As soon as the plane gets hijacked and the conference room is breached in the mission "Turbulence", kill the guys that enter and make sure you pick up one of the AK-74U's before you head into the next room. Once you've done that, head into the next area and you'll suddenly be thrown into the air. You're now in the zero-gravity part of the mission.
While floating about, you'll need to kill the enemy forces that are also in the air with you. This can be quite tricky because of the camera angles and you getting turned upside down, so try and be as fast as possible, but don't just spray bullets. Also remember that swapping weapons is faster than reloading. It's also a lot easier to just hip-fire, rather than zooming in. You should also have killed 3 guys before you get thrown about the 2nd time before the remaining 2 can be dispatched. If you don't get 3 then 2, restart the checkpoint otherwise you probably won't have enough time.