Do airlines boroscope engines as part of regular mx, if so how frequently.
Yes, engine boroscope is a scheduled maintenance item - frequency depends on the engine and where in the engine (hot section is more frequent), typically cycle based. When there are known issues the interval is often shortened - often if there is a finding, they are allowed to continue operation but need to re-check at a very short interval (I've seen as little as 10 cycles) to make sure it's not getting worse. Obviously at some point the problem will drive the engine off-wing.