Do pilots know the unique ICAO 24-bit hexadecimal code for their aircraft? Do they ever give it directly to ATC? For example, to controllers who make the flight strips with destination, departure, type of aircraft etc. I know for a fact that some flight plans are sent out that include the unique hex code for the aircraft.


3 Answers 3


Typically pilots do not know the 24-bit aircraft address of their aircraft.

The 24 bit address is intended for data protocol level of communications and is not used in voice communication. Therefor there is no need for pilots to be aware of their ICAO 24-bit aircraft address. They use the aircraft's registration if they need to identify the aircraft they are using for the flight.

The 24-bit address is used by transponders for communication with Mode-S radars, ADS-B transmissions, by TCAS for tracking traffic and coordinating Resolution Advisories and by Data-Link communications.

  • $\begingroup$ Thanks for the fast reply! This actually makes sense as the 24bit address (even when converted to the 6 digit code) would be kinda difficult to remember. However it would be nice if they knew it, so you can link the aircrafts registration to the 24bit address. $\endgroup$
    – gary
    Dec 2, 2016 at 16:44
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    $\begingroup$ Some countries use algorithms that uniquely map the aircraft registration to the 24-bit aircraft address (and vice versa), but many countries do not have any mathematical relation between the registration and the address. There are databases online that keep track of the mappings. $\endgroup$
    – DeltaLima
    Dec 2, 2016 at 16:53
  • $\begingroup$ @gary At least in the US, the hex codes are public information. If you look up an aircraft's registration, it will show you the hex code. $\endgroup$
    – Pondlife
    Dec 2, 2016 at 18:50
  • $\begingroup$ @Pondlife the USA is one of countries that use an algorithm to map between aircraft registration and 24-bit aircraft address. Most countries have public aircraft registers that contain the 24-bit address. $\endgroup$
    – DeltaLima
    Dec 2, 2016 at 18:53
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    $\begingroup$ @gary One source that contains many aircraft and their 24-bit addresses is airframes.org $\endgroup$
    – DeltaLima
    Dec 2, 2016 at 19:02

I will agree that pilots generally do not know the ICAO 24 bit address for their aircraft. However, with the advent of the ICAO format flight plan, operators who want to take advantage of ADS-B provided ATC services (such as where only ADS-B - no radar - is provided), will have to include the hexadecimal equivalent of their aircraft's 24-bit ICAO address.


Do pilots know the unique ICAO 24-bit hexadecimal code for their aircraft?

Answer depends on the pilot really. Mode S Address(24 bit ICAO address as you say) can be seen as IP of the planes. It is included in some of the Mode S Mode S interrogations/replies as you pointed out.

For commercial airplanes, Mode S address is not changed during lifetime, generally but for some military scenarios, I have read that Mode S Address can be changed per flight for obvious reasons. So you can say, some military pilots definitely know Mode S address of their aircraft.

In addition to that, some platforms may provide you with display and control of Mode S Address.

  • $\begingroup$ After reading the question in the title, answer would be no. My answer was for your question in the first sentence of your post. $\endgroup$ Jan 17, 2017 at 16:34
  • $\begingroup$ Well, the transponders I have experience with allowed modifying their Mode S address on the fly. This is the reason i use IP Address analogy. In that sense, Mode S address allocation is not strict as MAC address. I do not have experience with civilian trandponders though. Do they have unchangeable mode s address? $\endgroup$ Feb 7, 2017 at 21:02
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    $\begingroup$ ICAO address on a civil aircraft is assigned as a registration attribute. Transponders can be reprogrammed by maintenance to meet the registration address of the host aircraft when moved to a new airframe. If addresses could be changed in flight this would possibly create a risk of collision due to ACAS perturbation (same for multilateration). $\endgroup$
    – mins
    Feb 7, 2017 at 21:19

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