Millions of people fly on thousands of planes every day.

Photo courtesy Federal Aviation Administration (FAA)

Introduction to How Airport Security Works

Terrorism has been a problem for airlines and air travelers since the 1970s, when hijackings and bombings became the method of choice for subversive, militant organizations around the world. Although security at airports has always been tight, the 9/11 attacks woke many people up to a harsh reality -- it wasn't tight enough.

On that day, men armed with simple box cutters took over four passenger jets and used them as flying bombs. What security measures might have stopped them? How has airport security changed since then? According to the Department of Homeland Security, 730 million people travel on passenger jets every year, while more than 700 million pieces of their baggage are screened­ for explosives and other dangerous items. In this article, we'll find out how high-tech solutions are being used to make flying as safe as possible -- and we'll also consider if what we are doing is enough.

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Thank You

Special thanks to Paul Hurd of PerkinElmer Detection Systems for his assistance with this article.

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The First Line of Defense

Imagine for a second that you are a terrorist who wants to blow up or hijack a plane. You know that once you get inside the airport, you will have to pass through metal detectors, bomb-sniffing dogs, and possibly a search of your clothes and luggage. How could you bypass all of those security measures? You could climb a fence or drive a truck to a sensitive area of the airport.

For this reason, the first line of defense in airport security is the most obvious: fences, barriers and walls. Tall fences that would be difficult to climb enclose the entire airport property. Security patrols regularly scan the perimeter in case someone tries to cut through the fence. Especially sensitive areas, like fuel depots or the terminals and baggage handling facilities are even more secure, with more fences and security checkpoints. All access gates are monitored by either a guard station or surveillance cameras.

Another risk is that someone could drive a truck or car containing a bomb up to the airport terminal entrance and just blow up the airport itself. Airports have taken several steps to prevent this. Large concrete barriers, designed to block vehicles up to the size of large moving trucks, can be deployed if a threat is detected. Loading zones, where people once parked their cars to get their baggage in or out of the trunk, are now kept clear of traffic. No one is allowed to park close to the terminal.

The photo-identification page of a U.S. passport

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Who Are You?

One of the most important security measures at an airport is confirming the identity of travelers. This is done by checking a photo ID, such as a driver's license. If you are traveling internationally, you need to present your passport.

Simply taking a look at a photo ID isn't enough, however. The high-tech buzzword in airport security today is biometrics. Biometrics essentially means checking fingerprints, retinal scans, and facial patterns using complex computer systems to determine if someone is who they say they are - or if they match a list of people the government has determined might be potential terrorists.

A new system called CAPPS II could help accomplish some of this. Short for Computer Assisted Passenger Prescreening System, CAPPS II will require more personal information from travelers when they book their flights, which will lead to a risk assessment of no risk, unknown risk, elevated risk, or high risk. Passengers considered risky will be further screened. Although the system has been delayed and isn't in place yet, the Department of Homeland Security (DHS) predicts that CAPPS II will make check-in faster for the average traveler.

You may have noticed the public address system at an airport replaying an automated message telling you not to leave your bags unattended. And you've probably noticed that check-in attendants are asking some questions that sound a little odd:

  • Has your luggage been in your possession at all times?
  • Has anyone given you anything or asked you to carry on or check any items for them?

These are very important questions. A tactic used on occasion by terrorists is to hide a bomb inside an unsuspecting person's luggage. Another tactic is to give something, maybe a toy or stuffed animal, to someone who is about to board a plane. That innocent-seeming object may actually be a bomb or other harmful device.

Just a month after the 9/11 attacks, the President signed a new law that restructured and refocused the airport security efforts of the U.S. The Aviation and Transportation Security Act established a new agency, the Transportation Security Administration (TSA). The TSA is part of the Department of Homeland Security. The TSA's mission is to:

  • Prevent attacks on airports or aircraft
  • Prevent accidents and fatalities due to transport of hazardous materials
  • Ensure safety and security of passengers

While the TSA deals with all forms of transportation, the Federal Aviation Administration (FAA) is devoted entirely to the operation of the U.S.'s civil aviation. FAA agents are located at every major airport for immediate response to possible threats. Most major airports also have an entire police force, just like a small town, monitoring all facets of the facility. Background checks are required on all airport personnel, from baggage handlers to security-team members, before they can be employed. All airport personnel have photo-ID cards with their name, position and access privileges clearly labeled.

