Zack
Lv 7
Zack asked in Cars & TransportationAircraft · 3 months ago

# What prevents an airliner door from being opened at low altitude?

An airplane door can't be opened at cruising altitude due to the pressure difference between the inside and outside, but what prevents some crazy passenger from opening the door at altitudes below 10,000 feet?

Relevance
• 3 months ago

An airplane is pressurized almost from the moment the engine starts. I had this altitude indicator with me once I flew about 20 years ago (I am an aerospace engineer, and was curious about this, since pressurization is not my field -- I am a performance engineer), and I noticed that the flight display was claiming we were at 6000 feet while my gauge was showing about 2000 feet pressure altitude in the cabin.

So in essence, the cabin pressure is not the same as the exterior one unless you are below 1000 feet or so. Above that, the pressure is some kind of ratio.

As a consequence, the cabin pressure is keeping the door shut almost from the start.

• 3 months ago

Its called the  D.B. Cooper switch.

• 3 months ago

The door is actually pushed hard up against the surrounding fuselage of the plane once the cabin is pressurised and pressurisation takes place on the ground well before take off.  The door is not held against the outside of the fuselage, it's pushed against the inside.

At engine start the cabin pressure is held at ambient (outside).

As the plane climbs the outside pressure falls but so also does the cabin pressure (to reduce stress on the airframe) so that the cabin pressure falls to the equivalent of 8,000ft.  There is always a difference in pressure between the cabin (higher pressure) and outside (lower pressure).

The plane spends 90% of the time above 8,000ft.

As the plane descends the cabin pressure is increased ( 6,000ft equiv., 5,000ft equiv. etc, 4,000 etc etc ) to maintain a pressure difference on the door.   So there is always at least 1lb - 2lbs per square inch pressure difference even until the plane touches down.

After landing during taxing the pressure is equalised.

Even just 1lb per square inch pressure difference creates about 1.6 tonnes of force holding the door shut.  2lbs per square in pressure difference creates 3.2 tons of force.

• michinoku2001
Lv 7
3 months agoReport

That's a solid answer. It's the exact reason why racing sail boats and "go-fast" offshore power boats don't have solid doors on the head. You couldn't open the head door when the hull was stressed anyways.

• 3 months ago

Complex locking systems.

• 3 months ago

They are locked.

• 3 months ago

I remember a few years ago some nut opened the door of a Fiji Airlines Trilander when it was in flight.

No one hurt and the plane landed safely, but the passenger spent a while in one of Fiji’s luxuriously appointed (not) mental health wards.

• noname3 months agoReport

this must have ben a Long time ago, since a short while after D.B. Cooper's  hijacking in '71;, all airliners were  required to have an   interlock; informally called ' D.B. Cooper switch'

• 3 months ago

I would prevent some crazy passenger from trying it while knocking his d!ck in the dirt.

• 3 months ago

A couple of things.

1 - Most of the time an airliner spends at low altitudes is either during takeoff or approach/landing.  When all the passengers are supposed to be in their seats with belts buckled.

2 - Cabin pressurization actually starts on the ground before takeoff.  And given the area of the door, even that low of a pressure differential means it would take someone with the strength of a gorilla to force the door open.

• Nancy
Lv 6
3 months ago

Other than the flight attendants and people on the plane, nothing.  At low altitude, a passenger could go up and do what it takes to open the door and actually open it if nobody stopped them.  It wouldn't cause the plane to crash but it could result in him getting sucked out if he didn't somehow secure himself.  Alarms, of course, would go off in the cockpit and the plane would descend to the lowest safe altitude until it could safely land.

• Rick
Lv 6
3 months ago

They are bolted CLOSED .............................