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So what exactly are the reasons:

  • policy by the airlines?
  • slowing down security?
  • What if it's 100mL of water filled in one of those tiny travel bottles that people use to downsize their toiletries?

I was just discussing this topic with a friend & then realized while I know of the rule I would like to know all the aspects as to why.

hippietrail
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verve
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    http://en.wikipedia.org/wiki/2006_transatlantic_aircraft_plot

    it's a security theater, basically

     – JonathanReez Feb 17 '15 at 09:52
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    AFAIK you can take a bottle of water on the plane, just not through security. You could for instance take an empty bottle through security and then fill it up at the gate. – drat Feb 17 '15 at 09:56
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    @drat: that is what I do all the time - empty my bottle before security, fill it up before getting on the plane (e.g. Chicago airport even has bottle-filling stations airside) – Jonas Feb 17 '15 at 12:22
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    @Jonas: In various places, such "bottle-filling stations" are commonly known as "taps" :) – O. R. Mapper Feb 17 '15 at 14:10
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    @O.R.Mapper: The fact that there are bottle filling stations specifically to fill water bottles indicates that bringing filled bottles onto the plane is acceptable practice. Of course, there are a few airports with only warm water taps, presumably to encourage buying water. – Jonas Feb 17 '15 at 14:13
  • @Jonas: Why would taps be limited to warm water? In any case, I think I didn't explain my intentions clearly; I totally agree it is considered an acceptable practice to bring filled bottles onto the plane. I just wanted to point out that, having grown up in a region where tap water is generally drinkable, there is a much more universal way of refilling bottles (i.e. publicly accessible taps, e.g. in bathrooms) and the installation of specific appliances such as "bottle-filling stations" is not necessary and would rather be a waste of resources, because water is already available from taps. ... – O. R. Mapper Feb 17 '15 at 14:27
  • ... Maybe some dedicated bottle-filling stations are indeed provided to encourage filling bottles and bringing them into the plane. On the other hand, those could also just as well be meant for general convenience in the security area, similar to the food stalls in the security area, which are used by people waiting for their departure, and which can be used by people to buy food to bring on the plane. However, I'm quite sure the food stalls in the security area are not in any way intentionally meant to encourage passengers to buy their food on the ground rather than on the plane. – O. R. Mapper Feb 17 '15 at 14:33
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    https://www.google.com/search?q=security+theater – Foo Bar Feb 17 '15 at 14:51
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    Just as a note, while the comments so far are correct that you can usually buy or fill a water bottle in the terminal and take it on with you, this is not always the case. In particular, some international flights actually confiscate water bottles at the gate or even inside the boarding bridge. See this question and this question. – reirab Feb 17 '15 at 16:25
  • @reirab Confiscation at the boarding bridge? How is that a security thing? Lol. – verve Feb 18 '15 at 02:36
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    I once read (source: the internet!) about a guy who tried to bring a bottle frozen water through security, claiming that, hell, "it's not liquid!"! – Pierre Arlaud Feb 18 '15 at 10:28
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    It's all risk ratio. If there is a risk that some less than stable person might hide an explosive as a bottle of liquid, then banning all liquids of a usable size is an easy win. It's easy to do, the beaurocrats can publicize that they've "done something" to prevent "terrorism"... the weak minded can applaud their "something"... it's win all around. – CGCampbell Feb 18 '15 at 13:20
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    Relevant xkcd and TSA's response to the comic. – Léo Lam Feb 18 '15 at 13:23
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    @LéoLam That's hilarious that TSA officially responded to xkcd. I'm not certain that I agree with their conclusion that a water bottle full of liquid explosives is really more dangerous than a lithium battery, though. Lithium is extremely reactive. Inadvertently mixing (relatively small amounts of) Lithium with water is how people blow themselves up with meth labs. – reirab Feb 18 '15 at 18:35
  • Security staff work strictly to the rule book. Apparently no discretion is permitted. In this situation common sense has no place. As a regular traveler I've always thought the more they inconvenience innocent clients, families,old folk etc the more they justify their jobs. Airports and airlines are simply there to make money. Aggravation through the airport achieves this. Customers are secondary. Does anyone really know if I can take an empty Thermos flask through security please? – john toone Jul 02 '17 at 09:56
  • Personally, I believe the only right answer is (because of) inertia. Once implemented by a bureaucracy, a rule takes on a life of it's own and rescinding it is much harder. Making a rule for safety is an easy decision for a government employee or agency, after all, it's for safety. Rescinding it is exponentially harder. After all, I know I would not want to be the government person/agency that said a rule is no longer needed, only to have that what was protected against happen. Then you have to prove why you considered it no longer a threat. ... – CGCampbell Feb 16 '18 at 19:47
  • Nitrated alcohol looks so like water. One good shake can blow all away. – J Bergen Jul 02 '17 at 23:18

4 Answers4

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Because it's difficult to tell apart a bottle full water from a bottle full of a chemical like hydrogen peroxide that could be used to make liquid explosives. There was one hare-brained terrorist plot that apparently tried this in 2006, and because "passenger convenience" will always lose out to "bureaucratic ass-covering" when it comes to security theater, all liquids of all kinds were banned by the TSA. (Unless they're under 100ml, so yes, you can take a 100ml bottle of water on board... if you can find one!)

