Definition
A single letter placed after the aircraft type designator in Item 10 of the ICAO flight plan (or the equivalent field in a domestic FAA flight plan) that tells ATC what navigation, communication, and surveillance equipment the aircraft has on board and is approved to use. The suffix identifies whether the aircraft is RNAV-capable, what level of RNAV or RNP it supports, and whether it has transponder, DME, GNSS, and other equipment relevant to routing and separation.
Plain English
It is a code letter you put on the flight plan to tell controllers what your aircraft's navigation gear can do. The letter tells them whether you can fly direct routes using onboard navigation, and what kind of approaches and procedures you are allowed to fly.
Context Anchor
Seen when filing IFR flight plans for random RNAV routes or other routing that depends on the aircraft’s RNAV approval.
Derivation
Suffix comes from Latin sub- (under, after) and figere (to fix or attach), meaning something attached at the end. The suffix is literally a letter attached at the end of the aircraft equipment field on the flight plan.
Why Pilots Care
It determines whether ATC will clear the aircraft for direct routing and RNAV instrument procedures rather than ground-based airways.
Analogy
It is like a short approval mark on a form. The mark does not fly the airplane, but it tells the system what the airplane is approved to do.
Intuition Check
Do not treat “suffix” as just a piece of spelling at the end of a word. Here it means an official flight-plan code that carries operational meaning.
Example Sentence 1
Before filing the flight plan, the pilot confirmed the correct RNAV capability certification suffix for the aircraft's installed GPS and transponder.
Example Sentence 2
Without the proper RNAV capability certification suffix, the aircraft was routed along airways instead of receiving a direct clearance.