Complex sentence: He served as Chief of the Air Staff during the late 1980s at the time of the ordering of the Boeing Airborne early warning and control system and the development of the European Fighter programme and then served as Chief of the Defence Staff during the Gulf War.
Split into simpler sentences: He served as Chief of the Air Staff during the late 1980s, when the Boeing Airborne early warning and control system was ordered and the European Fighter programme was being developed. He then served as Chief of the Defence Staff during the Gulf War.