Airport metal detectors rely on pulse induction.

Photo courtesy L-3 Communications

Step Through, Please: Metal Detector

All public access to an airport is channeled through the terminal, where every person must walk through a metal detector and all items must go through an X-ray machine.

Almost all airport metal detectors are based on pulse induction (PI). Typical PI systems use a coil of wire on one side of the arch as the transmitter and receiver. This technology sends powerful, short bursts (pulses) of current through the coil of wire. Each pulse generates a brief magnetic field. When the pulse ends, the magnetic field reverses polarity and collapses very suddenly, resulting in a sharp electrical spike. This spike lasts a few microseconds (millionths of a second) and causes another current to run through the coil. This subsequent current is called the reflected pulse and lasts only about 30 microseconds. Another pulse is then sent and the process repeats. A typical PI-based metal detector sends about 100 pulses per second, but the number can vary greatly based on the manufacturer and model, ranging from about 25 pulses per second to over 1,000.

If a metal object passes through the metal detector, the pulse creates an opposite magnetic field in the object. When the pulse's magnetic field collapses, causing the reflected pulse, the magnetic field of the object makes it take longer for the reflected pulse to completely disappear. This process works something like echoes: If you yell in a room with only a few hard surfaces, you probably hear only a very brief echo, or you may not hear one at all. But if you yell into a room with a lot of hard surfaces, the echo lasts longer. In a PI metal detector, the magnetic fields from target objects add their "echo" to the reflected pulse, making it last a fraction longer than it would without them.

A sampling circuit in the metal detector is set to monitor the length of the reflected pulse. By comparing it to the expected length, the circuit can determine if another magnetic field has caused the reflected pulse to take longer to decay. If the decay of the reflected pulse takes more than a few microseconds longer than normal, there is probably a metal object interfering with it.

A demonstration of PI technology

The sampling circuit sends the tiny, weak signals that it monitors to a device call an integrator. The integrator reads the signals from the sampling circuit, amplifying and converting them to direct current (DC).The DC's voltage is connected to an audio circuit, where it is changed into a tone that the metal detector uses to indicate that a target object has been found. If an item is found, you are asked to remove any metal objects from your person and step through again. If the metal detector continues to indicate the presence of metal, the attendant uses a handheld detector, based on the same PI technology, to isolate the cause.

Many of the newer metal detectors on the market are multi-zone. This means that they have multiple transmit and receive coils, each one at a different height. Basically, it's like having several metal detectors in a single unit.

In the next section, we'll discuss what happens to your carry-on items while you're going through the metal detector.

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Your carry-on items are sent through a machine that X-rays the contents.

Photo courtesy L-3 Communications

Step Through, Please: X-Ray System

While you are stepping through the metal detector, your carry-on items are going through the X-ray system. A conveyor belt carries each item past an X-ray machine. X-rays are like light in that they are electromagnetic waves, but they are more energetic, so they can penetrate many materials. The machine used in airports usually is based on a dual-energy X-ray system. This system has a single X-ray source sending out X-rays, typically in the range of 140 to 160 kilovolt peak (KVP). KVP refers to the amount of penetration an X-ray makes. The higher the KVP, the further the X-ray penetrates.

In a dual-energy X-ray system, the X-rays pass through a detector, a filter and then another detector.

Image courtesy L-3 Communications

After the X-rays pass through the item, they are picked up by a detector. This detector then passes the X-rays on to a filter, which blocks out the lower-energy X-rays. The remaining high-energy X-rays hit a second detector. A computer circuit compares the pick-ups of the two detectors to better represent low-energy objects, such as most organic materials.

