Complex sentence: In the early 1980s, Trago Mills elected to design and build its own aircraft that could be sold onto the British military as a trainer to replace the then aging "Bulldog" fleet.
Split into simpler sentences: In the early 1980s, Trago Mills elected to design and build its own aircraft. The basic premise was to design an aircraft that could be sold onto the military as a trainer to replace the then aging "Bulldog" fleet.