Complex sentence: Most aircraft had been disposed of by 1981 with the manufacturer buying the aircraft and simultaneous ending support of the aircraft, only the luftwaffe aircraft remaining in service, being disposed of in 1999.
Split into simpler sentences: Most aircraft had been disposed of by 1981, with the manufacturer buying the aircraft and simultaneously ending support of the aircraft. Thereafter, only the luftwaffe aircraft remained in service, being disposed of in 1999.