Your Not-So-Secret ID Number

An example of the ICAO Aircraft Address Code on registration certificate of EAA’s Cessna Skycatcher.

Did you know that your airplane has a distinctive identification number that is not your N number? And depending on what type of transponder you have, that special number may be broadcast automatically without you even knowing it.

The special identification number is assigned to every registered airplane. The number is sometimes called the ICAO (International Civil Aviation Organization) number, or can be labeled the Mode S number. The reason it is called the ICAO identification is because unlike the registration (N number) it is unique for the world, not just a single country.

The eight-digit ICAO number is assigned to the airplane and the owner. The ICAO number is essentially invisible; it is like the VIN number on your car while the N number is the license plate. The authority that issues the N number or other registration number matches it with the ICAO number the same way the DMV has methods to make sure the VIN and license plate numbers match the records.

Because different countries use combinations of letters and numbers to identify airplanes there was no easy way to translate something like an N number, or a C and letters from Canada, into an electronic code. The 8 digit ICAO number can be unique around the globe no matter what the registration or call sign may be.

The ICAO identification number system dates back to sometime in the 1970s when the parameters for Mode S transponders were being established. Mode S stands for selectable, as compared to the other transponder Modes of A and C. Mode A is the four-digit code you dial into the transponder, and Mode C is your pressure altitude measured by the altitude encoder.

Modes A and C are really pretty dumb modes. Whenever a ground based radar or an airborne traffic warning system sends out an interrogation a Mode A/C transponder just yells back with its code and altitude. A Mode S transponder can respond with Mode A/C in the same way, but it also has a smart “selectable” mode that waits to be called by name.

The reason Mode S was developed is because it looked like radars interrogating, and Mode A/C transponders all replying, would be stepping all over each other the same way a crowded Unicom frequency becomes unusable when too many pilots transmit at once. Mode S, on the other hand, only responds when it is called by name.

The idea was that the controller’s radar would periodically “call the roll” by sending a general interrogation asking which transponders are within range and all transponders would respond. After the roll call the radar would interrogate specific Mode S transponders by name only as often as needed to keep pace with changes in relative aircraft positions. That would free up a lot of talk time on the transponder frequency.

The FAA planned to phase out Mode A/C transponders by the 1980s, and then by the 1990s and allow only Mode S. A number of other countries, mostly in Europe, actually did require Mode S for much of the airspace, but in the U.S. the transponder frequency crowding didn’t reach the crisis levels that were predicted in the 1970s, so the proposed Mode S requirement deadlines came and went and were never were enforced.

However, Mode S found a new and crucial use when TCAS, the collision avoidance system required on jets, came along. TCAS operates by interrogating transponders on other nearby airplanes. The TCAS calculates the range to the other transponder by timing the round trip of the interrogation and reply, and uses directional ADF-type antenna technology to calculate a bearing to the other airplane. TCAS range is quite accurate, but the bearing is less so.

TCAS knows the range – distance if you want to think of it that way – to an airplane that may be a collision threat, and it knows the altitude of the other airplane because of transponder Mode C, so TCAS can calculate a vertical escape maneuver to avoid a collision threat. TCAS creates what is called a resolution advisory (RA) telling a crew to either climb or descend, and how fast to go up or down, to resolve a collision threat. TCAS can’t calculate a turn to escape because the bearing accuracy just isn’t good enough to provide the time necessary to initiate and complete a heading change.

As you can imagine, when two TCAS-equipped airplanes are a threat to each other the TCAS in each could decide that a descent, for example, is the best avoidance maneuver. But that is a potential nightmare if both TCAS computers tell the crews to escape in the same vertical direction. As both descend or climb together, the box would calculate that an even greater vertical rate of descent or climb is needed and the collision risk would actually increase, not be resolved.

