I don't know about anyone else here, but as well as needing the experience to judge when a student is ready for solo (as FlyingFarmer says), I know I made a few bad calls for other reasons when I first qualified as an instructor. Like the student whom I sent for solo circuits less than an hour before official night. Or the time I sent an early student to "get a couple of circuits in before that weather arrives".
Just like everything else in aviation, or in life, you need a bit of practice before you know how to do it properly, and sending students solo is one of those things that I think takes a bit of practice. I'm glad that those few times I was sweating in the tower hoping my student gets back on the ground before things turn nasty were not first-solos.
FFF
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