Both airplanes and helicopters handle like trash. Why do the airplanes twitch on their own? Even with 100/100 flying level. In real life, you can leave an airplanes yoke alone for minutes at a time and it will keep steady. Realism aside, if Rockstar was trying this, WHY? We're using gamepads not flight simulator setups. As for turbulence, it rarely happens at low altitudes. The rudders slam the aircraft making precise movements impossible since the bumpers are just clicks. Everything is way to twitchy.
As for helicopters, they are near impossible to be steady without using all of your limbs at once to control but once again, we're using gamepads not flight simulator setups. GTA IV's helicopter control was perfect. The auto-leveling is also extremely annoying.
This is easily fixable by just tweaking a few numbers in a .dat file but knowing Rockstar, we're going to have to wait until it comes out for PC and someone makes a handling mod.
TL;DR - The aircraft handling in this game is worse than Just Cause 2 and that's saying something