This wasn't checked before the plane was worked on? You mean to say that the instructor had the power applied without noticing that the throttle was at full position rather than idle?

Sounds like a slight lapse of good judgement to me.

I've done something similar, but my throttle position was at normal idle, not full open. When I went to full open to "arm" the ESC, it was actually already armed. So, when I went from full open to idle, the motor rotated and, before I could react, it sliced my index finger to the point where I needed 5 stiches. Not near as bad as what happened in your situation, but somewhat different in terms of what caused the problem.

What I've suggested in the past was that motor checks be done with the prop removed. It won't hurt electric motors to do so. And, if that HAD been done, it would have been obvious that the throttle setting on the transmitter were not set right.

CGr.