For manufacturer's info go to either Maint manual Chap 4 (for scrap lives) or Chap 5 for main recommendations, generally speaking.
For PT6 look in Service Bulletins, early numbers (Bulletin series depends on PT6 model)
Phase checks versus 150 hour problem is terminology - Beech talk about phase check (means interval between inspections) currently recommended at 200 hr interval e.g. Ph 1 @ 200, Ph 2 @400, repeating 4 Phases at 2yr cycle if the hours don't accrue.
Old heavy iron types often talk about 'C' checks or 'D' checks for big inspections - meaningless names unless you have a copy of the maint schedules. You could equally well call it a sausage check or a dog check, it just depends what the schedule writer decided to call it!
As for how do they decide - best ask the chaps wot make 'em...
Gizza day or two and I'll check current Kingair situation and let you know.
