Hello,

The eligibility criteria to appear for AFCAT for Flying branch is that a candidate must have passed 10+2 with  Mathematics and Physics with minimum of 50% marks individually in Mathematics and Physics from a recognized board. Also the candidate must have passed 3 year bachelor's Degree with minimum 60% marks from a recognized University  or must have passed BE/B Tech degree with minimum 60% marks from a recognized University or must have cleared Section A & B examination of Associate Membership of Institution of Engineers (India) or Aeronautical Society of India with a minimum of 60% marks from a recognized University. The age must be in between 20-24 year however, for CPL license holder issued by DGCA (India) the upper age limit is 26 years.

You haven't mentioned the subjects that you have studied in NIOS but check the above mentioned eligibility criteria, if you have fulfilled it you can appear for AFCAT when you are in the final year of the qualifying exam and after passing in the qualifying exam.