5h 20m
2026-06-21 UAL78 NRT → EWR Jun 21, 09:00 UTCJun 21, 21:10 UTC12h 10m
2026-06-20 UAL79 EWR → NRT Jun 20, 16:02 UTCJun 21, 04:33 UTC12h 31m
2026-06-20 UAL79 - Jun 20, 16:05 UTCJun 21, 04:34 UTC12h 30m
2026-06-20 UAL2416 SFO → EWR Jun 20, 07:51 UTCJun 20, 11:49 UTC3h 58m
2026-06-20 UAL2416 SFO → EWR Jun 20, 07:34 UTCJun 20, 11:54 UTC4h 20m
2026-06-19 UAL862 HKG → SFO Jun 19, 04:11 UTCJun 19, 16:28 UTC12h 17m
2026-06-19 UAL862 HKG → SFO Jun 19, 03:55 UTCJun 19, 16:29 UTC12h 35m
2026-06-18 UAL877 SFO → HKG Jun 18, 08:26 UTCJun 18, 22:05 UTC13h 39m
2026-06-18 UAL877 - Jun 18, 08:20 UTCJun 18, 22:20 UTC14h 0m
2026-06-17 UAL810 - Jun 17, 14:15 UTCJun 18, 02:20 UTC12h 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 N2352U page for operator, model, and routing details .

Mode-S A21DC4 frequently asked

What is Mode-S code A21DC4?
A21DC4 is an ICAO 24-bit transponder address. Allocated to United States. Assigned to N2352U.
Where is Mode-S A21DC4 registered?
Mode-S code A21DC4 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 A21DC4 broadcasting right now?
No active broadcast right now. Last signal was 2h ago.
Hex, octal, and decimal Mode-S codes: what's the difference?
The same 24-bit identifier expressed in different bases: hex A21DC4, octal 50416704, decimal 10,624,452.
What flights does A21DC4 operate?
Mode-S A21DC4 has broadcast 20 distinct callsigns, including UAL857, UAL858, UAL878, UAL877, UAL869.