Airport security screeners have an impossible task: Be correct 100% of the time.

The difficulty of keeping people safe and moving. Part 1of a series on the challenges associated with X-ray security screening. At Synapse Technology, we’re using AI to make this process more effective and efficient.

Fig. 1: Sample set of 30 X-rays of baggage. This set contains at least two firearms and three knives.

The holiday season is one of the busiest for air travel, and anyone who’s traveled during the past couple of weeks can agree that for passengers, “busy” quickly becomes synonymous with “stressful”: Traffic, last-minute ticket changes, and crowded gates compete for a hectic pre-flight experience.

And yet, often the most jarring experience one might have at an airport is at the security checkpoint. You wait in a tremendously long line before having to quickly follow an often-confusing protocol for putting your belongings into trays. (Do I take my shoes off? Is my iPad big enough to stay in my bag? Why is PreCheck not written on my boarding pass this time?) Passengers are confused, officers are shouting instructions, and frequent flyers often just stare off into the distance going through the motions, trying to get it over with as quickly as possible.

Fig. 2: A busy day at Denver Airport’s security checkpoint.

It’s easy to have a cynical view of the entire process, particularly when reading articles about how the TSA fails to detect prohibited items 95% of the time during penetration tests where agents are actively trying to hide threats.

In 2018, the TSA seized a record 4,000+ guns at security checkpoints according to CBS News. This marks a 17% increase from last year, and averages 11 guns found every day at airports across the US. Yet there were only three reported instances in that time where guns were missed by TSA officers; all instances where the passenger later realized it and alerted the authorities.

With test failure figures like 95%, it’s difficult to believe that only three guns were missed. One survey conducted by Stratos Jet Charters of 1,000 passengers who had successfully snuck prohibited items onto flights showed that over 150 people had previously brought weapons or ammunition through airport security unnoticed. And while guns are prohibited past security, some airports are in jurisdictions that allow firearms to be carried right up until the checkpoint, startlingly close to the thin barrier guarding the “sterile” area of an airport.

TSA Officers Have A Difficult Job

Having spent the past two years in and around security checkpoints as both a frequent flyer and working with our airport security customers, I’ve come to appreciate that TSA agents have an incredibly difficult job. Not to mention that the job is currently made worse by having to work without pay during the government shutdown.

The security officer’s task is to stop anything prohibited from getting through the checkpoint while also processing passengers as quickly as possible. The TSA’s mission is to “protect the nation’s transportation systems to ensure freedom of movement for people and commerce,” necessitating a balance between security and efficiency. A lack of efficiency can lead to angry passengers, missed flights, and other negative effects that could hamper such commerce.

Inefficiency also costs the TSA money. Those instances where an operator needs to pat down a passenger in response to a false hit on the swab test, or pull a bag aside for further inspection, add up to millions of extra dollars spent keeping checkpoints open for longer than necessary.

Since 9/11 these officers have been motivated by the mission of preventing a terrorist or other ill-intentioned individual from getting through with a weapon that could cause mass harm to passengers or attempt to bring down a plane.

Officers are also on watch for passengers that may be carrying items that could cause unintentional harm to other passengers or aircraft, including items like lithium batteries or hoverboards that could blow up mid-flight or lighters that could ignite part of a bag. The Australian government catalogues many such incidents. Below is are two examples from those case studies:

The dental supplies included a glass container of about 100 mls of mercury. The glass container broke during transport and leaked into the hold of the aircraft. Fortunately it was discovered and cleaned up — very costly. Had it not been discovered, significant weaknesses in the skin of the aircraft could have occurred.

Sometimes, you might even think you have your own bag but be accidentally carrying someone else’s belongings:

Federal Police were called to a baggage carousel at an International Airport regarding an unclaimed bag. An inspection of the contents revealed a fire extinguisher (dangerous goods), a blue pullover and a packet of sandwiches… It was finally discovered that the passenger had inadvertently taken the taxi driver’s bag from the boot and had not noticed that he checked in an additional bag. … Also it is assumed that the taxi driver was wondering what happened to his lunch.

These two examples are both cases of items difficult to spot because of their rarity. How do you catch 100mL of Mercury, or even expect it? Mercury and a fire extinguisher are one of over 100 types of items that are prohibited.

