Complex sentence: Nigeria Airways became Airbus' 40th customer in 1981, when it placed an order for four Airbus A310-200s; these aircraft were incorporated into the fleet in late 1984 and early 1985.
Split into simpler sentences: Nigeria Airways became Airbus' 40th customer in 1981, when it placed an order for four Airbus A310-200s. The A310-200s were incorporated into the fleet in late 1984 and early 1985.