To land a plane safely make sure you approach the runway straight on. Slow down the plane with the same button you would use in a car. Drop the landing gear with the same
button you use to enter stealth.
If the plane gets too low to reach the runway be sure to accelerate.
auluftwaffles.com, short video game guides 
Or given in game prompt for lowering landing gear.
Rockstar could have made this easier goddamit