How are the fighter plane generations determined?

A Sopwith Camel, one of the most successful British fighters of  World War One

1st Generation fighters were the canvas-covered, fixed fixed landing gear, bi- and triplanes of World War One and early World War Two. They had open cockpits and only the smallest amount of instrumentation. They were also lightly armed, usually with only one or two machine guns. These were the first fighters and were the planes in which aerial warfare was learned, with their lessons still relevant today.

2nd Generation

A P-51 Mustang, one of the most ubiquitous 2nd Generation fighters

Second Generation fighters were the aircraft of World War Two. When you think of a fighter, a plane like this probably pops into your mind. These aircraft were the first all-metal, enclosed cockpit monoplanes to come into wide use. They also featured retractable landing gear, and other avionics advances combined with more powerful engines meant that these planes could go two or three times faster than their predecessors. They also had much heavier armaments, either 6 machine guns (most common on American aircraft) or some combination of machine guns and cannon (most non-American aircraft were equipped this way). This generation saw the greatest use in their intended role in World War Two, but was also the most short lived, coming to prominence in the late 1930s and ending in the early 1950s.

3rd Generation

A MiG-15, the most common Eastern Bloc fighter of the early 1950s, in Polish markings

3rd Generation aircraft represent something completely different from previous generations of fighters- jets. These aircraft could fly higher and faster than piston engined aircraft, being nearly invulnerable to 2nd Generation aircraft unless the jet pilot was exceptionally stupid or unlucky. However, that was in the air. Early jet engines accelerated poorly. This generation saw the introduction of guided missiles, though their teething troubles meant that gun battles were still common. Early fighters of this generation could not go supersonic, though that changed later. This generation began with the first jets being introduced in late World War Two and ended in the 1970s and 1980s for most airforces, though a few still soldier on in minor air forces and as targeting drones.

4th Generation

A MiG-29 of the German Air Force, formerly of the East German Air Force and absorbed into the German Air Force with the Reunification of Germany

4th Generation aircraft represent the computerization of fighter aircraft. These aircraft have fly-by-wire and HUD systems that completely change the experience for the pilot, allowing them to focus less on flying the aircraft and more on fighting. They also have more advanced avionics and radar, greatly increasing dogfighting capabilites. Missiles also became more advanced and are now highly reliable, though countermeasures have increased side-by-side. These aircraft represent the main fighter force of most nations and were introduced in the 1970s and 1980s, still being in widespread use today.

5th Generation

Two American F-22s, the only 5th Generation figher in active service

5th Gneration fighters have the same advantage over 4th Generation fighters as 4th Generation fighters had over 3rd Generation ones: they have increased avionics and computerization. However, they also have one critical advantage over 4th Generation fighters: they have stealth. 5th generation fighters are virtually undetectable by their 4th Generation counterparts, which makes them nigh on untouchable. These aircraft are the most advanced aircraft ever created. While the F-22 is the only 5th Generation fighter on active duty, several other fighters, most notably the F-35, PAK FA, and the J-20, are scheduled to come into service in the late 2010s and early 2020s.