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