Q:

Banner's control of Hulk in Avengers

During the climatic battle, Banner says 'That's my secret. I'm always angry', and transforms into the Hulk. Does this imply that Banner learned how to control Hulk? 
If this is the case, why did he transform during the Helicarrier attack? 

A:

My theory is this: 
Banner can already control The Hulk at the beginning of Avengers
This is shown when he is about to show his 'secret' to Black Widow the first time.  They are then interrupted (I forget by what already) before he can show her (presumably, his instant transformation).  It is then shown (in a very epic fashion) that he can turn into The Hulk at will when the giant Chitauri is approaching them stating to Widow "I'm always angry".
However, when Loki was escaping and caused the floor to pop out from underneath him and Widow, he then loses this control due to the sudden nature of the event and therefore doesn't have full control over The Hulk, hence the battle with Thor.

A:

My impression was everyone kept assuming the hulk was something he was constantly fighting for control, and whenever he got angry enough to lose his focus he'd run wild, when the reality was Banner doesn't "lose" to the Hulk, he calls him out. Once he transforms he doesn't have a great deal of control over what the Hulk decides to smash, because like he keeps saying, it's 'another guy' taking over at that point, but the choice to change is Banner's. 
It makes sense if you accept Banner the man isn't necessarily a great person to begin with, which I don't think he's ever claimed to be, and can get to the point where he actually wants to unleash the Hulk and let the consequences be damned. This also explains why he'd run off to hide in one of the most densely populated areas of the world, where the collateral damage would be massive if he ever let the Hulk out. It's not to get himself away from stress, it's to put himself in a position where the cost of calling the hulk would be more than he's willing to pay. 
On the airship Loki manipulates them all into wanting to fight each other, which explains why Banner would let the Hulk come out to play, he wasn't losing control over a monster, he was just as eager to throw down as the Cap and Stark were a few seconds earlier. 
Just a thought. 

A:

When Banner and Stark were in the lab, Stark poked Banner with something.  Then in the same scene he said something along the lines of "I learned to control Iron Man with this thing (points to light on chest) - you can control the Hulk with something too." Banner then says something like: "That won't work for me." Tony then says something like, "Yeah you will, you'll see." Then Banner turns into Hulk on the ship, but whatever Tony did hadn't kicked in yet (at least not fully).  
This is also why Tony knew that Banner would show up at the final battle , saying "Let me know when Banner gets here, okay?" to everyone's surprise, because he knew that Banner should be feeling the effects of whatever Tony did to him.  Then, at the very end, Bruce and Tony ride off together - because now they are close friends who understand each other and share scientific acumen as well. 

