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I’m quite young – young enough that I have never flown before the terrorist attack on the World Trade Center in New York.

I have doubts about the movie United 93: Was it possible to enter an airplane with a knife and/or a (fake) bomb? Was the hijacking possible due to some errors in the security system, or was the system simply not reliable enough to avoid a hijacking?

So, what security checks on passengers entering the restricted area changed after that event?

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    You folks can probably give a more comprehensive answer from a passenger perspective than most of us over at Aviation - and certainly a less snarky one than I could muster. All I have to offer is this delightful song by a bunch of lawyers from Texas. :-) – voretaq7 Aug 20 '15 at 17:20
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    To help clarify, are you looking for changes in the passenger's experience only? versus all the stuff that goes on in the IT world? – Gayot Fow Aug 20 '15 at 17:29
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    Is there a particular country you're interested in? As I note below, the "before" security situations differed from country to country, as well as the "after" situations. – Michael Seifert Aug 20 '15 at 17:45
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    Also to clarify, what place(s) are you referring to? The changes are different depending on where you are. As might be expected (considering the attacks were in the U.S.,) the most pronounced differences that resulted from 9/11 are probably in the U.S. – reirab Aug 20 '15 at 17:47
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    I lived on a farm as a kid. Carrying a pocketknife was completely normal. I flew with one more than once and no one had a problem with it. Then 9/11 happened. – Mason Wheeler Aug 21 '15 at 19:01
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There were a bunch of changes to US airport security screening after 9/11. I'm not sure if there's a comprehensive list anywhere, but here are a few highlights:

What Didn't Change

  • You still walk through a metal detector
    Airlines generally don't want their passengers starting gun, knife, or chain fights on the airplane. For one thing you might damage the aircraft, and for another it's REALLY hard to get blood out of the seats and carpet.
    Also a survey of flight crews reveals that they overwhelmingly prefer not being shot or stabbed while working.

  • Your carry-on still gets X-rayed
    The airlines want to know what's in your bag - they're not only concerned about weapons (see above) but also other dangerous or prohibited items.
    (Sorry, but you can't take your pet crocodile on the plane unless it's already been made into a crocodile-skin bag.

  • Your checked luggage is still subject to X-ray and/or hand inspection
    See above for carry-ons. A survey of baggage handlers reveals that they prefer not to be bitten by crocodiles that escaped into the cargo hold.


The Sensible Changes

  • ID checks are more stringent
    It's been a LONG while since you could run up to the counter 10 minutes before departure and say "I need to be on that plane, print me a boarding pass! I'll pay in cash!" (that era predates me), but now the ID you present needs to match the name your ticket is under.

  • Only ticketed passengers are allowed into the secure area
    As Michael noted this is standard practice in some other countries.

  • The "Explosive Trace Detection" machine gets used more
    There are several models of this device, the most common use a swab or a "puff chamber" and a spectrometer to look for chemicals normally associated with explosives. These devices are pretty accurate and good at what they do, and because effectively cleaning explosives residue from clothing and equipment is non-trivial they're a nice tool in the screening arsenal.

    • There are "whole body" trace detectors that blow a column of air around a passenger to check for explosive traces, but I've not encountered one myself. I have however had a lot of technical equipment swabbed.
  • The baggage X-Rays are WAY better now
    Many airports had older-generation X-Ray systems for examining carry-ons and checked baggage. These were single-density X-Ray systems with low-resolution sensors that produce a pretty poor image.
    I haven't seen one of those at an airport in years - the new systems are variable-density (they can deal with a mix of things inside your bag) and much higher resolution. I'm sure the old machines would eventually have all been retired and replaced, but the pace of adoption was probably accelerated.

  • "Random" enhanced screenings (SSSS)
    Whether or not it's truly random, some portion of passengers receive "Secondary Security Screening Selection" - an extra-thorough (and somewhat more time-consuming) screening process.
    Diverting some percentage of passengers through this process is theoretically an effective deterrent (though in practice if you're planning something nefarious and discover you've been selected for secondary screening there are several opportunities for you to abort your nefarious plan and avoid capture).

