I guess that could mean either couldn't it?
If I'm riding a Greyhound and the bus driver has a mental breakdown, and takes everyone on a joyride, I don't call that a "carjacking". I mean, I'm sure the press and officials might refer to either as a "hijacking" in this case, but I just find it an awkward way to describe the scenario when a pilot doesn't do what he's supposed to with his aircraft.
Anyways, I'm being pedantic.
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This whole thing is going to be a bit unnerving if say, the young co-pilot turned out to be a sleeper agent for a terrorist organization. I'm obviously not saying there's any evidence to support that conclusion, but in the case of the US, that might be one of the ways to successfully hijack a plane, as doing it the conventional way would be less likely to succeed post 9-11 (see: Richard Reid).