Aircraft weight is important not just for calculating things like how much runway, thrust, and fuel an aircraft will need, but also for other things like calculating how much they disturb the air by creating vortices. These vortices can be dangerous for other aircraft flying in their wake. Commercial aircraft are divided into weight categories by the ICAO to account for the Wake Vortex Turbulence they generate. The FAA also has categories for aircraft based on weight that mirror those of the ICAO, except for smaller aircraft.

Larger, heavier aircraft generate more vortices than lighter, smaller ones. Larger aircraft are also less impacted by the wakes of smaller aircraft, while smaller aircraft are more impacted by the vortices generated by heavier aircraft. Consequently, an aircraft coming behind a 'Super' category Airbus A380 must have a distance of five to eight nautical miles, depending on the size of the succeeding aircraft.

Aircraft Weight

The weight of an aircraft is of critical importance for a range of things. For example, an overloaded aircraft may not be able to leave the ground, and if it does, the weight may result in unusually poor flight characteristics. Increased total weight is undesirable for the aircraft's performance, and manufacturers strive to make aircraft as light as possible without having to sacrifice strength or safety. Increased weight reduces the flight performance in almost every respect.

Aircraft are built to handle different levels of weight and carry various levels of payload. While it's possible to build giant double-deckers like the Boeing 747 and Airbus A380, these large quad-engine designs have become inefficient and have fallen out of production. Nowadays, the largest commercial aircraft in production are the Airbus A350-1000 and the upcoming Boeing 777X.

The FAA and ICAO classify aircraft into four weight categories, with the ICAO's category called Wake Turbulence (WTC). This is based on an aircraft's maximum take-off weight (MTOW). The FAA used the categories Super, Heavy, Large, and Small. Meanwhile, the International Civil Aviation Organization (ICAO) classifies aircraft into Super, Heavy, Medium, and Light.

Wake Vortex Turbulence

All aircraft generate vortices at their wing tips from the aircraft producing lift. The heavier the aircraft is and the slower that aircraft flies, the stronger the vortex will be. Another important factor that influences the size of the vortex is the span of the aircraft generating it. At low altitudes, these vortices normally persist for up to 80 seconds, while in very calm wind conditions, they can last for up to two and a half minutes. They normally dissipate quickly in windy conditions, and cross winds can carry a vortex away from the flight path.

According to Skybrary, Wake Vortex Turbulence is generated from the point where the nose landing gear leaves the ground at takeoff to the point where the nose landing gear touches the ground during landing. When another aircraft encounters the turbulence generated by another aircraft, a Wake Vortex Encounter is said to have occurred.

The ICAO lists aircraft in categories based on the certified maximum takeoff weight (MTOW) of the aircraft. It divides commercial aircraft into Heavy, Medium, and Light, with another special category, Super, for the Airbus A380. The FAA also has its own Aircraft Weight Class category, where most commercial passenger jet designations overlap with the ICAO's list.

Super (Category J)

Super is a weight category (Category J) reserved only for the largest aircraft: even the Boeing 747-8 doesn't fit into this category. The only commercial passenger jet in the Super category is the Airbus A380 'superjumbo.' Other noncommercial aircraft in this grouping include the Antonov An-225 Mriya and the bizarre twin-boom Scaled Composites Model 351 Stratolaunch (called Roc).

While 251 examples of the Airbus A380 were built, only one example of the An-225 and Model 351 Stratolaunch was built. The Ukrainian An-225 military transport aircraft, which was originally built by the Soviet Union to carry the Buran, the Soviet counterpart to the American Space Shuttle. However, that aircraft was destroyed in fighting in early 2022, leaving only the A380 and Model 351 in the Super category. Aircraft designated as 'Super' include the likes of:

• Airbus A380.
• Scaled Composites Model 351 Stratolaunch.
• Antonov An-225 Mriya (destroyed).
• Antonov An-124 Ruslan (classified by the UK Civil Aviation Authority as Super).

Additionally, the An-124 Ruslan, the other large Soviet military transport aircraft, is classified by the UK Civil Aviation Authority as Super. It is classified by the ICAO as Heavy, as is its smaller American rival, the Lockheed C-5M Super Galaxy. The Airbus A380 has an MTOW of 1,268,000 lbs, while the An-225 had an MTOW of 1,410,000 lbs.