In Japan, they've already got bottle scanners that can identify suspect liquids. These are increasingly being adopted by other countries, and once they're widespread enough, the liquid silliness will hopefully end.

Note that while you can't take a partly or completely filled >100ml water bottle through security, it's perfectly fine to take an empty bottle through security, and fill it up before you board the plane. Or you can buy a bottle once you've passed security.

Jonas
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lambshaanxy
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    Are 100ml of hydrogen peroxide not sufficient to make liquid explosive? Or is it just that the explosion might not kill everyone on the plane? Ah you gotta love security theatre. – JoErNanO Feb 17 '15 at 11:16
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    @JonStory For maximum security, you could live in a bunker and never fly! Discussions on if the current rules are actually in any way sensible or proportional probably belong on Skeptics.SE though – Gagravarr Feb 17 '15 at 12:37
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    I like how, having confiscated what could be a bottle full of explosive, they (often) throw it into a box containing a lot of other previously confiscated potential explosives. – bye Feb 17 '15 at 17:11
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    This is getting a bit off-topic. Perhaps someone should post the question, "Ways of blowing up a plane with at most 100ml of liquid". – Cary Swoveland Feb 17 '15 at 23:36
  • Comments are not for extended discussion; this conversation has been moved to chat. – RoflcoptrException Oct 25 '16 at 20:22
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tl;dr: Airports like to sell drinks (and it may increase security)

As described by @jpatokal it is indeed possible to make explosives out of some liquids, and thus airlines used security as a reason to ban all liquids.

Though the amount of terrorist attacks prevented by this is probably in the range of 0-1 (which could of course make it worth the effort), there is a side effect that is easily noticeable:

Since it is harder for people to arrange their own drinks, the number of drinks purchased at an airport increases. I have been unable to find a citation for this but can attest to this from personal experience.

So, preventing people from taking bottles may increase security, but it will definitely increase revenue. And this is at least part of the reason why the rule got enough support to be implemented.

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Dennis
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    This is skeptical, but anyhow the revenue increase is a positive side effect and wasn't the original plan. – Nean Der Thal Feb 17 '15 at 13:47
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    In general we can of course never know why certain things are done. But the economical benefits here are clear, and those tend to result in support. Therefore I think the economical angle should not be overlooked when someone asks about all aspects leading to the current situation. – Dennis Feb 17 '15 at 14:13
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    What is really annoying is that, even though once you are past the check in stuff is tax-free, a bottle of water will cost you more than double the "external" price... – algiogia Feb 17 '15 at 15:21
  • You should put some emphasis (italics) on "may" in the bold header. – Cole Tobin Feb 17 '15 at 17:31
  • @algiogia And yet you hear more people complain about the prices at movie theaters prices than airports. – Cole Tobin Feb 17 '15 at 17:32
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    @MeNoTalk did you mean "cynical", rather than "skeptical"? – Golden Cuy Feb 18 '15 at 00:10
  • At one airport in South America I had post-security bought bottles of water confiscated at the gate. I since found that this is a common occurrence in various places. I have also seen the TSA "test" liquids being carried onto a plane at the gate. – Peter M May 18 '16 at 14:36
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The policy is actually fairly reasonable, and the base reason is that they can't run an analytical chemistry laboratory at the checkpoints (nor find staff that can both understand the results and work for a government salary).

The hydrogen peroxide mentioned in another answer is one possibility. Looks just like water. The stuff you buy at the drug store makes a great antiseptic or toothpaste, stronger mixes make good rocket fuel. Gasoline looks a lot like apple juice - light up 500ml of Regular Unleaded and ask yourself if you want that happening in the window seat. 100ml bottles of apple juice are rather rare and will attract attention. Other chemicals not mentioned here can start roaring blazes on contact with air - you just have to open the cap.

