You can break it down and carry it on in a small hard case or soft type bag (20" or less in length). Detector, batteries, charger, pinpointer are OK to take in the carrying, just make sure to put all the pointy metal things: digger or trowel, push probe, shovel into check in luggage.
Take out the batteries but have them available in the bag in case they want to see it work.
Clean everything well before you pack it at both ends of the trip. Sometimes grass has been freshly fertilized and can leave traces of nitrogen on the equipment and this could get you in trouble.
I flew at the end of June with mine as a carry on. It was no problem leaving Atlanta but coming home they took everything out of the bag, wiped it down looking for something. So make sure to allow a little extra time in security.
Also check with your airline as to how many carry on bags you can have as some have recently changed the number or size.
Have a great flight!
Jeff