There is no percentage barrier to be eligible for NDA. For NDA examination, one needs to successfully qualify 10+2 from a recognized educational board. And for Air Force and Naval Wing, candidates who have studied and passed Maths and Physics in intetmediate is are eligible. So you must have studied Physics and Mathematics if you want to appear for NDA for Air Force. You have passed 12th PCM with 51% marks which makes you eligible for this. Also keep in mind the age criteria. The minimum age limit is 16.5 years while the upper age limit is 19.5 years.