Sales were certainly not a consideration - every airport I've been to charged market rate for drinks after security, one openly advertises (since the 1990s) that prices after security are exactly the same. And I've had no problems at all bringing a freshly rinsed thermos through. It gets a quick glance to see if its empty.

paul
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    If your explanation should hold, there has to be a complete ban on bringing liquids through security. If five terrorist buddies are allowed to bring 500ml of unleaded petrol through security and it is a real problem, we have an immense security risk. – Tor-Einar Jarnbjo Feb 17 '15 at 18:05
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    I always thought they could just do the trick I've seen done at many non-corporate music festivals: You say it's just water in that bottle? Shake it up then swallow two fingers' worth of the bottle in one gulp while the security guard watches, and keep it down. No need for a chemistry lab - the person's stomach will very quickly let you know if it was hydrogen peroxide. – user56reinstatemonica8 Feb 17 '15 at 19:38
  • @user568458 ...and what if I don't want to drink it yet? Unnecessary harassment, and still not foolproof. – nmclean Feb 17 '15 at 20:06
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    @user568458 - that's exactly what happens if you bring through water in a baby's bottle - one of the few exceptions to the liquid rule. – paj28 Feb 18 '15 at 11:01
  • @Tor-EinarJarnbjo 5 x 100ml is certainly an issue. But it's also a much larger conspiracy than one nutcase acting alone. There are other methods to detect the conspiracies. – paul Feb 18 '15 at 12:54
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    @user568458 it's not all that difficult to create a nearly-invisible barrier inside a bottle. Or an inner container. Visit a magician's supplier for examples. – paul Feb 18 '15 at 12:56
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    @paj28: best comment ever on this topic: "fresh milk for infants: no limit when transported in the original containers" – paul Feb 18 '15 at 12:57
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    If this was really about terrorism, then there would be just a limit on the total amount of liquid you are allowed to bring. The wired rule that liquids have to be in max 100 ml containers (even though you are allowed several of them, so if you split the forbidden 500 ml bottle among 5 smaller bottles its suddenly fine), leads to the conclusion that the official reasons are bullshit, and its just about increasing revenue from overpriced drinks. A dangerous 500 ml doesn't suddenly become safe by splitting among 5 bottles, Especially that you can combine it back once on the plane. – lowtoxin Jul 28 '16 at 01:59
  • @paul No larger conspiracy is required. You can bring 5x 100 mL bottles on yourself, as long as you can fit all 5 of them in a 1L zipper bag. That said, with gasoline, you need an ignition source and those are banned. Also, to create an explosion with gasoline, you need a spray nozzle or some such thing to make a mist out of it. If you just light a 500mL bottle of gasoline on fire, it will burn, but it won't explode in real life like it does in Hollywood. And, with jet fuel, it wouldn't even do that. You can drop a lit match in a bucket of jet fuel and it won't burn. – reirab Jul 28 '16 at 03:37
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    "every airport I've been to charged market rate for drinks after security" Haha. Hahahahaha. Hahaha. Ha. Nice joke. I mean, er, wow, you go to airports that are friendly to their customers. Every airport I've been to charges a noticeable mark-up. Some charge a ludicrous mark-up: I paid EUR3.50 for a 750ml bottle of water at Lyon acouple of years ago, for example (it might even have been 500ml). – David Richerby Sep 22 '18 at 18:13
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If you ask them, it's to prevent terrorist attacks in the form of bombings. Apparently several people posting agree. I would answer that it is to appear to prevent terrorist attacks. I say that because tsa is slightly more effective than mall cops.
Could there be bombs created through that nefarious methods that get caught by the liquid limitations? Maybe. Probably not. TSA got caught missing something like 95% of things getting past them by homeland security. So hope it's that 5%. It isn't like multiple people couldn't pool together small bottles of liquid.
The purpose is to appear busy and to make people feel safe by being very intrusive. The reason we haven't seen more attacks is because of increased intelligence operations, and because we reinforced the cockpit door. Without the ability crash the plane into something, we're back to the pre-911 days, where they can attack the plane, but it's not nearly as good a target.

LRom
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  • The reinforced cockpit door was the cause of one of the worst attacks of 2015, that security measure clearly backfired. And increased intelligence hasn't helped much. Terrorists still fly (and many that aren't are incorrectly banned). What has helped to some extend is increased security. Many reports show prevented attacks and captured terrorists at the gates. But as the 'underwear bomber' showed, the intrusive checks in the airport are not foolproof yet. – Abel Jan 06 '17 at 21:06
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    Why do people always say "you can just get 5 people to get together and..." - when even trying to make software in teams like that leads to squabbling and fighting. The more people need to team up to attack, the more likely the plot will collapse or be discovered in advance. – Kate Gregory Jul 02 '17 at 13:14