How did security checks at airports change after September 11th, 2001?

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.

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.

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.

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.

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.

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.