Can an aircraft fly upside down?

Aerodynamically, yes, any aircraft can fly upside down, but for most but is very inefficient. Also, some have structural limitations that won't take the strain. Some aeroplanes that are designed for aerobatic manoeuvres have specialized wings, control surfaces, and engines. Practically, flying upside down is limited to small aircaft, acrobatic and jet fighters, and the like. Many aircraft, particularly large aircraft are not designed nor build to fly upside down

References:

“I sort of understand how an airplane wing works in terms of Bernoulli's principle (although I have only a vague idea why Bernoulli's principle is true). I know the wing is shaped to produce more air pressure on its lower surface than its upper surface and thus provide lift. If that's so, how can stunt planes fly upside down?” – asks John from Illinois, and gets an answer from Cecil @ Straight Dope. Interesting and detailed answer.

If the greater curvature on top of the wing and the Bernoulli effect are evoked to explain lift, how is it possible to fly an aircraft upside down? The illustrations in this page attempt to show that an increase in airstream velocity over the top of the wing can be achieved with airfoil surface in the upright or inverted position. It requires adjustment of the angle of attack, but as clearly demonstrated in almost every air show, it can be done

Interesting page: Basic Principles of Aircraft Flight

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