When we estimate an STC, people look first at engineering hours and the Agency's fees and charges. In my experience, the line that decides whether the project makes money is another: the prototype aircraft.
Most STC applicants don't own the aircraft they modify. They borrow a slot from a launch customer for the prototype installation, conformity checks, ground tests and, where needed, flight tests under a permit to fly. That slot is negotiated months ahead, around the operator's flying programme and maintenance checks.
Miss it, and the cost isn't a few extra engineering days. It's waiting for the operator's next window, sometimes the next season, with team, kit and flight test crew on hold. A late flight conditions approval or an open point on the test programme can cost months when engineering slipped by weeks.
I now treat aircraft availability as the critical path and work backwards from it: what must be agreed with the Agency, and approved internally, before the aircraft rolls into the hangar? A compliance management tool linking each requirement to its open activities flags the gaps months ahead, not the week before.
How do you protect the prototype aircraft slot on your STC projects?