i had a appendix surgery in 2015. i am studying aeronautical engineering 3rd year..so i want to appear for the AFCAT exam this year in September. Can I be eligible for airforce as i went through the surgery long back that is in 2015??

As long as your past medical history doesn't affect your overall performance on the function of that part, you are considered fit. Your scars, residual injuries, fractures will be assessed for the functional limitation. If yours has been a simple appendectomy involving a minor surgical intervention, that is ok. If you can meet the other eligibility criteria, then Appendectomy should not be a matter of concern. Appendectomy is not included in the list of surgeries that can cause you to be unfit for flying so you need not worry .