The TSA has 20 layers of security encompassing the full “curb-to-gate” journey from your car to your flight and beyond. The security checkpoint itself has several measures that a flyer might encounter during their journey. An ID and boarding pass check, a metal detector to look for metallic threats like guns or knives, a body scanner that can further look for concealed explosives or contraband, X-ray screening of your carry-on luggage, a swab test to look for traces of explosives, and potentially a pat-down, among other measures.

The most dangerous items confiscated by the TSA are often discovered by the X-ray screener. (Many of these items end up being highlighted on the TSA’s entertaining blog). In the X-ray screening process, a belt carries your carry-on bag into the X-machine and through the path of an X-ray beam that produces an image of your bag in a manner similar to a finish-line camera. Operators have control over this belt and can stop the belt to freeze the bags currently on-screen, make the belt go backward to re-scan a bag, and make the belt go forward again to keep the process going.

People who aren’t familiar with the specifics of airport screening may wonder why the TSA screeners’ jobs are so difficult if they have the support of X-ray machines. Don’t X-rays provide a clear image of the bag, allowing for easy threat identification? Not exactly.


Not All Threats Are Created Equal.

Fig. 3: The two monitors that an X-ray screener typically sees. On the left monitor is the “top” view, with the corresponding “side” view for each bag being shown on the right monitor. The first bag contains a bullet among an innocent-looking set of coins, the second bag contains a knife, and the third bag contains a replica firearm.
Fig. 4: An X-ray security operator screening passengers.

The belt carries a steady stream of bags onto the screen for the operator to scrutinize at the rate of about one bag every 2.5 seconds.

These machines typically use two X-ray beams to distinguish the materials in the bag into two major categories: organic objects which show up as orange, and metal objects which show up as blue. Everything else takes on a color somewhere in between, often a greenish hue. Low-density items like clothing are highly translucent while dense items (particularly metallic) are more opaque.

The operator can see multiple views of your bag from different angles, which can allow them to roughly discern what the shape of an object might be. Since each X-ray is captured from one direction, some items can obscure other items and require an operator to manually search the bag to see what’s really in it.

Fig. 5: The “side view” of a very cluttered bag with several dense metallic objects that appear as black or dark blue, including coins, a buckle, a tablet. The tray and bag, low-density and made of organic material, appear as a faint orange. A pocket knife is shown in the red box, which would be very difficult to spot with only a “top” view.
Fig. 6: Some items can blend in much more easily with their surroundings. Pictured is a bag containing a lighter. In Japanese airports, passengers are not allowed to carry more than one lighter onto a plane

New CT machines tackle some of these X-ray limitations by forming a 3D image of a bag, allowing operators to see objects that may otherwise be hidden or obscured. These machines aren’t a silver bullet though, and have their own human, structural, and cost considerations.

Inherently, the screening task presents a challenge. If an officer sees a dark area of the screen, they must ask themselves: Could there be a gun hidden behind this laptop? Taped to the rail of this suitcase? Is this thin piece of metal just the insert in a shoe or could it be a razor blade? Is there anything off about the electronics in this hairdryer?

Operators are worried about the 0.00000001% or lower chance that a particular bag might belong to a terrorist trying to do harm or an oblivious passenger risking the safety of a flight with a bottle of mercury. But why are we so reliant on operators to detect these threats in the first place?

X-ray Machines Leave The Decision Largely Up To The Operator.

X-ray machines were first mandated for use in US airports in 1974. Fast forward 45 years, and despite some real improvements in hardware, surprisingly little has changed in how humans interact with X-ray machines. Specifically, detection of prohibited items is still carried out exclusively by the operator that happens to be looking at the screen.

Aside from some basic software assistance in detecting some explosives and liquids, it is up to the operator and the operator alone to decide if there is a prohibited item in a suitcase. They have to look at the X-ray and make the judgment call on whether to pull a bag or let it through.

It’s incredible that in the age of self-driving cars and facial recognition technology, a pair of fallible, human eyes are usually the only barrier between a lethal threat, like a gun, and passengers.

In describing why at least three guns were missed this past year, TSA Administrator David Pekoske cited potential technology issues, along with potential procedural issues or operator training issues.

What Pekoske fails to mention is the cognitive issues that make screening just damn hard. Issues that new machines won’t fix.

Continued in Part 2: The flaws of security screeners