I’m considering booking a trip involving two separate bookings with a connection in Istanbul (IST) on different airlines.
I’m aware of the risks involved, and am eligible to enter Turkey visa-free if necessary, but I’d prefer to avoid the hassle of exiting and re-entering the airport.
Assuming that I can check in for both flights online and will have only hand luggage, is it permitted and physically possible to transit while staying airside on separate tickets at IST?
Yes, it is - as long as you have no checked bags.
Upon arrival in IST there will be 2 or 3 potential paths you can take.
For some flights, if you already have a boarding pass for your connecting flight, you will be allowed directly into the departure lounge. This option is not always available (it depends on a number of factors including the airport you arrived from), but if it if available for your flight then you can proceed directly to the gate for your next flight.
Otherwise you will be sent to an area with 2 paths out - one via immigration to enter the country (which will require a visa/etc), and a second for connecting passengers that where not able to take the direct exit to the departure area as mentioned above.
Connecting via this second option will require you to show a boarding pass, and to pass through security - after which you will be allowed into the departure area. Presuming you were able to check-in and obtain boarding pass online then you will be able to pass through this path.
If you were not able to obtain a boarding pass online, then depending on the airline you are flying you may be able to collect one from the transfer desk on arrival (before passing through security/immigration), but it will depend on the airline you are flying. Turkish Airlines definitely has this as an option, however I'm not sure that any other airlines do.
Note that all of this is predicated on you not having checked bags. If you have checked bags then you will need to clear immigration in order to collect them, and then proceed to the check-in counter to check them again for your connecting flight.
