The Movie: On the deck of the famed Helicarrier, Natasha Romanoff warns her fellow Avengers that it might get difficult to breathe. Shortly afterwards, the whole ship reveals itself to be a massive chopper and takes flight. The outer paneling of the ship transforms into a patchwork of reflective mirrors, acting as a cloaking device.
Real Life: Props to the film for nodding to a real life invisibility technique, called "optical camouflage," as the method of clocking the Helicarrier. You can test out optical camouflage for yourself: tape a webcam to your back, and put your laptop in front of you, facing some friends. It will seem as if they are looking right through you! In The Avengers, however, the panels look to be just normal mirrors reflecting the sky and clouds around the ship. That might cloak the ship from certain angles, but not from below, because the mirrors on the bottom would reflect whatever ocean or landmass it's flying over. Also, mirrors with that kind of sensitivity would catch a lot of sunlight, effectively signaling to everyone around that a giant ship is trying to be sneaky and failing hard. So if you ever see a Helicarrier-shaped piece of ocean, city, or meadow flying above you, sparkling in the sun, be careful! The Avengers have been assembled for some reason (probably interdimensional aliens).