what is the cut off marks for aeronautical engineering ?

For IIT-B, the cutoffs in the past few years have been in between 1.5k and 2k for Aerospace. The same have been ~2.5k-2.7k for IIT-K, and ~3k for IIT-M and IIT-KGP.

Regarding marks, scoring slightly above 50% of the max marks will surely land you with a rank close to 2k, which is sufficient for getting Aerospace at IIT-K, IIT-KGP, IIT-M but maybe not IIT-B.

Get around 10–15 marks more, and you will nail IIT-B Aerospace.

Still, the best advice (which I received from my tutors) will be to TRY YOUR BEST, and get AS GOOD A RANK AS YOU CAN.

WHAT IF YOU CAN SOMEHOW WORK YOUR WAY INTO TOP 100 AND JUST BECAUSE AEROSPACE CUTOFF IS AROUND 1.5K–2K YOU DO NOT PREPARE TO THE FULL EXTENT?

If you get AIR-1 and then take up Aerospace, you will surely have a THUG LIFE at whichever IIT you go to.

Although on a personal bias, I would go with IIT-K for the world class Aerospace facilities it has (including its own airstrip and huge wind tunnels).

But then, Aerospace Industry is ABSENT in India, unless you find some job as a scientist with ISRO/DRDO or maybe if you happen to become an Aircraft Engineer for some airlines, or you pursue MS/PhD from some top American University in Aerospace, in which case you may even get a chance to go and work with NASA.