Apparatus

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sensors Mode-S transponder

A5521C

ICAO 24-bit transponder address · allocated to United States

Currently assigned to N442AS · BOEING 737-900 · operated by ALASKA AIRLINES INC

Hex
A5521C
Octal
51251034
Decimal
10,834,460
Country
US
Tail
N442AS
Type
B739
Broadcasting now · callsign ASA507 · 37,000 ft
Flights observed
448
Distinct callsigns
20
Country
US
Last broadcast
Jul 26, 23:00 UTC

Current position

A5521C is currently broadcasting. Map zoomed to live position.

Country allocation

The ICAO 24-bit address A5521C 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
ASA280 - 9 2026-06-30
ASA515 Alaska Airlines 7 2026-05-24
ASA298 - 5 2026-04-17
ASA748 - 4 2026-07-06
ASA62 - 4 2026-07-23
ASA577 Alaska Airlines 4 2026-05-10
ASA503 Alaska Airlines 4 2026-07-02
ASA501 Alaska Airlines 4 2026-07-25
ASA497 Alaska Airlines 4 2026-07-24
ASA478 Alaska Airlines 4 2026-05-12
ASA442 Alaska Airlines 4 2026-07-25
ASA31 Alaska Airlines 4 2026-05-07
ASA299 Alaska Airlines 4 2026-06-03
ASA1397 Alaska Airlines 4 2026-05-10
ASA1330 Alaska Airlines 4 2026-05-20
ASA12 Alaska Airlines 4 2026-05-17
ASA682 - 3 2026-06-28
ASA657 Alaska Airlines 3 2026-05-30
ASA629 - 3 2026-07-14
ASA623 Alaska Airlines 3 2026-07-24

