There are three basic parts to each aircraft type field: Weight Category / Type Designator / Equipment Suffix.
The weight category is only applicable to 'Heavy' aircraft, defined as aircraft capable of takeoff weights of 300,000 pounds or more, whether or not it is operating at this weight during a particular phase of flight. For practical purposes, consider anything bigger than a Boeing 757 to be heavy (for example, B767, A330, A340, B747). A heavy aircraft receives a prefix of 'H/' in front of its aircraft type abbreviation. For the other aircraft categories (e.g. small and medium), no prefix is included.
Every aircraft has an aircraft type abbreviation, which is normally, but not always, four characters. A list of aircraft type abbreviations and their corresponding aircraft and wake category is available below. The list contains airplanes that are commonly used throughout BVA's server, but is my no means complete. For a more comprehensive list, see the FAA's Database.
Finally, the equipment suffix tells controllers of your aircraft’s navigation capabilities. A list of possible equipment suffixes, as well as an interactive tool to help you determine your aircraft's suffix, is available on the Equipment Suffixes page.
Below are some correct examples of aircraft type abbreviations. Notice how each of the aircraft types are four characters, each has a specific equipment suffix, and only the last example has a prefix because it is the only aircraft classified as 'Heavy'.
not using a
Air with GPS capability
|Boeing 737-800 using GNSS
|Boeing 757-300 using an FMS with limited RNAV capability
|Heavy Boeing 767 with GNSS