The solution to that problem is to use Mode S and the ICAO identification of each airplane. The TCAS in each airplane asks for and receives the ICAO number of the other over Mode S. Now TCAS knows the name, as it were, of the threat airplane and can make a deal. TCAS in one airplane says, “Hey, you, No. 51010010 (or whatever the number is), you tell your pilots to climb, and I’ll tell my pilots to descend.” Without Mode S to selectively address the specific threat airplane, and without the unique ICAO identification number to know for sure who is who, it would not be possible for TCAS to resolve a potential collision threat between two TCAS-equipped airplanes.

The ICAO number will now be a big part of ADS-B that is slated to replace ground-based radar in 2020. ADS-B will broadcast your ICAO number along with a host of position and velocity information so each and every ADS-B-equipped airplane is positively identified.

Many of us already have Mode S transponders and the ICAO number is programmed (by the avionics installer) into the transponder even if we don’t have TCAS or ADS-B installed. You can see your ICAO (Mode S) number on your aircraft registration card if the card has been issued in the past few years. The number begins with a 5 for U.S. airplanes. When we key the mic we still call ourselves by our registration number or flight number, but before long that almost-invisible eight-digit global identification will be telling controllers and other airplanes in the area exactly who we are without us even knowing that it is happening.

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45 Responses to Your Not-So-Secret ID Number

  1. Jim says:

    So, my question is, Mac, speaking of information being not so secret, why did you redact the ICAO number and the serial number for EAA’s Skycatcher? We all know that info is available at

  2. Mac says:

    Hi Jim,

    That was a decision at EAA headquarters. You are right. Anyone can see the ICAO or Mode S number, or any other airplane registration info on the FAA site. The not so secret number is not a secret at all. If you want to see my Mode S number look up N45 FM. The number is programmed into my Garmin 330 transponder and is being broadcast over Mode S, but you can also see the number on the FAA site. I just can’t remember what it is.


    Mac Mc

  3. Dewey Nichols says:

    I’ve been schooled.

  4. With modern computers, or even paper systems the “tail numbers” would be sufficient world wide. They simply can not be entered into a base 10 system like the mode C transponder. IOW each digit and only be 0-9 (IE a 10 digit code) although valid numbers are less than that. OTOH as far as electronic systems go the represented “character” could be any alphanumeric character which can also be alphabetized. Early computers would have had some problems handling both Cyrillic and the English Characters. Some system conventions have become a bit strange with no real standardization. Virtually all science uses the metric system, but many European systems use a comma for a decimal.Why? I don’t know.

    Mode S as it’s presently implemented and “next gen” equipment (GPS) can know the position of the aircraft to a relatively high precision. Good enough that aircraft could easily negotiate anti collision maneuvers between more than just a couple of planes. However I still see no real, or valid need for using other than the tail number. Watching your clearance come up on a little screen is sorta handy too, particularly when you are intercepting the ILS and ATC comes on with further clearance sounding like an auctioneer. You have to write it down while bouncing around, trying to follow the ILS AND read back the clearance. IOW, write and read in a bumpy flight while trying to fly a precision course. Now we can just hope that they finally get smart and tell LightSquared to “take a hike.”

  5. Tye Hammerle says:

    My favorite conspiracy theory related to this is that some day, when mode-s is mandatory, not only will they know who you are but they’ll be able to bill you for that 5 minutes you spent transitioning class D airspace yesterday and the 4 hours you were in the ATC system last week going IFR to grandma’s house.

    • Chris says:

      Luckily you don’t have to get a Mode S transponder in the USA. You can get a UAT system that supports anonymous mode, when you have 1200 dialed into your mode c transponder the UAT system can use a random ICAO address so no one can directly identify you (and send you a bill for flying through their airspace).

      • Mac says:

        Hi Chris,

        Yes, the “anonymous” mode is in the spec for the UAT ADS-B transceiver. However, if you fly in regulated airspace where an assigned transponder code is required, UAT will transmit your ICAO number along with all other position and velocity data once you enter anything but 1200 in the transponder. If you never fly in regulated arispace where a Mode C transponder is now required you won’t need ADS-B of any kind. But you can’t leave the country because no other nation has accepted the UAT concept for ADS-B, and you have to stay out of airspace where controllers now assign you a squawk.