The less-sensible changes

  • Your laptop probably can't stay in its bag
    Unless you have a TSA-approved laptop bag you must remove your laptop from its carry-on bag and place it in an X-Ray Transparent bin for screening.
    This is "less sensible" as it's time-consuming, but there is logic behind it (radiopaque items in some laptop bags make it difficult to discern "normal laptop-looking bits" from "bomb-cleverly-disguised-as-a-laptop bits".

  • Backscatter X-Ray or Pat-Down of passengers
    Perhaps the TSA's most controversial policy, frequently derided as "the nudie-scanner" and "freedom gropes", and plagued by stories of misses (where prohibited items made it through the checkpoint despite the supposedly-thorough screening elements).
    As Nate points out the backscatter x-ray devices have been largely (if not entirely) replaced with millimeter wave scanners - same concept, but theoretically lower risk (less ionizing radiation exposure) and possibly slightly better resolution.
    While I consider them to be largely security theater in the sense that they're not as good (cost, time, or detection rate) as the TSA would have us believe there is some merit to employing the techniques. Our indiscriminate and universal use of them is what lands them in my "less sensible" bin.

The "Are you serious? This is pointless!" changes

  • Please remove your shoes
    Yes, there was once a time when you didn't have to smell the feet of your fellow passengers. But then some guy tried to light his shoe on fire and that ruined it for everyone.
    This lands in the "Are you serious?" bin because not only is it protecting us from yesterday's threat, it is also full of loopholes: Children under 12 don't have to take their shoes off, nor do passengers who go through the "TSA Pre-Check" screening path (which is sometimes opened up to non Pre-Check passengers if the main screening path is backed up).

  • Sorry, no water bottles
    More specifically, some pretty draconian restrictions on "liquids". Where you used to be able to bring your coffee or water bottle purchased outside the secure area in with you, now you must abandon it and buy a new one inside the checkpoint.
    The water you bought outside the checkpoint was delivered on the same truck that delivered the water inside the checkpoint (probably by the same person), which makes the whole thing pretty silly. But even ignoring that, there are again loopholes here (like "infant nourishment").
    The overall effectiveness of this rule is highly debatable, but it does have the benefit of getting people to put their liquids in sealed baggies so if a container breaks or leaks it doesn't mess up anyone else's carry-on.

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    Note that a few of these changes are "Post-Post-9/11", like backscatter X-Ray of passengers and the "Please remove your shoes" routine, but I'm lumping them in under the larger heading of "9/11 induced security hysteria" around airports and air travel. – voretaq7 Aug 20 '15 at 18:39
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    Good summary. Note that the "no liquids" restrictions were also "post-post-9/11", as they were largely a reaction to the "liquid explosive plot" of 2006. – Michael Seifert Aug 20 '15 at 18:44
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    You might also add restrictions on sharp objects. Prior to 9/11 passengers were allowed to carry small pocket knives or (ahem) box cutters on the plane. – Nate Eldredge Aug 20 '15 at 18:53
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    @voretaq7: I'm with reirab: I'm pretty sure the rules actually used to be more permissive. Pre-9/11, I used to carry a Swiss Army knife with me everywhere, and I have pretty clear memories of bringing it on planes (putting it in the bin to go through the metal detector; nobody objected). I think I used it to open bags of peanuts. I have a recollection of a 3-inch blade length limit, and the Swiss Army knife had maybe a 2.5 inch blade. – Nate Eldredge Aug 20 '15 at 20:32
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In US airports, the biggest change from a "civilian's" perspective was probably the fact that only ticketed passengers are now allowed in the secure area. Before 9/11, anyone could go through the security checkpoint and into the gate areas. You might see this in older movies, with people greeting passengers right as they get off the jetway instead of coming through opaque doors into some arrivals hall.

Note that not all countries allowed people without tickets into the secure area before 9/11; for example, I'm pretty sure that Canada only allowed ticketed passengers past security. In such countries, this would not have been the biggest change.

