How does airport luggage reach the correct plane?

1. At check-in, a printer makes a long sticky tag for your suitcase, and it is looped around the handle. Then the bag rides away on a belt, through a little door in the wall. Where is it going?
2. The stripes on the tag are a barcode. It stands for a number that belongs to this bag alone. The airport's computer remembers which flight goes with that number.
3. The belt carries the bag through a ring of scanners. Their red laser lines sweep over it from every side, so they can read the barcode however the bag is lying. Beep! The computer now knows its flight.
4. Every bag also goes through a scanner that looks inside it without opening it, a bit like an X-ray at the doctor's. Safety officers check the picture to make sure everything is safe to fly.
5. Now the sorting begins. As each bag reaches the turn for its flight, an arm swings out and pushes it onto a side belt. The computer knows exactly when to push, because it follows every bag along the belt.
6. Handlers load the bags onto carts, and a small tractor pulls them out to the plane. A sloping belt carries each bag up into the hold. When you land, the same thing happens in reverse.

You are looking down on the baggage hall from above. Tap each part to see what it does.

• Bag tag. The tag carries a barcode: black and white stripes that stand for a number. No other bag in the airport has the same number today.
• Tag scanners. A ring of scanners sweeps thin red laser lines across each bag. Light bounces back from the white stripes but not from the black ones, and that pattern spells out the number.
• Screening machine. The screening machine makes a picture of what is inside each bag without opening it. Safety officers make sure nothing dangerous is going on the plane.
• Sorting computer. The sorting computer looks up each tag number to find the flight, then works out which belt leads to that plane. It follows every bag as it moves, so it knows when to push.
• Sorting point. At each sorting point, an arm called a pusher waits beside the belt. When the right bag arrives, it swings out and slides the bag onto the side belt.
• Side belt. Each side belt leads to the loading area for one flight. Every bag that is pushed onto it is going on the same plane.
• Cart and tug. Handlers lift the bags into carts. A small tractor called a tug pulls a train of carts across the airport to the plane.
• Hand-check station. Sometimes a tag is crumpled or torn and the scanners cannot read it. That bag rides to a person, who reads the number, types it in and sends the bag on its way.

Watch the bags with the scanner off, then tap the scanner to switch it on. Try crumpling a tag, or tap a pusher to push a bag yourself.

Experiment: What happens to a bag whose tag cannot be read?

1. Watch a few bags with the scanner off. Where do they end up?
2. Tap the scanner to switch it on and watch the pushers.
3. Press Crumple the next tag and follow that bag.

What we found: A bag that has not been read cannot be sorted, because the computer does not know its flight. It rides to the end of the belt, where a person reads the tag and types the number in. With the scanner on, almost every bag is pushed to the right plane automatically.

Turn on JavaScript to try it yourself.

Read the flight on each bag's tag, find that flight's pier on the board, then tap the pier. Load all 6 bags.

When you check in, a printer makes a long tag for your suitcase. The stripes on it are a barcode, and they stand for a number that only your bag has. At the same moment, the airport's computer is told: this number goes with this flight.

Behind the wall, the bag rides on conveyor belts. It passes through a ring of scanners that shine thin red laser lines on it from every side. White stripes bounce the light back and black stripes do not, so the scanner can read the number. The computer looks it up and finds the flight.

The bag is also screened by a machine that looks inside it without opening it. Then comes the sorting. The computer follows each bag along the belt. When the bag reaches the turn for its flight, a pusher swings out and slides it onto a side belt that leads to that plane's loading area.

Handlers lift the bags into carts, and a tug pulls them to the plane, where a sloping belt carries them up into the hold. If a tag is too crumpled to read, the bag goes to a person who types the number in. Our letter comparison is close, but a letter has its address written on it. A bag tag only has a number, and the address is kept in the computer.

Key words

Barcode

A pattern of black and white stripes that stands for a number a machine can read.

Scanner

A machine that reads a barcode by sweeping a beam of light across it.

Diverter (pusher)

An arm that swings out to push a bag onto a side belt.

Pier

The loading area where all the bags for one flight are collected.

Database

The computer's big list. Here it matches every tag number to a flight.

Check your thinking

1. How does the sorting computer find out which flight a bag is on?

   Show the answer

   A scanner reads the tag's number and the computer looks it up. The barcode is a number. The computer keeps a list that matches each number to a flight.

2. Why must a pusher swing out at exactly the right moment?

   Show the answer

   Because the bags keep moving, and pushing early or late would send the wrong bag. The belt never stops. The computer follows each bag so the arm meets the right one.

3. A tag is so crumpled that the scanners cannot read it. What happens to the bag?

   Show the answer

   It rides to a person who reads the number and types it in. Without a number the computer cannot sort the bag, so a person identifies it by hand.

4. The belt is sped up to sort more bags per hour. What finally limits the speed?

   Show the answer

   A diverter needs time to swing out and back before the next bag arrives. If bags follow each other more quickly than the paddle can cycle, the second bag misses its branch.

5. Why is every bag looked at by a screening machine?

   Show the answer

   To check that nothing unsafe goes on the plane, without opening the bag. The machine shows the shapes inside, so officers can check each bag quickly and safely.