Which LCD are you using? An SPI interface'd unit as suggested in reply #2, or something else>?
SPI display
only needs SS, MOSI, SCK connected (altho MISO is also tied up).
Another SS can be used to drive MAX7221, it will drive 4 digits and then up to 32 other LEDs (plus 4 decimal points from the 4 digits if not being used).
So 2 SS's, SCK, MISO, MOSI, 5 pins.
2 pins for serial interfacing with the Flight Sim.
Leaves you 13 pins free for buttons or whatever.