We had a recent flight change, lost another few minutes of our GC layover, and were already under your recommended 90 minute minimum. Going to be a bit tight for us on the return trip. Worst case scenario we will spend another day or two in GC. I hear there might be some diving there....
I agree that the return flights from the sister islands are usually pretty quick, especially if you are traveling in the morning. In my experience the GCM airport is usually the most busy on Sat and Sun from around 12 pm to 3 pm and it's pretty quiet on Saturday and Sunday mornings.
Just a few comments:
As stated by others, you do not have to go through Customs and Immigration when returning to GC from either Little Cayman or Brac, but sometimes the airport staff try to herd you into the incoming I&C lines, especially when there is another large plane (or several large planes) landing on GC around the same time with loads of newly arrived visitors. Don't let them force you into the incoming immigration line; get someone's attention and tell them you just returned from Little Cayman and you've already been through immigration.
The last time we were on LC the plane was a little late leaving LCM and it stopped on Brac. It was still a short flight and we had plenty of time, and our bags showed up quickly, but if you have a tight connection, be aware that it might actually be a little tighter than expected. However, the GCM airport does offer a Meet and Greet service for a fee, see the link below. I have never used it but I have read good things about it - if you are concerned about missing your connection then it might be worth checking it out.
https://www.caymanairports.com/airport-advertising/meet-greet-service/
When you have a generous layover time, flying to/from Little Cayman or Brac (or just flying home after a trip to Grand Cayman), keep in mind that the George Town Yacht Club is only a few minutes from GCM and they offer a free shuttle, see the links below.
https://www.gtyachtclub.com/
George Town Yacht Club
After you check in at the airport for your next flight and check your bags, instead of going through Security, walk outside the Ticketing Hall to catch the GTYC shuttle. They make regular rounds about every 15 minutes but we usually just call and tell them we are waiting and they show up in a few minutes.
If you have a carry-on bag they will stash it for you at the restaurant. Tell the driver when you want to get back to the airport and she will be waiting for you. Keep in mind that you will have to go through Security when you get back to the airport in order to access the Departure Lounge and catch your next flight.
The GTYC opens for breakfast at 8:30 am on Sat and Sun and they open at 11 am for lunch on weekdays, so you can enjoy a nice meal in a lovely setting (indoors or outside) and grab a little more vacation before you have to leave.