Since different materials absorb X-rays at different levels, the image on the monitor lets the machine operator see distinct items inside your bag. Items are typically colored on the display monitor, based on the range of energy that passes through the object, to represent one of three main categories:

  • Organic
  • Inorganic
  • Metal

While the colors used to signify "inorganic" and "metal" may vary between manufacturers, all X-ray systems use shades of orange to represent "organic." This is because most explosives are organic. Machine operators are trained to look for suspicious items -- and not just obviously suspicious items like guns or knives, but also anything that could be a component of an improvised explosive device (IED). Since there is no such thing as a commercially available bomb, IEDs are the way most terrorists and hijackers gain control. An IED can be made in an astounding variety of ways, from basic pipe bombs to sophisticated, electronically-controlled component bombs.

An X-ray of a bag Notice that all organic items are a shade of orange.

Photo courtesy L-3 Communications

A common misconception is that the X-ray machine used to check carry-on items will damage film and electronic media. In actuality, all modern carry-on X-ray systems are considered film-safe. This means that the amount of X-ray radiation is not high enough to damage photographic film. Since electronic media can withstand much more radiation than film can, it is also safe from damage. However, the CT scanner and many of the high-energy X-ray systems used to examine checked baggage can damage film (electronic media is still safe), so you should always carry film with you on the plane.

Electronic items, such as laptop computers, have so many different items packed into a relatively small area that it can be difficult to determine if a bomb is hidden within the device. That's why you may be asked to turn your laptop or PDA on. But even this is not sufficient evidence since a skilled criminal could hide a bomb within a working electronic device. For that reason, many airports also have a chemical sniffer. This is essentially an automated chemistry lab in a box. At random intervals, or if there is reason to suspect the electronic device that someone is carrying, the security attendant quickly swipes a cloth over the device and places the cloth on the sniffer. The sniffer analyzes the cloth for any trace residue of the types of chemicals used to make bombs. If there is any residue, the sniffer warns the security attendant of a potential bomb. In addition to desktop sniffers like this, there are handheld versions, that can be used to "sniff" lockers and other enclosed spaces and unattended luggage. Walk-through models, such as GE's Entry Scan 3, are also available. These sniffers can be used to detect explosives and narcotics.

Now that you have passed through security and are waiting to board your plane, let's see what is happening with your checked baggage.

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Your luggage goes through a larger X-ray system.

Photo courtesy L-3 Communications

Check Your Bags: X-ray Systems

In addition to passenger baggage, most planes carry enormous amounts of cargo. All of this cargo has to be checked before it is loaded.

Most airports use one of three systems to do this:

  • Medium X-ray systems - These are fixed systems that can scan an entire pallet of cargo for suspicious items.
  • Mobile X-ray systems - A large truck carries a complete X-ray scanning system. The truck drives very slowly beside another, stopped truck to scan the entire contents of that truck for suspicious items.
  • Fixed-site systems - This is an entire building that is basically one huge X-ray scanner. A tractor-trailer is pulled into the building and the entire truck is scanned at one time.

In some airports, medium X-ray facilities are set up to scan an entire pallet of luggage at a time.

Photo courtesy L-3 Communications

One old-fashioned method of bomb detection still works as well or better than most hi-tech systems -- the use of trained dogs. These special dogs, called K-9 units, have been trained to sniff out the specific odors emitted by chemicals that are used to make bombs, as well the odors of other items such as drugs. Incredibly fast and accurate, a K-9 barks at a suspicious bag or package, alerting the human companion that this item needs to be investigated.

In addition to an X-ray system, many airports also use larger scanners. Let's take a look at those next.

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This diagram shows how the X-ray system in a CT scanner rotates around a bag.

Image courtesy L-3 Communications

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Check Your Bags: CT Scanners

The first security check that your checked bags go through depends on the airport. In the United States, most major airports have a computer tomography (CT) scanner. A CT scanner is a hollow tube that surrounds your bag. The X-ray mechanism revolves slowly around it, bombarding it with X-rays and recording the resulting data. The CT scanner uses all of this data to create a very detailed tomogram (slice) of the bag. The scanner is able to calculate the mass and density of individual objects in your bag based on this tomogram. If an object's mass/density falls within the range of a dangerous material, the CT scanner warns the operator of a potential hazardous object.

CT scanners are slow compared to other types of baggage-scanning systems. Because of this, they are not used to check every bag. Instead, only bags that the computer flags as "suspicious" are checked. These flags are triggered by any anomaly that shows up in the reservation or check-in process. For example, if a person buys a one-way ticket and pays cash, this is considered atypical and could cause the computer to flag that person. When this happens, that person's checked bags are immediately sent through the CT scanner, which is usually located somewhere near the ticketing counter.

