How can airplanes fly upside down? I've read many times, including here on this very site that the commonly known explanation of flight is wrong, and that airplanes can fly because the shape of their wings deflects air down. This makes sense, but as far as I can tell it doesn't explain upside down flight or symmetric wings.
The images I've seen show an inclined wing, which forces the air to go downwards. But how can planes fly upside down then?
 A: Upside-down or right side up, flight works the same way. As you stated, the wing deflects air downward. When inverted, the pilot simply controls the the pitch of the aircraft to keep the nose up, thus giving the wings sufficient angle of attack to deflect air downwards.
Most airplanes are designed with some positive angle of attack "built-in," meaning that there is some angle between the wings and the fuselage so that the wings have a small positive angle of attack while the fuselage is level. This is why the floor isn't tilted tailwards when you're in an airliner in level flight. So when upside down the nose has to be held a bit higher than usual, and the other flight systems (including the pilot!) must be designed to handle it, but there is nothing really special about upside-down flight.
A: Colin's answer is right. Let me see if I can clarify a little bit.
First, forget that old Bernoulli explanation.
It's not wrong, but it confuses everybody.
If you create a simple symmetrical teardrop-shaped airfoil, and place it in a wind stream, then the air will flow past it, and if you turn it at an angle to the wind, it will deflect the wind stream, and it will feel a lateral force.
The vertical stabilizer of any airplane (with or without a rudder) works that way.
Now make an airplane's wings the same way - symmetrical airfoil.
Then the plane can create positive lift by tilting the nose up, and negative by tilting it down.
Some aerobatic aircraft are designed exactly that way, as in this example.
They can fly inverted just as easily as normally.
Most airplanes are not designed for inverted flight, so they use an airfoil designed for efficient upright flight, but they can still develop negative lift if called upon to do so.
(Example: The Cessna 172, a typical small plane, is stressed for 4g positive, and 2g negative.)
ASIDE: That means if the airplane, as loaded, weighs 1000kg, it is designed to withstand upward force of 4000kg and downward force of 2000kg. The latter can happen if the pilot stumbles into a downdraft at even a normal cruising speed.
Fun Fact: Inverted flight is interesting for other reasons as well.

*

*the ground is above you, and you have to press "nose down" to maintain altitude or climb.

*to turn left, you have to bank right, and vice-versa, because you're in negative-g.

*to turn to a heading, you have to realize that the compass is backward. If you're pointed North, East is on your left, not your right.

So it takes practice! 
