Special thanks to Paul Hurd of PerkinElmer Detection Systems for his assistance with this article.
Millions of people fly every day. The vast majority of them are law-abiding folks who have no intention of harming anyone. But there is always the possibility that a terrorist or a criminal is hidden among the masses. Also, many people with no intent to cause harm may accidentally carry a hazardous material onto the plane. To avoid these problems, airport security is an important part of any airport.

Photo courtesy Photo courtesy Federal Aviation Administration (FAA)
Millions of people fly on thousands of planes every day.

Imagine a terrorist trying to blow up or hijack a plane. What are all of the different techniques that the terrorist might use to get a bomb into position? A terrorist could:

  • plant a bomb in an unsuspecting passenger's luggage.
  • smuggle a bomb in his luggage
  • strap a bomb or gun onto his body
  • walk onto the tarmak by hopping a fence and approach a plane from the ground
Airport security tries to cut off all of these different routes. Now, you'll learn about the metal detectors and X-ray systems used by airport-security personnel. You will also find out what you definitely can't carry on a plane.

Who Are You?
The first line of security at an
airport is confirming your identity. This is done by checking a photo ID, such as your driver's license. If you are traveling internationally, you need to present your passport.

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

During the check-in process, the attendant asks you a couple of security questions:

  • 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.

The guidelines and requirements for airport security are established by Civil Aviation Security (CAS), a division of the Federal Aviation Administration. CAS has three main objectives for airport security:

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

FAA agents working under CAS 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, and require background checks 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.

Step Through, Please
The entire perimeter of an airport is secured. A fence restricts access to the runways, cargo-handling facilities and terminal gates. All public access 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.

Photo courtesy PerkinElmer Detection Systems
Airport metal detectors rely on pulse induction.

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.

Photo courtesy PerkinElmer Detection Systems
Your carry-on items are sent through a machine that X-rays the contents.

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.

Image courtesy PerkinElmer Detection Systems
In a dual-energy X-ray system, the X-rays pass through a detector, a filter and then another detector.

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.

Photo courtesy PerkinElmer Detection Systems

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

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.

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

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.

Photo courtesy PerkinElmer Detection Systems
Your luggage goes through a larger X-ray system.

Most airports use one of three systems to do this:

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

Photo courtesy Federal Aviation Administration (FAA)
Dogs are used to sniff for bombs.
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.

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.

Image courtesy PerkinElmer Detection Systems
This diagram shows how the X-ray system in a CT scanner rotates around a bag.

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?

Now Boarding
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 4 inches or longer
  • 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
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.

Another thing you don't want to carry on a plane is a dark sense of humor. 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 jest. Everyone who works in aviation, from flight attendants to security personnel, are trained to react immediately to those words.

Check out the next section for links to more articles about airports, as well as to other great resources.

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