sensors Mode-S transponder
A21633
ICAO 24-bit transponder address · allocated to United States
Currently assigned to N2331U · BOEING 777-300ER · operated by UNITED AIRLINES INC
- Hex
- A21633
- Octal
- 50413063
- Decimal
- 10,622,515
- Country
- US
- Tail
- N2331U
- Type
- B77W
- Flights observed
- 212
- Distinct callsigns
- 20
- Country
- US
- Last broadcast
- 55m ago
Last known position
Country allocation
The ICAO 24-bit address A21633 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 |
|---|---|---|---|
| UAL838 | - | 13 | 2026-07-05 |
| UAL878 | - | 11 | 2026-07-09 |
| UAL871 | - | 11 | 2026-07-02 |
| UAL809 | - | 11 | 2026-06-25 |
| UAL869 | - | 10 | 2026-07-08 |
| UAL951 | United Airlines | 9 | 2026-05-22 |
| UAL872 | - | 9 | 2026-07-03 |
| UAL190 | - | 9 | 2026-07-11 |
| UAL858 | - | 8 | 2026-07-08 |
| UAL837 | United Airlines | 8 | 2026-07-04 |
| UAL810 | - | 8 | 2026-06-26 |
| UAL989 | - | 7 | 2026-06-19 |
| UAL988 | - | 7 | 2026-06-19 |
| UAL862 | - | 7 | 2026-06-28 |
| UAL43 | - | 7 | 2026-06-20 |
| UAL857 | - | 6 | 2026-07-06 |
| UAL42 | - | 6 | 2026-07-14 |
| UAL950 | United Airlines | 5 | 2026-05-21 |
| UAL79 | - | 5 | 2026-07-12 |
| UAL189 | - | 5 | 2026-07-10 |
Recent flights last 50
| Date | Flight | Route | Takeoff | Landing | Duration |
|---|---|---|---|---|---|
| 2026-07-14 | UAL42 | - | Jul 14, 00:30 UTC | Jul 14, 06:45 UTC | 6h 15m |
| 2026-07-13 | UAL78 | - | Jul 13, 08:45 UTC | Jul 13, 20:45 UTC | 12h 0m |
| 2026-07-12 | UAL79 | EWR → NRT | Jul 12, 16:02 UTC | Jul 13, 05:06 UTC | 13h 4m |
| 2026-07-12 | UAL79 | - | Jul 12, 15:55 UTC | Jul 13, 05:05 UTC | 13h 10m |
| 2026-07-12 | UAL2416 | - | Jul 12, 07:35 UTC | Jul 12, 12:10 UTC | 4h 35m |
| 2026-07-11 | UAL190 | MNL → SFO | Jul 11, 01:01 UTC | Jul 11, 13:40 UTC | 12h 39m |
| 2026-07-11 | UAL190 | - | Jul 11, 01:10 UTC | Jul 11, 13:50 UTC | 12h 40m |
| 2026-07-10 | UAL189 | - | Jul 10, 08:55 UTC | Jul 10, 22:25 UTC | 13h 30m |
| 2026-07-09 | UAL878 | HKG → SFO | Jul 9, 14:51 UTC | Jul 10, 02:55 UTC | 12h 4m |
| 2026-07-09 | UAL878 | - | Jul 9, 14:40 UTC | Jul 10, 03:00 UTC | 12h 20m |
| 2026-07-08 | UAL869 | SFO → HKG | Jul 8, 21:12 UTC | Jul 9, 10:39 UTC | 13h 27m |
| 2026-07-08 | UAL869 | SFO → HKG | Jul 8, 21:05 UTC | Jul 9, 10:35 UTC | 13h 30m |
| 2026-07-08 | UAL858 | PVG → SFO | Jul 8, 04:44 UTC | Jul 8, 15:22 UTC | 10h 38m |
| 2026-07-07 | UAL858 | - | Jul 7, 06:50 UTC | Jul 8, 06:45 UTC | 23h 55m |
| 2026-07-06 | UAL857 | - | Jul 6, 21:05 UTC | Jul 7, 08:15 UTC | 11h 10m |
| 2026-07-05 | UAL838 | NRT → SFO | Jul 5, 08:31 UTC | Jul 5, 17:10 UTC | 8h 39m |
| 2026-07-05 | UAL838 | - | Jul 5, 08:15 UTC | Jul 5, 16:45 UTC | 8h 30m |
