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There's something below the door on almost every commercial aircraft. What is inside that box? The protrusion can be seen from the interior of the aircraft. Source a320

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On Youtube: Emergency slide deployment on British Airways 747-400. The trick is that the slide is fixed to the cabin floor and the container is on the door. On some planes, the door slides upwards. The slide must be separated from the floor before the door is opened for normal use. Voice orders from the purser to arm and "disarm" (don't know the word in English) slides can be heard on the cabin public address. – mins Feb 19 at 17:19
    
@mins i always hear cabin crew instruction "arm the cabin door&cross check" and "disarm cabin door&cross check" in English everytime I fly. Then they may proceed to announcement in local language. Source:experience of flying in southeast asia. – vasin1987 Feb 19 at 19:57
    
The copyright on the page for the image is pretty clear that it's not permitted to be reposted here: This photo is copyright protected and may not be used in any way without proper permission.. They further go on to say _ You may not use these photos on any web page, commercial or non-commercial, for profit or non-profit, without written permission from the photographer. Contact the ASN staff to request permission_ – Johnny Feb 19 at 23:22
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@kepler22b - Wikipedia is a good place to find images that are licensed for sharing (generally either public domain or CC license), here's one that even has a caption that says what that box on the door is, here's another – Johnny Feb 20 at 1:52
up vote 12 down vote accepted

It contains the evacuation slide. From airspacemag:

An escape slide sits inside a carbon fiber pressure cap covered by a casing of material similar to the aircraft interior walls—that big square box at the bottom of an airliner’s interior door.

Here's an image from Boeing:

Evacuation

Image from Boeing.com

According to Boeing, the positioning of evacuation slides inside the doors reduced the deployment time.

By 1963, improvements in materials and inflation systems reduced the weight and bulk of the slide system, making it practical to move the slides out of the ceiling to the lower inboard face of the cabin doors (see fig. A). This location resulted in a still more efficient escape system, reducing the time needed to ready an escape slide from between 20 to 50 seconds to between 18 and 24 seconds, including door opening time.

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