sensors Mode-S transponder
A21D90
ICAO 24-bit transponder address · allocated to United States
Currently assigned to N2351B · PIPER PA-28-140/150/160/180
- Hex
- A21D90
- Octal
- 50416620
- Decimal
- 10,624,400
- Country
- US
- Tail
- N2351B
- Type
- P28A
- Flights observed
- 5
- Distinct callsigns
- 1
- Country
- US
- Last broadcast
- 2h ago
Last known position
Country allocation
The ICAO 24-bit address A21D90 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 |
|---|---|---|---|
| N2351B | - | 5 | 2026-04-18 |
Recent flights last 5
| Date | Flight | Route | Takeoff | Landing | Duration |
|---|---|---|---|---|---|
| 2026-04-18 | N2351B | - | Apr 18, 21:35 UTC | Apr 18, 22:15 UTC | 40m |
| 2026-04-18 | N2351B | - | Apr 18, 10:30 UTC | Apr 18, 11:50 UTC | 1h 20m |
| 2026-04-14 | N2351B | - | Apr 14, 16:50 UTC | Apr 14, 17:20 UTC | 30m |
| 2026-04-14 | N2351B | - | Apr 14, 16:25 UTC | Apr 14, 16:45 UTC | 20m |
| 2026-04-14 | N2351B | - | Apr 14, 14:55 UTC | Apr 14, 15:25 UTC | 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 N2351B page for operator, model, and routing details .