  • This changed long before 2001, at least in some airports. I was refused access to the gate as a non-traveler, to accompany my girlfriend, who was traveling, in 1994 or 1995. – phoog Dec 28 '15 at 19:38
  • @phoog At most U.S. airports, non-passengers were allowed to go through security prior to 9/11. I did so many times myself during the 90s. – reirab Jan 6 '16 at 17:25
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Overall, I think changes in Europe (which had prior experiences with international terrorism) were limited at first. What happened is that norms and procedures for US domestic flights were brought in line with those of other countries.

In the US changes included the access restrictions mentioned by Michael, stricter ID requirements, the creation of the TSA and of the No Fly List, and a ramping up of the Federal Air Marshal Service.

Further changes were introduced later, like Advance Passenger Information Systems (useful to enforce no-fly lists and for screening programmes), the ban on liquids and gels and new scanning devices.

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What changed after that event in the passenger security check, before entering in the restricted area?

Virtually nothing changed. You were still asked the 3 key questions you are asked today, Did you pack the bag? Has the bag been out of your direct awareness since you packed it? Did you pack anything you shouldn't have?

The difference is that whereas before if you said no or I dont know to any of the above questions, they went through your bag. Now, regardless of what is said, they will probably cut your locks and go through your bag.

Since 9-11, a lot more of your stuff gets stolen before the bag makes it to the plane, and there is nothing you can do about it.

Could be possible now to enter in an airplane with a knife and/or a (fake) bomb?

Yes, there are actually a bunch of people that have done this over and over just to make a mockery of the TSA.

Before 9-11, you walked through a metal detector, and then you waited. If you set off the metal detector, you might get patted down.

Now, you have to take off your belt and shoes, you have random odds of "enhanced pat downs" based on "random number generators". If youre super lucky, the TSA takes you to their special hidden room for an enhanced security check (Sky Harbor has a doorless underground room for this.)

It's a whole crap ton of hassle and privacy invasion, but you can still hide a knife bundled in something in your bag, and the xray scanner wont get it. Even with the full body scanners, ANYTHING can be hidden in a fat roll.

All that hassle, no real gain.

Was the hijacking possible for some errors in the security system, or it >simply wasn't reliable enough to avoid a hijacking?

It wasn't an "error in the system". Stop war. Stop murder. Stop violence. You can't. People have tried since the dawn of man, and there is just no way to stop it.

The TSA is an act of paranoia. Those who are hyper paranoid and afraid of the world enjoy the inconvenience because they feel like they are trading freedom for security. In reality, they are trading freedom for a placebo.

We are no better off in the airport than we were pre 9-11, it just takes 5 times as long to get through the airport, and it is now an often humiliating and angering experience.

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Was it possible to enter an airplane with a knife and/or a (fake) bomb?

Before 9/11 the US allowed blades less than four inches long as well as knitting needles, glass bottles and other sharp objects.

When you went through the metal detector you'd either leave it in your carry on or put it in a dish. Security would often have a ruler or mark on the side of a table to check the blade against.

You would not be allowed on with a bomb if they caught you. The security systems were far less able to detect bombs then, no fancy chemical detection machines. Checked and carry on luggage was X-rayed. Trained dogs could sniff out some types of explosives. Luggage and carry-ons would be randomly searched by hand.

If your carry-on contained a lot of wires or electronics under the X-Ray, for example a laptop and charging cables (relatively rare in 2000), it might be searched. You might be asked to turn the computer on to demonstrate it's not a fake.

Was the hijacking possible due to some errors in the security system, or was the system simply not reliable enough to avoid a hijacking?

The system was looking for means to take over or destroy the plane. It did not consider you could do that with box cutters and small knives. It was looking for guns and bombs. Rather than defeat security head-on, the hijackers exploited one of its assumptions.

The 9/11 attackers got away with it, in part, because before 9/11 hijackers were not there to destroy the plane, they were there to take hostages and make demands. In a hostage situation you'd cooperate and let the authorities handle it. The 9/11 passengers did not realize they were in immediate danger.

