As others have mentioned, the purpose of Batter Saver mode is to cut power as much as possible to extend battery life. Radios are big power hogs, so wi-fi, Bluetooth, and GPS are candidates to get shut down.
I don't often use Battery Saver mode on my Tab S2, so I'm not sure if you can turn the wi-fi back on while in Batt Svr mode or not.
Easiest thing to do is just not use Battery Saver mode when using iFly. Make sure the tablet's fully charged before flight, and use either a USB power adapter (if you have a cigarette lighter) or a portable battery to extend the tablet's battery life while flying instead of Battery Saver mode.