Actually if you donīt have APU you donīt need GPU, start engine2 with batts and it will be less noisy than the APU at only 200lbs/hour, just double from APU, that simple and common use. The LJ60 itīs a great aircraft if you treat it well and, on some T/O, you have to derate the power at 400ft because lots of pax donīt like that much rate of climb.