Here is the full list of all 9 Zombies on a Plane achievements.
Unlock the 3rd aircraft liveary slot by accumulating 500,000 points
Reach 20,000 score in shooting modes
Stack up a high combo of x50
Unlock Brett by accumulating 1,000,000 points
Rid the skies of zombies
Crash into a ship, an island or the sea
Finish all 10 waves in either 'Defend the Cockpit' or 'Flight of 10 Waves'
Press repair key to repair a damaged part
Stay alive in either 'Survive the Horde' or 'Flight of Survival' for 60 whole minutes