What are the most relevant KPIs to measure the quality of the training of students pilots?
I think the right starting point would be to list the qualities of a good pilot. Just Google for this phrase and you’ll see a lot of options. Look at the qualities that resonate with your perception and look important/relevant for your training program.
The next step is to do root-cause analysis and find out the leading factors of those qualities (that what will help you to build a better training program), as well as indicators that will help to validate achieved qualities (lagging indicators). Also, it’s a good idea to keep in mind the multi-layer nature of any training.
Have a look at the training scorecard and KPIs here.