It began losing altitude, Iron Man managed to get the engine damaged by Hawkeye's arrow running again before it crashed. (IIRC, it was at around 10,000 feet when the engine began running again.) Which brings up my nitpicky complaint that Joss (or whoever) seemed to forget that walking around outside at high altitude isn't exactly possible. The Hellicarrier was flying over the clouds which would put it at the same altitude commercial jets fly at, around 30,000-40,000 feet. At that height it's not only very cold but there's practically no air to breathe. Yet we see Captain America, Hawkeye and Nick Fury all outside while the Hellicarrier is flying at altitude, there were also times when a plane or something was at altitude and someone would open a door or something which, again, would de-pressurize the craft which would be bad news for the exposed mortals inside. Iron Man in a pressurized suit wouldn't be bothered by the lack of air, nor would Hulk or Thor likely be bothered but Hawkeye, Black Widow, Nick Fury and even Captain America are still mortal humans that come with all of the expected weaknesses. (IIRC Captain America is only at peak human capabilities and not strictly "super powered.") So it stood out as odd to me to see the mortal human characters outside at high altitude (again with very thin air and very cold temperatures) without really being bothered by it. That's probably one thing that took me out of the movie if even only a little bit. I believe a Black Widow film with ScarJo actually is planned. As for a solo Hulk film with Ruffalo? I'd be interested in seeing it but I think the biggest hurdle is I think the Hulk works best with someone else. When Banner is Hulk he's not much more than ass-punching and quips. Which really only worked here because of the way he played off the others.