        Mac Mc

  6. Mac says:

    Hi Tye,

    No need for Mode S or the ICAO number to bill for airspace use. Canada and the rest of the world are doing just fine in charging for ATC service with the existing Mode A/C transponders.


    Mac Mc

  7. Hey Roger. The mode A/C transponders are base 8, not base 10. If ATC says squawk 0138, you’ll know he’s made an error. I read back “squawk 0140″ and get a grateful response.

    • The transponder doesn’t appear to come close to 8 bits. but they chose and entry system *appears* as base 10 to the operator. The early systems were all incapable of counting to 10. It only takes 3 bits to count to 7 (0-7) and 8 bits can count to 256. Transponders are a kludge that I believe even predates the 8 bit 6502 or 8088 (by quite a bit) which were available in 1980 and more basic processor were available at least 10 years before that. I’ve never had one apart, but they appear to transmit a sequence of four, 3 bit codes.

      • I didn’t say 8-bit, I said “base 8,” that’s “octal” and the three bits per digit pack nicely into the 12 bit squawk code response.
        If you want fun, try to decode the Gray Code used for the Mode C altitude encoding; derived from a glass disk driven by a copper bellows. No airborne computation required, just parallel to serial conversion. (I worked on this stuff from 1956 to 1988 and taught number theory and codes as part of my Introduction to Computers class in 1964.)

        • My Bad…I understood 8 bit. Well, I see why the altitude is only 100 foot resolution<:-))

          I really disliked base conversions. We worked mainly with decimal, binary, base 16, and BCD. I had a devil of a time convincing engineers (Double E's no less) that I could control 10 valves with 4 output lines. (BCD). That was a long time ago and in another life…bout early to mid 80's.

  8. Mac says:

    Right you are, Roger. There are 4,096 codes available from the four digits you can dial into a transponder. And the Mode A and C responses are combined using a very rudimentary system so no extra data, such as a specific aircraft identification number, can be transmitted via the A/C reply. That should be no surprise when you consider the Mode A/C format dates back something like 50 years.


    Mac Mc

  9. Tye Hammerle says:

    Interesting, what can they use off mode a/c to bill you? I’d have thought they’re simply going by the paperwork the Europeans have to file before they fly. With mode s, there’s no need to file a flight plan or request to fly to be able ID you.

  10. Mac says:

    The transponder reply only shows that you were in the airspace. There is nothing in a Mode A/C reply that identifies a specific airplane until it is compared with an ATC clearance. You’re right, it is the flight plan filing and the clearance that triggers an airspace user bill. However, much of Europe airspace requires a Mode S transponder so your 24-bit ICAO identification number will automatically be broadcast no matter what code is selected in the transponder.

    Mac Mc

    Mac Mc

  11. Phil says:

    So why are EAA, NBAA, etc making such a big deal of the BARR program going away? In effect, BARR or not, anyone can get live tracking data just by passively listening to Mode-S and ADS-B transmissions, and correlating it to publicly available registration information. There are already “apps for that” that will identify and track aircraft in your area, and there is a growing new breed of plane-spotters engaged in this activity. BARR is already largely irrelevant; why the big fuss?

  12. Mac says:

    Hi Phil,

    Yes, you could receive Mode S or ADS-B data, but you would need to be within line of sight reception range of the airplane you want to track. What BARR blocks is access to the national traffic tracking system that the FAA operates. All aircraft in the IFR system, and even some receiving radar advisories under VFR, are piped to a central FAA computer so traffic flow can be monitored and routes adjusted to help avoid congestion. This national data is also made available to the operators of for-profit websites so anyone anywhere with internet access can see airplanes en route, and also a history of an airplane’s flights. BARR blocks an N number from being sent to the for-profit websites.

    So, any dedicated snoop can listen for VFR voice conversations between pilots and controllers, or scan the 1090 MHZ frequency for Mode S transmissions of the ICAO number. But that’s very hard work, expensive and hardly a national system. Without BARR the FAA simply hands all of that data to the entire world and anyone who can get on the internet can track your flights and flight history.

