General Aviation aircraft have keys. This is because they are often left parked overnight in un-towered airports and in airports overseas where security is lax.
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Opening an aircraft door is impossible while the plane is at cruising altitude or above 10,000 feet due to air pressure. However, as the plane gets lower, experts say it is possible for a door to open as the pressure outside equalizes with the pressure inside the plane.
Why? For the simple reason that cabin pressure won't allow it. The cabin pressure is far too strong for anyone to open them. At a typical cruising altitude, up to eight pounds of pressure are pushing against every square inch of the interior fuselage.
According to the Federal Aviation Administration, the exit doors are built to only open on the ground. Even if the plane is at a lower altitude, the air pressure would be too much for a person to open an emergency exit mid-flight.
An airplane bathroom does not use the water like a regular bathroom. Instead, it uses a vacuum system, and when it flushes it moves into a septic tank that is on the airplane. The pipes used are much smaller than regular plumbing pipes and they can be placed in any direction since they do not use gravity and water.