sensors Mode-S transponder
A21DCB
ICAO 24-bit transponder address · allocated to United States
Currently assigned to N23521 · CESSNA T182 Turbo Skylane · operated by AIRCRAFT HOLDINGS LLC
- Hex
- A21DCB
- Octal
- 50416713
- Decimal
- 10,624,459
- Country
- US
- Tail
- N23521
- Type
- C82S
- Flights observed
- 19
- Distinct callsigns
- 1
- Country
- US
- Last broadcast
- Jul 26, 01:50 UTC
Last known position
Country allocation
The ICAO 24-bit address A21DCB 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 1 distinct
| Callsign | Airline | Flights | Last seen |
|---|---|---|---|
| N23521 | - | 19 | 2026-07-26 |
Recent flights last 19
| Date | Flight | Route | Takeoff | Landing | Duration |
|---|---|---|---|---|---|
| 2026-07-26 | N23521 | - | Jul 26, 00:40 UTC | Jul 26, 01:50 UTC | 1h 10m |
| 2026-07-25 | N23521 | - | Jul 25, 13:40 UTC | Jul 25, 14:05 UTC | 25m |
| 2026-06-28 | N23521 | - | Jun 28, 16:55 UTC | Jun 28, 18:44 UTC | 1h 50m |
| 2026-06-28 | N23521 | - | Jun 28, 12:19 UTC | Jun 28, 15:29 UTC | 3h 10m |
| 2026-06-26 | N23521 | - | Jun 26, 15:29 UTC | Jun 26, 15:45 UTC | 15m |
| 2026-06-20 | N23521 | - | Jun 20, 16:34 UTC | Jun 20, 17:39 UTC | 1h 5m |
| 2026-06-18 | N23521 | - | Jun 18, 11:45 UTC | Jun 18, 12:05 UTC | 20m |
| 2026-06-13 | N23521 | - | Jun 13, 15:45 UTC | Jun 13, 16:05 UTC | 20m |
| 2026-06-11 | N23521 | - | Jun 11, 16:45 UTC | Jun 11, 17:45 UTC | 1h 0m |
| 2026-06-11 | N23521 | - | Jun 11, 15:40 UTC | Jun 11, 15:50 UTC | 10m |
| 2026-06-11 | N23521 | - | Jun 11, 13:15 UTC | Jun 11, 14:15 UTC | 1h 0m |
| 2026-06-09 | N23521 | - | Jun 9, 16:45 UTC | Jun 9, 18:05 UTC | 1h 20m |
| 2026-06-04 | N23521 | - | Jun 4, 15:45 UTC | Jun 4, 16:40 UTC | 55m |
| 2026-06-02 | N23521 | - | Jun 2, 18:15 UTC | Jun 2, 18:45 UTC | 30m |
| 2026-06-02 | N23521 | - | Jun 2, 16:30 UTC | Jun 2, 17:05 UTC | 35m |
| 2026-06-02 | N23521 | - | Jun 2, 14:20 UTC | Jun 2, 15:25 UTC | 1h 5m |
| 2026-06-02 | N23521 | - | Jun 2, 12:20 UTC | Jun 2, 12:40 UTC | 20m |
| 2026-06-01 | N23521 | - | Jun 1, 17:35 UTC | Jun 1, 17:50 UTC | 15m |
| 2026-06-01 | N23521 | - | Jun 1, 11:10 UTC | Jun 1, 13:35 UTC | 2h 25m |
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 N23521 page for operator, model, and routing details .