    BARR is far from irrelevant.


    Mac Mc

    • John (Jiggs) says:

      It should not be prohibitively expensive to monitor aircraft that fly into the Washington, D.C. area to see who arrives to influence Congress or Regulators. Maybe our Congress Critters also would be interested to see if they get their fair share. Of visitors, that is.


  13. As far as those worried about being charged. If you are in an area that charges for ATC services (IE Canada) Transponder or no transponder you’d best be on a flight plan if you want to get home without a fighter escort at the border. <:-))

    I remember not so long ago, a group of us out in the hangar when over the radio, what should we hear, but "MBS Approach, this is Cherokee (something or other), I have an F-16 hanging off my wingtip, what's going on?" We knew right away, here was a pilot flying VFR who had not received a flight briefing, one who was going to be greeted by the "black SUVs" when he landed, and a pilot who "at the very least" was in for some remedial training.

  14. jeff says:

    I figure someone somewhere knows everything I do or even think about doing, this is just one more.

  15. BARR has been in in use for quite some time. I think just some of the big boys used it, but I think they also have a valid reason.

    BARR only blocked the N#s when requested.

  16. Which begs the question, why does the FAA give out all the traffic?

  17. Mac says:

    Hi Roger,

    You ask the important question. Why change the BARR system that has worked well for many years. The FAA says that because it lost a Freedom of Information request about the travels of an airplane it should now make the flight history of all airplanes in the system public. The FAA says it will now default to an open information stance unless airplane owners can prove that they do have a valid security concern, not just a wish for the privacy granted to people who travel in other ways. Obviously, the FAA needs to collect data on a national scale about the movement of air traffic in the system, but it doesn’t need to give that data to internet sites who then post it for the entire world to see. And those sites are intended to make a profit by using government collected data about the movements of all airplane owners. It really doesn’t make sense.

    Mac Mc

  18. And now that begs the question as to how they determine when for a business or individual privacy becomes security? Where and how do they draw the line. Privacy in your travels from your competition can certainly be viewed as business security and possibly worth a great deal of money, or do they limit security to safety for life and limb? What about a man or woman with marital troubles/problems taking a charter, rental, or their own plane? The ex knowing where they are could sometimes be viewed as a security issue<:-)), but would they need a restraining order to qualify?

    If you are a public figure and would like some privacy on a vacation. Now the FAA advertises where you are going?

    Me? I really don't care whether any one can track my personal flights or not and I refuse to fly commercial. Once I can no longer fly it looks like I'll have to save up for a charter.

  19. Dave Passmore says:

    I disagree with the blog comment that the ICAO number is a unique identifier different from your “N” number. In fact, you can easily convert your N-number into the ICAO “Mode S code” and visa versa, since the ICAO identifier is simply your N-number expressed as an octal number (computer assembler language coders are quite familiar with octal encoding).
    The reason this is important is that many pilots forget to change their transponder’s ICAO identifier when they change their plane’s N-number, such as when the aircraft is sold. The ICAO id should match the N-number expressed in octal digits.
    There’s a website (Google for it) that will do the conversion for you if you want to test this.

  20. Tye Hammerle says:

    Dave, are talking about the same thing? Comparing in windows calulator in programmer mode, my FAA assigned “Mode S code” for N58712 is 51711177, 58712 in octal is 162530. Quite a difference. If I get creative and prepend an ascii 078 (N) it still comes up way off, 35765030, in octal from the mode S code.

    • Dave Passmore says:

      For fun, take your current FAA registry Mode S transponder code, go to the website, and plug it into the space that says “ICAO24 address”. To the right of the space, select the little button that says “oct” (since the code is actually an octal number), and then hit “submit”.

      The result should be your “N” number.

  21. Jim says:

    Interesting, but their database is WAY out of date

  22. Tye Hammerle says:

    I’m not buying it. There’s more to it that we appear to know. A plain translation doesn’t reveal my N-number, they’re doing more than just a oct to dec translation.

