Apparatus

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sensors Mode-S transponder

0AC121

ICAO 24-bit transponder address · allocated to Costa Rica

Currently assigned to HK-4541 · BRITISH AEROSPACE Jetstream Super 31

Hex
0AC121
Octal
02540441
Decimal
704,801
Country
CR
Tail
HK-4541
Type
JS32
No current signal · last broadcast 2h ago
Flights observed
56
Distinct callsigns
1
Country
CR
Last broadcast
2h ago

Last known position

Most recent position reported by Mode-S 0AC121.

Country allocation

The ICAO 24-bit address 0AC121 falls within the block assigned to Costa Rica (CR) by ICAO Annex 10, Volume III. Every aircraft registered in Costa Rica 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
HK4541 - 56 2026-07-18

Recent flights last 50

DateFlightRouteTakeoffLandingDuration
2026-07-18 HK4541 - Jul 18, 00:04 UTCJul 18, 00:35 UTC30m
2026-07-17 HK4541 - Jul 17, 14:20 UTCJul 17, 14:39 UTC20m
2026-07-16 HK4541 - Jul 16, 22:30 UTCJul 16, 23:05 UTC35m
2026-07-16 HK4541 - Jul 16, 21:10 UTCJul 16, 21:25 UTC15m
2026-07-16 HK4541 - Jul 16, 04:40 UTCJul 16, 04:50 UTC10m
2026-07-15 HK4541 - Jul 15, 22:40 UTCJul 15, 22:50 UTC10m
2026-07-15 HK4541 - Jul 15, 19:30 UTCJul 15, 19:45 UTC15m
2026-07-10 HK4541 - Jul 10, 00:50 UTCJul 10, 01:05 UTC15m
2026-07-09 HK4541 - Jul 9, 18:20 UTCJul 9, 18:30 UTC10m
2026-07-09 HK4541 - Jul 9, 12:25 UTCJul 9, 12:40 UTC15m
2026-07-09 HK4541 - Jul 9, 06:15 UTCJul 9, 06:35 UTC20m
2026-07-09 HK4541 - Jul 9, 00:45 UTCJul 9, 00:55 UTC10m
2026-07-08 HK4541 - Jul 8, 06:10 UTCJul 8, 06:30 UTC20m
2026-07-08 HK4541 - Jul 8, 00:55 UTCJul 8, 01:15 UTC20m
2026-07-07 HK4541 - Jul 7, 20:50 UTCJul 7, 21:05 UTC15m
2026-07-02 HK4541 - Jul 2, 08:45 UTCJul 2, 09:00 UTC15m
2026-07-02 HK4541 - Jul 2, 06:20 UTCJul 2, 06:30 UTC10m
2026-06-22 HK4541 - Jun 22, 03:05 UTCJun 22, 03:14 UTC10m
2026-06-19 HK4541 - Jun 19, 19:49 UTCJun 19, 20:04 UTC15m
2026-06-18 HK4541 - Jun 18, 14:05 UTCJun 18, 14:20 UTC15m
2026-06-17 HK4541 - Jun 17, 11:30 UTCJun 17, 11:50 UTC20m
2026-06-13 HK4541 - Jun 13, 00:10 UTCJun 13, 00:40 UTC30m
2026-06-12 HK4541 - Jun 12, 19:40 UTCJun 12, 19:50 UTC10m
2026-06-10 HK4541 - Jun 10, 18:35 UTCJun 10, 18:55 UTC20m
2026-06-10 HK4541 - Jun 10, 16:00 UTCJun 10, 16:10 UTC10m
2026-06-10 HK4541 - Jun 10, 02:25 UTCJun 10, 02:40 UTC15m
2026-06-09 HK4541 - Jun 9, 22:55 UTCJun 9, 23:10 UTC15m
2026-06-09 HK4541 - Jun 9, 19:20 UTCJun 9, 19:30 UTC10m
2026-06-07 HK4541 - Jun 7, 23:20 UTCJun 7, 23:30 UTC10m
2026-06-07 HK4541 - Jun 7, 20:10 UTCJun 7, 20:20 UTC10m
2026-06-07 HK4541 - Jun 7, 12:40 UTCJun 7, 12:50 UTC10m
2026-06-06 HK4541 - Jun 6, 17:29 UTCJun 6, 17:40 UTC10m
2026-06-06 HK4541 - Jun 6, 10:09 UTCJun 6, 10:30 UTC20m
2026-05-25 HK4541 - May 25, 23:24 UTCMay 25, 23:35 UTC10m
2026-05-23 HK4541 - May 23, 19:00 UTCMay 23, 19:10 UTC10m
2026-05-23 HK4541 - May 23, 12:00 UTCMay 23, 12:10 UTC10m
2026-05-21 HK4541 - May 21, 01:10 UTCMay 21, 01:35 UTC25m
2026-05-20 HK4541 - May 20, 20:05 UTCMay 20, 20:20 UTC15m
2026-05-16 HK4541 - May 16, 22:15 UTCMay 16, 22:25 UTC10m
2026-05-15 HK4541 - May 15, 18:55 UTCMay 15, 19:10 UTC15m
2026-05-15 HK4541 - May 15, 14:40 UTCMay 15, 14:55 UTC14m
2026-05-01 HK4541 - May 1, 17:15 UTCMay 1, 17:30 UTC15m
2026-05-01 HK4541 - May 1, 14:35 UTCMay 1, 14:50 UTC15m
2026-04-27 HK4541 - Apr 27, 13:00 UTCApr 27, 13:10 UTC10m
2026-04-25 HK4541 - Apr 25, 19:45 UTCApr 25, 19:55 UTC10m
2026-04-23 HK4541 - Apr 23, 01:05 UTCApr 23, 01:15 UTC10m
2026-04-21 HK4541 - Apr 21, 17:01 UTCApr 21, 17:10 UTC10m
2026-04-19 HK4541 - Apr 19, 00:10 UTCApr 19, 00:20 UTC10m
2026-04-17 HK4541 - Apr 17, 06:50 UTCApr 17, 07:00 UTC10m
2026-04-16 HK4541 - Apr 16, 20:05 UTCApr 16, 20:20 UTC15m

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 HK-4541 page for operator, model, and routing details .

Mode-S 0AC121 frequently asked

What is Mode-S code 0AC121?
0AC121 is an ICAO 24-bit transponder address. Allocated to Costa Rica. Assigned to HK-4541.
Where is Mode-S 0AC121 registered?
Mode-S code 0AC121 is allocated to Costa Rica (CR). All aircraft registered there receive a Mode-S address from a fixed block defined by ICAO Annex 10.
Is 0AC121 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 0AC121, octal 02540441, decimal 704,801.
What flights does 0AC121 operate?
Mode-S 0AC121 has broadcast 1 distinct callsign, including HK4541.