Recent flights last 50

DateFlightRouteTakeoffLandingDuration
2026-07-26 ASA1574 DFW → SEA Jul 26, 21:30 UTCJul 26, 23:00 UTC1h 30m
2026-07-26 ASA545 PDX → JFK Jul 26, 17:15 UTCJul 26, 20:00 UTC2h 45m
2026-07-26 ASA1470 ORD → SAN Jul 26, 11:50 UTCJul 26, 15:20 UTC3h 30m
2026-07-26 ASA376 SFO → LAX Jul 26, 06:10 UTCJul 26, 09:35 UTC3h 26m
2026-07-26 ASA376 SFO → LAX Jul 26, 06:06 UTCJul 26, 09:20 UTC3h 14m
2026-07-25 ASA1787 PHL → PDX Jul 25, 22:15 UTCJul 26, 03:45 UTC5h 30m
2026-07-25 ASA442 SEA → PHL Jul 25, 15:40 UTCJul 25, 20:00 UTC4h 20m
2026-07-25 ASA501 PHX → SFO Jul 25, 01:00 UTCJul 25, 04:25 UTC3h 25m
2026-07-24 ASA497 ATL → SEA Jul 24, 20:50 UTCJul 24, 23:30 UTC2h 40m
2026-07-24 ASA623 PVR → SFO Jul 24, 14:45 UTCJul 24, 19:15 UTC4h 30m
2026-07-24 ASA416 SFO → DAL Jul 24, 00:45 UTCJul 24, 04:30 UTC3h 45m
2026-07-23 ASA62 KTN → SEA Jul 23, 21:30 UTCJul 23, 23:00 UTC1h 30m
2026-07-23 ASA62 KTN → SEA Jul 23, 19:45 UTCJul 23, 20:05 UTC20m
2026-07-23 ASA62 KTN → SEA Jul 23, 17:50 UTCJul 23, 18:05 UTC15m
2026-07-23 ASA62 - Jul 23, 15:30 UTCJul 23, 16:40 UTC1h 10m
2026-07-23 ASA1363 LAX → PVR Jul 23, 07:15 UTCJul 23, 10:30 UTC3h 15m
2026-07-23 ASA1375 LTO → LAX Jul 23, 03:30 UTCJul 23, 05:30 UTC2h 0m
2026-07-23 ASA1375 LTO → LAX Jul 23, 03:26 UTCJul 23, 05:20 UTC1h 54m
2026-07-23 ASA1604 SFO → LAX Jul 23, 01:00 UTCJul 23, 01:55 UTC55m
2026-07-22 ASA227 SJC → SEA Jul 22, 17:55 UTCJul 22, 23:30 UTC5h 35m
2026-07-21 ASA469 BNA → SEA Jul 21, 23:40 UTCJul 22, 04:10 UTC4h 30m
2026-07-21 ASA1376 PDX → PVR Jul 21, 17:40 UTCJul 21, 21:25 UTC3h 45m
2026-07-20 ASA1350 SEA → SFO Jul 20, 21:45 UTCJul 21, 01:50 UTC4h 4m
2026-07-20 ASA1345 SEA → BNA Jul 20, 16:55 UTCJul 20, 20:30 UTC3h 35m
2026-07-20 ASA563 - Jul 20, 11:30 UTCJul 20, 14:45 UTC3h 15m
2026-07-20 ASA564 SEA → MCI Jul 20, 01:50 UTCJul 20, 04:35 UTC2h 45m
2026-07-19 ASA439 - Jul 19, 20:05 UTCJul 20, 00:00 UTC3h 55m
2026-07-19 ASA1420 - Jul 19, 14:30 UTCJul 19, 18:20 UTC3h 50m
2026-07-19 ASA1787 PHL → PDX Jul 19, 02:05 UTCJul 19, 07:35 UTC5h 30m
2026-07-18 ASA442 SEA → PHL Jul 18, 18:10 UTCJul 18, 22:40 UTC4h 30m
2026-07-18 ASA591 CUN → SFO Jul 18, 04:05 UTCJul 18, 05:30 UTC1h 25m
2026-07-18 ASA504 OAK → SEA Jul 18, 01:15 UTCJul 18, 02:50 UTC1h 36m
2026-07-17 ASA1633 SJC → PDX Jul 17, 21:39 UTCJul 17, 22:55 UTC1h 15m
2026-07-17 ASA1632 PDX → SJC Jul 17, 18:44 UTCJul 17, 20:15 UTC1h 30m
2026-07-17 ASA1626 - Jul 17, 15:00 UTCJul 17, 16:59 UTC2h 0m
2026-07-17 ASA1626 DEN → PDX Jul 17, 15:01 UTCJul 17, 17:07 UTC2h 6m
2026-07-17 ASA752 ANC → DEN Jul 17, 08:00 UTCJul 17, 12:55 UTC4h 55m
2026-07-17 ASA87 ANC → ANC Jul 17, 00:14 UTCJul 17, 04:04 UTC3h 50m
2026-07-16 ASA1438 AKN → ANC Jul 16, 22:10 UTCJul 16, 22:55 UTC45m
2026-07-16 ASA1437 SFO → LAX Jul 16, 19:50 UTCJul 16, 20:39 UTC50m
2026-07-16 ASA79 SEA → ANC Jul 16, 15:19 UTCJul 16, 18:40 UTC3h 20m
2026-07-16 ASA461 SEA → SEA Jul 16, 03:45 UTCJul 16, 13:25 UTC9h 40m
2026-07-15 ASA214 - Jul 15, 01:05 UTCJul 15, 02:45 UTC1h 40m
2026-07-14 ASA92 - Jul 14, 22:40 UTCJul 14, 23:55 UTC1h 14m
2026-07-14 ASA629 LAS → SEA Jul 14, 19:20 UTCJul 14, 21:10 UTC1h 50m
2026-07-14 ASA777 - Jul 14, 15:35 UTCJul 14, 17:50 UTC2h 15m
2026-07-14 ASA9802 SEA → PAE Jul 14, 14:10 UTCJul 14, 14:15 UTC5m
2026-07-14 ASA366 LAS → SEA Jul 14, 04:50 UTCJul 14, 06:35 UTC1h 45m
2026-07-14 ASA1456 SEA → LAS Jul 14, 00:35 UTCJul 14, 02:50 UTC2h 15m
2026-07-13 ASA446 - Jul 13, 20:50 UTCJul 13, 22:50 UTC2h 0m

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 N442AS page for operator, model, and routing details .

Mode-S A5521C frequently asked

What is Mode-S code A5521C?
A5521C is an ICAO 24-bit transponder address. Allocated to United States. Assigned to N442AS.
Where is Mode-S A5521C registered?
Mode-S code A5521C is allocated to United States (US). All aircraft registered there receive a Mode-S address from a fixed block defined by ICAO Annex 10.
Is A5521C broadcasting right now?
Yes. Mode-S A5521C is currently airborne.
Hex, octal, and decimal Mode-S codes: what's the difference?
The same 24-bit identifier expressed in different bases: hex A5521C, octal 51251034, decimal 10,834,460.
What flights does A5521C operate?
Mode-S A5521C has broadcast 20 distinct callsigns, including ASA280, ASA515, ASA298, ASA748, ASA62.