Definition
The unique alphanumeric registration identifiers assigned by the Federal Aviation Administration to civil aircraft registered in the United States, all of which begin with the letter N followed by up to five characters (numbers, or numbers and letters).
Plain English
Every U.S.-registered aircraft has its own ID code painted on the outside, and because the code always starts with the letter N, these IDs are called N-numbers.
Context Anchor
Seen on the outside of U.S. aircraft, in aircraft paperwork, maintenance records, accident reports, and radio calls.
Derivation
The letter N was assigned to the United States by the International Telecommunication Union as the country's prefix for radio call signs after World War I. That prefix carried over to aircraft registration, so every U.S. civil aircraft ID begins with N.
Why Pilots Care
Proper identification of the aircraft is required for air traffic control, regulatory compliance, and accident reporting.
Analogy
An N-number is like a license plate for an aircraft, with the country code built into the front.
Intuition Check
Do not assume an N-number is the aircraft model number or serial number. It is the aircraft’s official U.S. registration identifier.
Example Sentence 1
The pilot read back the clearance using the aircraft's full N-number on first contact with the tower.
Example Sentence 2
During the pre-landing checklist the pilot verified the N-number matched the one cleared by the tower.