  23. I agree with Tye. They’re doing a table lookup at some stage. There is no way my ICAO code, 50063077 converts to N12495.

    • Wayne says:

      The ICAO code does convert to the tail number without a table lookup. I wrote an Excel Spreadsheet a few years back that converts the numbers, and gives the strapping for the transponder (or other device like your ELT), to make it easier to make the changes when the aircraft tail number get’s changed, or when we do a new Mode-S installation. It wasn’t easy, but it is just the tail number in an octal format, they defiantly over complicated the conversion process. The spreadsheet not necessary now that the FAA puts the octal code on the registration, but it was easy to get the code before they started putting it on the registration.

  24. When I was growing up in the 1930s, it was common for people in totalitarian states to be asked “Papers Please,” and woe betide the poor soul who didn’t have authorization to be where he was or to be going somewhere his papers didn’t authorize. Doesn’t this tracking system seem like the necessary precursor to such a scheme?
    Many of the control freaks associated with Pilot N Paws are unhappy that I don’t file IFR and therefore can’t be tracked as I move their precious pups (105 so far) towards a safer place.
    Personally I get tired of saying “Mother, may I?” when I want to dodge a building CB or get a better photo op or make a precautionary landing.
    It’s been eleven years since I’ve flown commercial.

  25. Mac says:

    Hi Roger,

    You are exactly correct. The ICAO number is an additional aid in knowing the exact location of your airplane. After all, that’s how the air traffic control system works. By knowing exactly where an airplane is located, it can be separated from other airplanes in the system. And it works great. There has not been a collision between two airplanes flying under IFR in the U.S. in decades. While you may find this to be threatening or sinister, it is actually the only way all-weather flying can be conducted safely. VFR, well that’s a different matter, and the collision threat under those rules is very real and constant, and about a dozen collisions under VFR take place over the U.S. in an average year.


    Mac Mc

  26. Setting aside the logistics, your logic leads straight to requiring a drive plan before leaving the garage in your car or motorcycle. The death toll on the highways is far higher than in the air. (The elite truly hate Henry Ford.)
    One of my most memorable near collisions occurred while under ATC’s control. (The pilot of the Lear coming off CVG as I skirted the area to the south was wearing blue sun glasses.)
    Depending on the efficacy of a government agency is foolish.

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  29. PL Baker says:

    Wow! I spend a great deal of time researching material for my novels and my current piece is full of airplanes and helicopters flown by private individuals and government agents. Needless to say, I am impressed and overwhelmed by this information, since all I wanted to find out was whether or not someone could fly anonymously over parts of the US and Canada. Guess not, huh?

    Thank you, thank you, everyone for your comments and truly informational guidance! Accuracy is important to me when I write, even fiction…unless, of course, I want to ‘invent’ something super-secret! ;-) Thanks, again.

    • Roger Halstead says:

      Fly small, low, and slow. Remember that on many 4 passenger aircraft the N numbers are too small to read while they are in the air. Actually I’ve seen glassair IIIs with small tail numbers.

  30. Genevieve says:

    You’re so awesome! I don’t think I’ve truly read through a
    single thing like that before. So nice to find someone with some unique thoughts on this
    subject matter. Seriously.. many thanks for starting
    this up. This site is one thing that is required on the web,
    someone with a bit of originality!

  31. Jerry Blank says:

    So, hidden in this seems to be the issue of who has access to the FAA data and how and for how long it is stored. Someone threaten the president with an airplane and you can bet law enforcement will get that data, and fast. And, somehow it does not seem to matter how long ago it was.

    So, Mac, how about tax authorities, now and after 12/31/2019? Can they get it now or then, too?

  32. I agree with your explanation, we are totally here
    empathize very intimately indeed it is real possible

  33. Joaquin says:

    I bought a second hand gtx327 transponder from the usa, and using it overseas in my experimental aircraft with a different “N” number than the original FAA registry, so my transponder code is not my “N” number, but previous owner, how can I change this?

  34. NBA2K16 says:

    Wow, lovely site. Thnx ..

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