All HCS squadrons are reserve squadrons and the officers are all TARs. I am not sure exactly how the squadrons evolved to be the way they are, but on paper at least they are supposed to be for training of reserve pilots. In actual practice, they are the squadrons who fly the majority of all Naval Special Warfare support missions. Their only platform is the HH-60H, and getting a TAR spot in an HCS squadron is probably toughest ticket in the helo world. The most junior guys in HSC squadrons are going to be senior LT's and there are not even that many of those. The wardroom in HCS squadrons is VERY senior. If you want in to an HCS squadron, HS is probably the best way to start out. HS squadrons also fly the HH-60H and they share the NSW mission with HCS and also get a lot of good experience with CSAR. After your first fleet tour, your best bet is to be an instructor at the RAG or an evaluator at the WTU (weapons and tactics unit).
There are rumors that HCS may be opened up to non reserve officers, but there is nothing set in stone yet. I think that the HCS squadrons like the way they have it set up right now, and they don't want to change. They pretty much get their pick of guys to bring in, and they have their happy little world with their Gucci mission. If you are still in flight school, you can pretty much forget about HCS for the next 8 years or so.
