Projects can run for several years, and the management can be reluctant to let staff leave prior to completion. Promotion seems to be down to luck of the draw: I’ve seen some graduates rise quickly, whilst equally gifted others have left after 4 or 5 years for a similar role before being promoted within a few months at their new company. It’s worth noting that at MBDA, a role of “Principal Engineer” is just used to denote approx ten years of time in the industry, seemingly rather than any particular technical knowledge or job grade. As of early 2018, there is a re-banding scheme underway to reduce the granularity of each “level”.