In most other countries, particularly in Europe, all baggage is run through a scanning system. These systems are basically larger versions of the X-ray system used for carry-on items. The main differences are that they are high-speed, automated machines integrated into the normal baggage-handling system and the KVP range of the X-rays is higher.

With all of these detectors, scanners and sniffers, it's pretty obvious that you're not allowed take a gun or bomb on a plane. But what else is prohibited?

You Can't Take it With You

There are a number of items that you cannot carry on a plane, and some of that can't be packed in your bags, either:

  • Explosives: Fireworks, ammunition, sparklers, matches, gunpowder, signal flares
  • Weapons: Guns, swords, pepper spray, mace, martial arts weapons, swords, knives with blades of any length
  • Pressurized containers: Hair spray, oxygen tanks, propane tanks, spray paint, insect repellant
  • Household items: Flammable liquids, solvents, bleach, pool chemicals, flammable perfume in bottles 16 ounces or larger
  • Poisons: Insecticides, pesticides, rat poison, arsenic, cyanide
  • Corrosives: Car batteries, acids, lye, drain cleaner, mercury

Now Boarding

While most of the things that you can't take on board an airplane are fairly obvious (guns, knives, explosives), there are some things that most people wouldn't think about. Who would have thought that a smoke detector could be considered hazardous? If you do transport a hazardous material on a passenger plane without declaring it, you could face a fine of up to $27,500! Make sure you contact the local airport authority if you have any concerns about an item you plan to carry with you on a trip.

Because terrorism is a constant and terrifying threat, this means that any mention of certain words, such as "bomb," "hijack" or "gun," can lead to your immediate removal from the plane and quite possibly your arrest, even if the word is said in an innocent manner. Everyone who works in aviation, from flight attendants to security personnel, is trained to react immediately to those words.

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Are we doing enough?

While billions of tax dollars are spent beefing up airport security, there are fears that things are still not safe enough. A March 2004 report by the Government Accountability Office (GAO, formerly the General Accounting Office) said that there were still problems "hiring, deploying, and training [TSA's] screener workforce. Staffing shortages and TSA's hiring process continue to hinder its ability to fully staff screening checkpoints."

The GAO also noted the extensive delays in the implementation of CAPPS II, which is far behind schedule and doesn't even have date of completion or cost estimates. The GAO report states, "TSA has not fully addressed seven of eight issues identified by Congress as key elements related to the development, operation, and public acceptance of CAPPS II."

The air marshal program also came under fire in the GAO report - the need for many additional marshals resulted in an abbreviated training program, and budget cuts have further crippled the program. A recent investigation by the DHS's inspector general found 753 reports of air marshal misconduct during an eight-month period in 2002, including sleeping and being drunk while on duty.

Finally, many security experts fear there are too many threats that aren't being addressed at all. Many baggage handlers, mechanics and other technicians with access to airplanes are not screened or searched. Handheld surface-to-air rocket launchers are another concern - currently, U.S. aviation has virtually no defense against such an attack.

Air Marshals

If fences and barriers are the first line of defense, the air marshals are the last. If everything else fails and a terrorist still gets onto a flight with a weapon, an armed air marshal can take control of a situation and restrain the attackers. Although the air marshal program has existed since the 1970s, it has never had as high of a profile as it has in the post-9/11 era.

An air marshal is a federal agent disguised to look like regular passenger. Each air marshal is authorized to carry a gun and make arrests. There are not enough air marshals to cover every flight, so their assignments are kept secret. No one knows which passenger is the air marshal, or even if an air marshal is present on the flight at all. Although their exact numbers are kept classified, airline insiders estimate that only five percent of U.S. flights have an air marshal on board. This is still a major increase - in the years before 9/11, a handful of marshals guarded just a few international flights.

In addition to policing the sky, new laws have forced the installation of locks on cockpit doors. This could prevent hijackings by terrorists who are trained to fly passenger jets by keeping them away from the plane's controls.

For more information on airport security and related topics, check out the links on the next page.

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