After being hired by the CIA as a mercenary, a man teams up with four other mercenaries to stop drug lords from taking a plane hostage.

You can't really take a plane hostage... What or who is on the plane, that is valuable enough to take hostage... The president's daughter (For instance)

I like the idea of outlaws (i.e. mercenaries) having to operate inside the law. And I like the idea of 4 loners coming together to form a team. So I'm on board with the concept.

What I'm wondering is: do you need the CIA in this scenario? Or does the CIA hire the 4 mercenaries IN ORDER TO stop the drug lords?

If so, I begin to wonder: are they ON the plane? Or is it a race to stop the plane? Or does it work some other way? And why these 4? How would mercenaries stop a plane -- or drug lords? Wait, why drug lords? Is it a plane heist or a shoot-em-up?

I think you can clarify some of these questions quite simply, though. Maybe focus on a) your protagonist and b) your timeline. So:

The world's most wanted mercenary is blackmailed by the CIA to take out a terrorist cell during a mid-air hijack.

Or however your story rolls!