Leak

[great cornholio]

It's been my understanding that a pressurized airplane needs to be leaky just so it won't overpressurize with constant bleed air being fed in. Is that right?

You are right if it were totally air tight then it would pop just like when you blow a balloon up too much. To make it "leaky" they have these things called outflow valves. Outflow valves open and close to regulate the cabin PSI diff, altitude, rate of climb etc. The more the valve opens you get a higher rate of climb and a lower PSID. The more the valve closes you get a higher rate of decent and a higher PSID. When you have to dump the cabin the outflow valve goes to full open. It is usually open while on the ground so that there is not much PSID on the plane on the ground so you can open doors and windows as needed.

The other leaks come from things that pass through the pressure vessele such as wires, control cables, etc. They try to make the holes these things make as air tight as they can using rubber grommetts and what not, but they still seem to leak. Also door and window seals leak quit a bit and thats why you hear a door or window squeal sometimes.