Complex sentence: He sold his second aeroplane (which he had used to teach himself to fly) to his new employer for 400 pounds-it became the F.E.1-the first aircraft to bear an official Royal Aircraft Factory designation.
Split into simpler sentences: He sold his second aeroplane (which he had used to teach himself to fly) to his new employer for 400 pounds. It became the F.E.1, the first aircraft to bear an official Royal Aircraft Factory designation.