Complex sentence: From April 1961 a TG-18 Auxiliary Power Unit was fitted for ground starting rather than the bank of lead-acid batteries, some aircraft were modified to allow the APU to be run in flight.
Split into simpler sentences: From April 1961 a TG-18 Auxiliary Power Unit was fitted for ground starting rather than the bank of lead-acid batteries. Some aircraft were modified to allow the APU to be run in flight.