Heavy (Category H)

Heavy aircraft are aircraft capable of takeoff of weights of 300,000 lbs (136,000 kilograms) or more, except for the A380. They are classified by the ICAO as Heavy, whether or not they are operating at that weight during a particular phase of flight. Heavy aircraft include almost all widebody aircraft types. The FAA, for its part, speaking of Weight Class, says that "any aircraft weighing more than 255,000 lbs, such as the Boeing 747 or the Airbus A340," is classified as Heavy.

It's not possible to list out all the aircraft the ICAO designates as Heavy, as it has 150 entries for that category. However, while this is a category mostly filled with widebody jets and large military transports, it does include some larger narrowbody aircraft. For example, the smaller Boeing 707-100 variant is classified as Large, but the bigger Boeing 707-300 is classified as Heavy. Aircraft classified as Heavy include the likes of:

• All widebody aircraft (except the Airbus A380).
• Most large military transports.
• Some large narrowbody aircraft (such as the Boeing 707-300).

Some other notable examples of Heavy aircraft include Concorde and the Airbus A400M Atlas. The A400M is the largest military transport currently in production in Europe and is at the lower end of the Heavy category. Heavy aircraft following behind an Airbus A380 (Super) need to have a minimum of five nautical miles' separation.

Medium (Category M)

Whereas the ICAO classifies this weight category of aircraft as Medium, the FAA uses the term Large. The FAA defines Large aircraft as including all aircraft with a maximum certified takeoff weight of over 41,000 lbs and up to but not including 300,000 lbs. The ICAO designates Medium aircraft types as less than 300,000 lbs and more than 15,500 lbs. This means more smaller aircraft meet the ICAO's Medium designation than the FAA's Large designation.

The Douglas DC-8 is an example where some variants are classified as Heavy and others are classified as Medium/Large by the ICAO and FAA. ICAO-designated Medium aircraft include most narrowbody aircraft, including the Airbus A321, which is the larger counterpart to the Airbus A320. It also includes Boeing's mid-sized Boeing 757 narrowbody, although the 757 is at the higher end of the category.

Indeed, both the 757-200 and 757-300 fall under this classification. Some states apply a higher category for the Boeing 757 than suggested by the aircraft's weight in case it is the preceding aircraft, and, therefore, the one generating the wake turbulence.

Almost all modern narrowbody commercial aircraft are in the Medium category, while all widebody commercial aircraft currently in production are in the Heavy category. Medium aircraft include Embraer E-Jets, the Airbus A220 (formerly the Bombardier CSeries), the ATR 72, the Fokker 100, the Boeing 717, and almost all other regional and commuter jets. A Medium jet following behind a Super A380 needs to have a minimum of seven nautical miles of separation.

FAA's Small Aircraft / ICAO Light (Category L)

The ICAO continues to classify aircraft designated by the FAA as Small as Medium, as its cut-off point is 15,500 lbs. The FAA defines small aircraft as "aircraft of 41,000 pounds or less maximum certificated takeoff weight." These aircraft include the ATR 42-500, Saab 340 (extensively used by Australia's regional Rex Airlines), Embraer 120 Brasilia, Pilatus PC-24, BAe Jetstream 41, and some other smaller aircraft. Those aircraft are classified as Medium by the ICAO.

An example of an aircraft designated as Light by the ICAO is the Embraer EMB-500 Phenom 100, which is a very light business jet that has a maximum takeoff weight of around 10,500 lbs. It is powered by two rear-mounted Pratt & Whitney Canada PW600 turbofans and can carry between four and seven passengers up to 1,178 nautical miles. Most light aircraft are small Cessna-like aircraft.

The ICAO lists Light aircraft types as being 15,500 lbs or less. A Light aircraft needs to have a minimal separation of eight nautical miles when following behind an A380. If the Light aircraft is following a Heavy aircraft, it is required to have a minimum separation distance of six nautical miles, while it is five nautical miles when the preceding aircraft was a Medium aircraft. It should be stressed that the Wake Vortex Turbulence is only one factor where the weight classification of aircraft is important.