| 2026-07-04 | UAL837 | SFO → NRT | Jul 4, 19:52 UTC | Jul 5, 06:00 UTC | 10h 8m |
| 2026-07-04 | UAL837 | SFO → NRT | Jul 4, 19:50 UTC | Jul 5, 05:55 UTC | 10h 5m |
| 2026-07-03 | UAL872 | TPE → SFO | Jul 3, 10:03 UTC | Jul 3, 21:05 UTC | 11h 2m |
| 2026-07-03 | UAL872 | - | Jul 3, 09:45 UTC | Jul 3, 10:40 UTC | 55m |
| 2026-07-03 | UAL871D | - | Jul 3, 07:00 UTC | Jul 3, 07:25 UTC | 25m |
| 2026-07-02 | UAL871D | - | Jul 2, 21:55 UTC | Jul 2, 22:10 UTC | 15m |
| 2026-07-02 | UAL871 | - | Jul 2, 04:40 UTC | Jul 2, 05:00 UTC | 20m |
| 2026-07-01 | UAL871 | - | Jul 1, 22:30 UTC | Jul 1, 22:50 UTC | 20m |
| 2026-07-01 | UAL838 | NRT → SFO | Jul 1, 08:31 UTC | Jul 1, 17:19 UTC | 8h 48m |
| 2026-07-01 | UAL838 | - | Jul 1, 08:35 UTC | Jul 1, 17:25 UTC | 8h 50m |
| 2026-06-30 | UAL79 | - | Jun 30, 16:00 UTC | Jul 1, 04:45 UTC | 12h 45m |
| 2026-06-30 | UAL2416 | - | Jun 30, 07:35 UTC | Jun 30, 12:15 UTC | 4h 40m |
| 2026-06-28 | UAL862 | - | Jun 28, 06:10 UTC | Jun 28, 16:49 UTC | 10h 40m |
| 2026-06-28 | UAL862 | - | Jun 28, 05:20 UTC | Jun 28, 05:34 UTC | 15m |
| 2026-06-27 | UAL877 | SFO → HKG | Jun 27, 07:19 UTC | Jun 27, 20:39 UTC | 13h 20m |
| 2026-06-26 | UAL810 | MNL → SFO | Jun 26, 13:40 UTC | Jun 27, 02:25 UTC | 12h 45m |
| 2026-06-25 | UAL809 | SFO → MNL | Jun 25, 21:43 UTC | Jun 26, 10:45 UTC | 13h 2m |
| 2026-06-25 | UAL809 | - | Jun 25, 22:04 UTC | Jun 25, 22:29 UTC | 25m |
| 2026-06-25 | UAL838 | - | Jun 25, 08:20 UTC | Jun 25, 16:59 UTC | 8h 40m |
| 2026-06-24 | UAL79 | EWR → NRT | Jun 24, 15:59 UTC | Jun 25, 05:25 UTC | 13h 25m |
| 2026-06-24 | UAL2416 | SFO → EWR | Jun 24, 07:59 UTC | Jun 24, 12:30 UTC | 4h 30m |
| 2026-06-23 | UAL810 | - | Jun 23, 13:45 UTC | Jun 24, 01:44 UTC | 12h 0m |
| 2026-06-22 | UAL809 | SFO → MNL | Jun 22, 22:08 UTC | Jun 23, 11:09 UTC | 13h 1m |
| 2026-06-22 | UAL809 | - | Jun 22, 22:05 UTC | Jun 23, 11:10 UTC | 13h 5m |
| 2026-06-22 | UAL838 | - | Jun 22, 08:25 UTC | Jun 22, 17:25 UTC | 9h 0m |
| 2026-06-21 | UAL837 | SFO → NRT | Jun 21, 19:55 UTC | Jun 22, 05:49 UTC | 9h 55m |
| 2026-06-20 | UAL1844 | - | Jun 20, 22:30 UTC | Jun 21, 04:14 UTC | 5h 45m |
| 2026-06-20 | UAL43 | FRA → EWR | Jun 20, 09:39 UTC | Jun 20, 17:34 UTC | 7h 55m |
| 2026-06-19 | UAL989 | - | Jun 19, 22:09 UTC | Jun 20, 04:44 UTC | 6h 35m |
| 2026-06-19 | UAL988 | - | Jun 19, 11:09 UTC | Jun 19, 19:25 UTC | 8h 15m |
| 2026-06-19 | UAL988 | FRA → IAD | Jun 19, 11:09 UTC | Jun 19, 19:26 UTC | 8h 17m |
| 2026-06-19 | UAL42 | - | Jun 19, 00:14 UTC | Jun 19, 07:04 UTC | 6h 50m |
| 2026-06-18 | UAL435 | SFO → EWR | Jun 18, 16:30 UTC | Jun 18, 21:00 UTC | 4h 30m |
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 N2331U page for operator, model, and routing details .