# What are the length requirement for US aircraft N-Numbers / registrations?

Many small aircraft registered in the US seem to have a registration that follows the pattern of N, 4 numbers, and ending in a letter. Sometimes they only have 4 characters, or less. And then there's the FAA Gulfstream N1.

Who decides on what the N-Number will be, and are there requirements for how long it should be?

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It's not just small aircraft in the US - all aircraft with US registration follow the same rules. The FAA has guidelines on how an N number must be formed. Basically, there are relatively few limits - you can have N + up to 5 characters, up to 2 of which can be letters (at the end).

Here's what the FAA has to say:

U.S. registration numbers may not exceed five characters in addition to the standard U.S. registration prefix letter N. These characters may be:

• One to five numbers (N12345)
• One to four numbers followed by one letter (N1234Z)
• One to three numbers followed by two letters (N123AZ)

To avoid confusion with the numbers one and zero, the letters I and O are not to be used.

### Other Requirements

• An N-Number may not begin with zero. You must precede the first zero in an N-Number with any number 1 through 9. For example, N01Z is not valid.
• Registration numbers N1 through N99 are strictly reserved for FAA internal use.

When an owner registers an aircraft, they can request an N number, provided it's not already registered.

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N-Numbers can be reused too (e.g. N3794N) –  Farhan Jul 7 '14 at 13:33
Yes, but only if the previous registration has expired or been removed. :) –  egid Jul 7 '14 at 15:32

Two other answers specify the algorithm for generating valid U.S. aviation IDs.

Many who view this question might also be interested to learn that the aviation scheme is part of the entire U.S. radio station identifier scheme. IDs beginning with AA-AL, K, N, and W are allocated to the U.S. (by ITU). CF-CK are used in Canada, DA-DR in Germany, etc.

Commercial broadcast stations have all letters (no digits) beginning with W, K, and N; W as the first letter is used for almost all transmitters east of the Mississippi River and (mostly) K to the west.

U.S. amateur radio operators are assigned IDs with mostly letters with one digit near the front. They are of the form L[L]DLL[L] where L is a letter and D is a digit (or occasional two digits). [L] means an optional letter. The whole ID is 4 or 5 characters. The digit is assigned by class of license and region according the this scheme.

Government, scientific, civil, and other users are assigned by other unique schemes. For example, the Oregon NOAA weather radio stations (see table of stations) all begin with 2 or 3 letters and end with 2 to 4 digits.

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D is Germany, not only DA-DR. I'm pretty sure the entire C range is for Canada as well. –  falstro Jul 6 '14 at 11:56
Standard amateur radio call signs can be up to six characters long, with a one- or two-character country prefix where one character may be a digit but one must be a letter. For example, WE1XAM would be a valid US amateur radio call sign. Prefix and suffix can be attached to this; for example, SM/WE1XAM/MM would indicate that WE1XAM is operating maritime mobile (/MM) in Sweden (SM/). 9A/WE1XAM/P would indicate portable operating (/P) in Croatia (9A/). –  Michael Kjörling Jul 9 '14 at 13:44

Per FAA.gov:

• An N-number can be in any of these formats
• One to five numbers (N12345)
• One to four numbers followed by one letter (N1234Z)
• One to three numbers followed by two letters (N123AZ)
• N-numbers do not have
• A zero (0) as the first number
• The letters "I" or "O""
• N1-N99 are reserved for the FAA/govt.

Someone who wants to register an aircraft, can either request a specific N number, if it's available, or they can just ask the FAA for a random one.

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I used to fly N4AM (Four-Alpha-Mike) regularly. I don't know why/how it ended up with such a short number, but it did prompt extra questions from ATC on several occasions. –  abelenky Jul 6 '14 at 20:57
Yeah, since in the US, we can shorten callsigns to the last 3 characters after initial contact, it does lead to a little confusion if someone registers their aircraft as Nnumber,2 letters. –  slookabill Jul 6 '14 at 22:03