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I saw a picture on Twitter after the loss of cabin pressure. I've seen these images many times before, but in this picture, there are some things on strings that I cannot identify.

enter image description here

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Maybe the oxygen-masks used to be mounted on this white plastic things and now removed? Not sure though. – jklingler Jan 8 at 12:00
    
This looks like some maintenance going on. – SMS von der Tann Jan 8 at 13:22
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I can't answer the question regarding what the white plastic devices are on the end of the strings; however, in a typical installation the oxygen masks would be attached to the end of the strings. Pulling on the oxygen mask, and thus the string, causes the oxygen generator (or oxygen candle) to start producing oxygen. Perhaps @smsvondertann is correct and this depicts temporary placeholders in place during maintenance procedures. – Jonathan Walters Jan 8 at 14:00
up vote 17 down vote accepted

They are the holders for the oxygen masks, they are packed into the plastic retainers and stored in the overhead, when the masks drop, they fall out of the containers. Not all systems have these retention pieces that I can find, but some do.

The "black" tubes (actually clear, but look black in the picture) you see are the oxygen mask tubes and the mask itself is dangling from the end of the tube, you can see they have green/red labels on them that signify they are oxygen. The oxygen generator for that group of seats/masks is under the label in the top "HOT HOT", oxygen systems get very hot when generating oxygen (just ask ValuJet).

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As a semi-related comment, I've seen some safety videos where they say "If an oxygen mask appears, pull down to start the flow of oxygen" or "if a string appears, pull down." There's a white string with a yellow tag that says "PULL." Why do some planes include a "PULL" string? Why would only a string appear as opposed to the mask itself? Is it in case the mask gets stuck because they're packed so tightly? – Stu Sztukowski Jan 8 at 14:38
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I guess they can't guarantee that the mask will fall out of the retainer, so they cover it either way. Pulling down on the mask triggers the firing pin in the oxygen container to begin the chemical reaction to produce about 15 minutes of oxygen, I think both the string and the tube are connected to that firing pin but I'd have to verify. I don't think only a string would appear, it would be both the string and the mask. – Ron Beyer Jan 8 at 14:41

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