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These are the cabin outflow valves for regulating cabin pressure.


Cost and reliability. At the dawn of the jet era, no jet engine was powerful enough to use just two. They also tended to fail a lot, so even once two was powerful enough, one still wasn’t, so quad-jets ruled the skies. However, power improved quite rapidly, and it soon became practical to build large twin-jets, which were significantly cheaper to buy and to ...

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