[tialaramex]

An airliner's external doors are designed such that they're held closed by internal pressure. Thus opening a door on the ground is pretty easy, which is convenient because that's the only place you should open them. But opening at cruise is extremely difficult.

Now - a strong person can do it when the plane is far enough up that it's very scary, after all some normal passenger airports are a few thousand feet above sea level and we want the doors to work there. But you won't open them at cruise.

All the normal doors on an aeroplane are fire doors so they can't be locked from the inside.

[schiffern]

  > after all some normal passenger airports are a few thousand feet above sea level and we want the doors to work there

That doesn't make sense.

Regardless of airport altitude, when the pilot shuts off the cabin pressurization and opens the cabin vent[s] there will be zero pressure across the door.

The cabin doesn't "remember" the pressure at the last airport, because it's not a sealed volume. The air is constantly being replaced from outside, and the pressure is constantly being regulated.

[tialaramex]

That's fair, maybe I could have explained it better.

[jandrese]

> Now - a strong person can do it when the plane is far enough up that it's very scary

Is this true? Some back of the napkin math: An airliner crusing at 30,000 feet is in air at about 0.3 ATM. Cabins are pressurized to about 0.8 ATM, so net air perssure is about 0.5 ATM. That's around 7 lbs/in^2. An airliner door is about 72"x42" for a total of around 3,000 in^2. So you need to be able to lift about 10 tons to open it. That would be a very strong person indeed.

[softskunk]

> after all some normal passenger airports are a few thousand feet above sea level

The parent commenter is referring to airports at high elevations. They mean to say that opening the doors is almost impossible at cruising altitude, which is far above any airports. Hence, the doors can only be opened when not on the ground during early ascent and late descent.

I hope my explanation makes sense.

[sebzim4500]

You are agreeing with him, he says you can't do it at cruising altitude.

He says you can do it high enough to be scary, but opening the door 100ft before landing would probably cause panic even if it isn't all that dangerous.

[MBCook]

It has happened at a few hundred feet, iirc. Nowhere near cruise but being at 3000 feet and having someone open the door would indeed be terrifying.

I’m guessing something like that is what the GP meant.