hi,

see you may take admission in direct second year of btech course after completing your diploma in engineering, in india there is few aeronautical engineering colleges offer lateral admission and some of the colleges has restriction in diploma stream, as diploma in mechanical or aeronautical engineering students are eligible for btech in aeronautical engineering. so you need to carefully check the eligibility criteria before you go for admission test. you may follow the below link to know about aeronautical colleges in india,

https://engineering.careers360.com/colleges/list-of-be-btech-in-aeronautical-engineering-colleges-in-india