It's my opinion that if there were another attempt to hijack an American plane 9/11 style, the hijackers would be overpowered by the passengers. Better to die trying than die when they drive the plane into a building.

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    This is not universally true. Before 9/11 European airports certainly didn't allow knives with blades less than 4 inches long, though I was surprised and rather shocked when I found US airports didn't seem to care about them. – Brian Drummond Aug 23 '15 at 21:26
  • @BrianDrummond You're right; I will qualify this as being for the US. – Schwern Aug 23 '15 at 21:36
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    After a wedding in the US, with my kilt, somehow my sgian dubh ended up in my hand luggage. (It's a knife, and a traditional part of the outfit). Raleigh-Durham didn't care; neither did Chicago or Minneapolis, but from Schiphol to Glasgow it had to be placed in an envelope and stowed in the hold. A very polite man with an MP5 said so... – Brian Drummond Aug 23 '15 at 21:42
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    Relatively rare? I, as an impecunious music student, bought a laptop in 1999. They can't have been that rare. Certainly nobody looked at me strangely when I took it through security. Also, one factor in 9/11 was the poor quality of the security staff in Logan. I flew through there in August 2001, and the bag screener was literally falling asleep while he was trying to observe the x-ray machine's video monitor. I thought about reporting it, decided not to, and have regretted that decision ever since the following month. – phoog Dec 28 '15 at 19:49
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In May 2001 I was able to greet visitors at the jetway in New York's La Guardia Airport. The rule at the time was ticketed passengers only, but the enforcement was obviously nothing like it is now. No one was matching IDs against boarding passes. Inspection of the passes was often cursory at best, and sometimes didn't occur at all. But the long TSA screening lines we know so well were only months away.

  • You still can greet visitors at the jetway. You just have to buy a ticket within a 24 hour span of their arrival. The new security procedures filter out terrorists who can not afford to buy an extra ticket. – emory Aug 23 '15 at 14:31
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    @emory While that's amusing, and I had to think about it for a moment, the new procedures are more about being able to better control who is inside security. Buying a ticket means an extra layer of security checks. But that just means you need a copy of Photoshop and a decent printer. The TSA hopes if they put enough leaky security layers up you'll screw up and get caught by one. – Schwern Aug 23 '15 at 21:42
  • @Schern You are correct. I was just noting that if you have disposable cash, you can still do whatever you want to do. I have noticed that some airports have special security lines for first class passengers - which remarkably are shorter than the coach class lines. If I was organizing a 9/11 sequel, my team would be flying first class. – emory Aug 24 '15 at 11:30
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    @emory it puts them closer to the cockpit, too. – phoog Dec 28 '15 at 19:51
  • @emory As I recall, the 9/11 terrorists actually flew first class, presumably for exactly this reason. – grkvlt Aug 11 '17 at 15:37
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Some other changes not mentioned here, but can be noticed:

  1. Cockpit doors are reinforced and entry during flights is forbidden (prior to 9/11 it was restricted, but was allowed).

  2. You cannot park near the airport terminal at most international terminals.

  3. Your electronic devices need to power up if you are traveling to the US and UK. This means, if your battery is dead on your mobile, you will not be allowed to take it on board (this is a new regulation, which I noticed on my recent trip); here is the relevant text:

Flying to and from the UK

Make sure your electronic devices are charged before you travel. If your device doesn’t switch on when requested, you won’t be allowed to take it onto the aircraft.

The BBC covered this in more detail.

  • I'm not sure about #3. I've never been allowed to store lithium batteries in check-in baggage, but I've had no issue carrying devices with them (with no indication whatsoever about how much battery was left) or carrying an extra one on my flights to/from the US. – Maroon Aug 23 '15 at 15:00
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    As I mentioned, its a new regulation (July 2015). – Burhan Khalid Aug 23 '15 at 15:13
  • I have flown between the Netherlands and the UK several times since July 2015 with phones and laptops and have never been asked nor seen others being asked to power up. – Willeke Dec 25 '16 at 11:47

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