So what’s the tradition behind the shirttail cut?
In the days of tandem trainers, students sat in the front seat with the instructor behind them. There were no radios, and in those “early” days of aviation, the instructor would tug on the student pilot’s shirttail to get his attention (and then yell in his ear). A successful first solo flight is an indication that the student can fly without the instructor. As there is no longer a need for the “shirttail,” it is cut off by the proud instructor and often displayed on a wall in the flight school.