FESS stands for functional endoscopic sinus surgery and consists of any operation on the sinuses done through an endoscope ('telescope'). The sinuses are holes in the facial bones which exist to stop your head weighing a ton and putting you in a permanent nose down attitude. They also give you the tone of your voice!
All but the most minor FESS is done under general anaesthesia and we usually use deliberate hypotensive anaesthesia to stop any bleeding.
Removing polyps is only one of the things we can do - we can also drain pus, remove chronically infected linings and operate on tumours.
This is NOT minor surgery and should only be done by a surgeon and anaesthetist who does hundreds. There are risks including blindness, but the risk is very small and the benefit significant.
Chronic infection and polyps can and do recur so the word 'cure' is not always appropriate. However the procedure can be repeated.
There is nothing specific to flying and so no reason IMHO why you cant fly after the proscribed time or a general anaesthetic.
Happy to answer general questions