dear student

Yes, you are eligible for aeronautical engineering after your diploma in mechanicall engineering.

You will get lateral entry into 2nd year of UG Aeronautical Engineering Course.

There are few Institutions which offer lateral admission to diploma holders in B.Tech. Aerospace/Aeronautical Engg. Google for the colleges which offer aeronautical engineering and contact the administrator regarding the admission procedure. Some of the college offer such courses on lateral entry basis. So you have less a chance to get the seat

hope this helps

All the best!!!