And if the passengers did notice that something was wrong earlier in the flight, and began to take action, the pilot could have taken swift action to silence them. "The pilot can raise the cabin altitude [meaning, reduce the pressure to match that of a specific height above the ground] up to cruise altitude," says 777 pilot Rick Solan. In essence, the pilot can depressurize the cabin, which at 30,000 feet will cause passengers to lose consciousness within seconds unless they grab their oxygen mask (which will automatically deploy if the pressure drops to such an extent). But sucking air from a tube will keep passengers immobilized in their seats, and at any rate the chemically-generated oxygen will run out after 15 minutes.
That's a depressingly grim scenario, however, and there's good reason to think that whoever took MH370 would be strongly motivated to avoid it. If the most obvious motive for abducting a planeload of Chinese nationals - ransoming them for political leverage - is indeed correct, then killing them would defeat the whole purpose of the mission. Given the cunning strategy of the plan's early phase, it's more likely (assuming that the flight didn't end up ditching in the southern Indian Ocean) that he was able to keep the passengers quiet the way that airline pilots all over the world do every day: by exerting his authority as captain and offering a plausible-sounding explanation for the rerouting and delay.