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
- Flights observed
- 218
- Distinct callsigns
- 20
- Country
- US
- Last broadcast
- Jul 26, 14:00 UTC
Last known position
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 | United Airlines | 22 | 2026-07-22 |
| UAL858 | United Airlines | 19 | 2026-07-24 |
| UAL877 | United Airlines | 14 | 2026-07-19 |
| UAL878 | - | 13 | 2026-07-02 |
| UAL872 | United Airlines | 13 | 2026-07-22 |
| UAL869 | - | 12 | 2026-07-01 |
| UAL862 | United Airlines | 12 | 2026-07-20 |
| UAL838 | - | 10 | 2026-07-18 |
| UAL837 | - | 9 | 2026-07-17 |
| UAL189 | United Airlines | 9 | 2026-07-25 |
| UAL871 | - | 8 | 2026-07-21 |
| UAL809 | - | 7 | 2026-06-16 |
| UAL190 | United Airlines | 7 | 2026-07-26 |
| UAL79 | - | 6 | 2026-06-20 |
| UAL78 | United Airlines | 6 | 2026-06-21 |
| 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
| Date | Flight | Route | Takeoff | Landing | Duration |
|---|---|---|---|---|---|
| 2026-07-26 | UAL190 | IAH → MIA | Jul 26, 01:25 UTC | Jul 26, 14:00 UTC | 12h 36m |
| 2026-07-26 | UAL190 | IAH → MIA | Jul 26, 01:16 UTC | Jul 26, 13:58 UTC | 12h 42m |
| 2026-07-25 | UAL189 | CLE → PHX | Jul 25, 09:20 UTC | Jul 25, 22:00 UTC | 12h 40m |
| 2026-07-24 | UAL858 | ICN → SFO | Jul 24, 05:30 UTC | Jul 24, 15:55 UTC | 10h 25m |
| 2026-07-23 | UAL858 | ICN → SFO | Jul 23, 07:05 UTC | Jul 24, 07:00 UTC | 23h 55m |
| 2026-07-22 | UAL857 | ICN → PVG | Jul 22, 20:50 UTC | Jul 23, 07:35 UTC | 10h 45m |
| 2026-07-22 | UAL872 | TPE → SFO | Jul 22, 03:50 UTC | Jul 22, 13:45 UTC | 9h 55m |
| 2026-07-22 | UAL872 | TPE → SFO | Jul 22, 02:15 UTC | Jul 22, 02:30 UTC | 15m |
| 2026-07-22 | UAL872 | TPE → SFO | Jul 22, 02:23 UTC | Jul 22, 13:41 UTC | 11h 18m |
| 2026-07-21 | UAL871 | - | Jul 21, 10:30 UTC | Jul 21, 10:40 UTC | 10m |
| 2026-07-20 | UAL871 | - | Jul 20, 22:35 UTC | Jul 21, 09:10 UTC | 10h 34m |
| 2026-07-20 | UAL862 | HKG → SFO | Jul 20, 04:30 UTC | Jul 20, 16:55 UTC | 12h 25m |
| 2026-07-19 | UAL877 | SFO → HKG | Jul 19, 07:35 UTC | Jul 19, 20:30 UTC | 12h 55m |
| 2026-07-18 | UAL838 | - | Jul 18, 10:35 UTC | Jul 18, 19:55 UTC | 9h 20m |
| 2026-07-18 | UAL838 | NRT → SFO | Jul 18, 10:41 UTC | Jul 18, 19:52 UTC | 9h 11m |
| 2026-07-17 | UAL837 | - | Jul 17, 20:00 UTC | Jul 18, 05:40 UTC | 9h 41m |
| 2026-07-17 | UAL858 | - | Jul 17, 06:05 UTC | Jul 17, 16:29 UTC | 10h 25m |
| 2026-07-17 | UAL858 | PVG → SFO | Jul 17, 04:44 UTC | Jul 17, 16:26 UTC | 11h 42m |
| 2026-07-16 | UAL858 | - | Jul 16, 07:05 UTC | Jul 17, 07:00 UTC | 23h 55m |
