Three years on contractors is longer than most founders expect, but in a regulated niche like airline maintenance it makes sense. The domain knowledge sits with the founders, and the engineering work comes in clear chunks tied to customer or compliance deadlines.
What decides whether that model holds up is how each chunk is defined. When contract work is scoped as one specific problem with a written outcome, the codebase stays understandable and the eventual first hire inherits a set of documented decisions. When it is scoped as open-ended hours, the first hire inherits archaeology.
The moment to hire full time, in my experience, is when the work stops arriving in chunks and starts needing someone who carries the whole system in their head every day.