Dear Sampath, let me clear your confusion first,IISc doesn't offer any course at undergraduate level - B.E/B.Tech- in Aeronautical engineering. Although BSc if offered in IISc.

M.Tech in Aeronautical engineering is offered but for that you have to give gate exam and after clearing that you will be called for personal interview.

On the basis of your performance you will be given admission.

All the best

Hope this helps