| 2026-07-15 | UAL857 | SFO → PVG | Jul 15, 20:57 UTC | Jul 16, 08:40 UTC | 11h 43m |
| 2026-07-15 | UAL857 | - | Jul 15, 20:50 UTC | Jul 15, 21:45 UTC | 55m |
| 2026-07-15 | UAL872 | - | Jul 15, 03:55 UTC | Jul 15, 14:00 UTC | 10h 5m |
| 2026-07-15 | UAL872 | - | Jul 15, 02:25 UTC | Jul 15, 02:35 UTC | 10m |
| 2026-07-13 | UAL871 | - | Jul 13, 22:15 UTC | Jul 14, 10:25 UTC | 12h 10m |
| 2026-07-13 | UAL1960 | - | Jul 13, 11:30 UTC | Jul 13, 16:20 UTC | 4h 50m |
| 2026-07-11 | UAL435 | SFO → EWR | Jul 11, 15:30 UTC | Jul 11, 21:10 UTC | 5h 40m |
| 2026-07-10 | UAL190 | MNL → SFO | Jul 10, 01:01 UTC | Jul 10, 13:25 UTC | 12h 24m |
| 2026-07-10 | UAL190 | - | Jul 10, 01:00 UTC | Jul 10, 03:00 UTC | 2h 0m |
| 2026-07-09 | UAL189 | SFO → MNL | Jul 9, 08:54 UTC | Jul 9, 22:00 UTC | 13h 6m |
| 2026-07-09 | UAL189 | SFO → MNL | Jul 9, 08:45 UTC | Jul 9, 22:05 UTC | 13h 20m |
| 2026-07-08 | UAL1844 | - | Jul 8, 22:30 UTC | Jul 9, 03:55 UTC | 5h 25m |
| 2026-07-08 | UAL43 | - | Jul 8, 09:45 UTC | Jul 8, 17:30 UTC | 7h 45m |
| 2026-07-07 | UAL989 | IAD → FRA | Jul 7, 22:40 UTC | Jul 8, 06:00 UTC | 7h 20m |
| 2026-07-07 | UAL988 | - | Jul 7, 11:30 UTC | Jul 7, 19:35 UTC | 8h 5m |
| 2026-07-07 | UAL42 | - | Jul 7, 00:50 UTC | Jul 7, 02:25 UTC | 1h 35m |
| 2026-07-07 | UAL42 | EWR → FRA | Jul 7, 00:00 UTC | Jul 7, 07:41 UTC | 7h 41m |
| 2026-07-06 | UAL435 | - | Jul 6, 16:05 UTC | Jul 6, 20:50 UTC | 4h 45m |
| 2026-07-06 | UAL1346 | - | Jul 6, 08:25 UTC | Jul 6, 12:45 UTC | 4h 20m |
| 2026-07-04 | UAL1175 | - | Jul 4, 21:15 UTC | Jul 4, 21:35 UTC | 20m |
| 2026-07-04 | UAL1175 | - | Jul 4, 16:45 UTC | Jul 4, 17:15 UTC | 30m |
| 2026-07-04 | UAL190 | - | Jul 4, 00:50 UTC | Jul 4, 13:00 UTC | 12h 10m |
| 2026-07-03 | UAL189 | SFO → MNL | Jul 3, 08:19 UTC | Jul 3, 20:35 UTC | 12h 16m |
| 2026-07-03 | UAL189 | - | Jul 3, 08:20 UTC | Jul 3, 20:40 UTC | 12h 20m |
| 2026-07-02 | UAL878 | - | Jul 2, 14:50 UTC | Jul 3, 02:40 UTC | 11h 50m |
| 2026-07-01 | UAL869 | - | Jul 1, 21:20 UTC | Jul 2, 10:40 UTC | 13h 20m |
| 2026-07-01 | UAL872 | - | Jul 1, 03:30 UTC | Jul 1, 12:55 UTC | 9h 25m |
| 2026-07-01 | UAL872 | TPE → SFO | Jul 1, 02:23 UTC | Jul 1, 12:53 UTC | 10h 30m |
| 2026-07-01 | UAL872 | - | Jul 1, 02:05 UTC | Jul 1, 02:15 UTC | 10m |
| 2026-06-29 | UAL871 | - | Jun 29, 22:24 UTC | Jun 30, 10:45 UTC | 12h 21m |
| 2026-06-29 | UAL858 | - | Jun 29, 05:20 UTC | Jun 29, 15:14 UTC | 9h 55m |
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 .