How does a pilot accidentally report a hijacking?

As history shows, the hijacking of an aircraft can be catastrophic, resulting in hundreds of deaths. 

So, as a precaution, pilots have a discreet way of alerting air traffic controllers of a hijacking, should one be taking place.

It's this process that likely resulted in the accidental alert that brought Amsterdam Schiphol Airport to a standstill on Thursday.

Military police were sent to the airport to deal with what authorities called "an incident or serious event with major consequences for the population".

Any fears of a possible terror attack were soon put to rest when the airline announced the hijack alert was a false alarm.

"Nothing has happened, all the passengers are perfectly looking forward to flying soon. We're sorry," Air Europa tweeted.

A special code is entered on a panel in the cockpit.
A special code is entered on a panel in the cockpit. Photo credit: Getty Images

How does this happen?

Every aircraft in the sky at any time is given a unique four-digit identification number by air traffic controllers.

This code is sent via a transponder from the aircraft back to air traffic controllers, allowing them to track its movements.

In New Zealand, the codes are randomly generated by a computer system at Airways New Zealand.  They are then issued to flights as they depart.

As part of the departure process, an aircraft's pilot enters their unique assigned four-digit code into a keypad located on the cockpit panel.

This is when an error can occur.  Whether it's due to distraction or clumsy fingers, sometimes a pilot can enter the wrong code.

It's usually no big issue, as they can just reenter the correct code. However, if they accidentally enter the number 7500, then their day is going to change, and quickly.

The 7500 code alerts controllers and other aircraft that a flight has been hijacked, and due to the nature of hijacking, a simple "sorry it was an accident' over the radio isn't going to bring an end to the full-scale emergency response.

There's no way of controllers knowing if a pilot is being forced to pretend everything is fine, or if it's a genuine mistake, so the worst is assumed until proven otherwise.

It's not the first time a fake hijack alert has occurred. In 2005, a Virgin Atlantic flight from London to New York was intercepted by fighter jets and forced to land in Halifax after a crew member accidentally sent out the hijack code.

Armed police raided the aircraft as soon as it landed but the situation was eventually resolved.

The only time the code has been used in New Zealand was in 2008, but it was no false alarm.

A woman hijacked a regional Air New Zealand flight using a knife. Two crew members were injured shortly after the aircraft had taken off from Blenheim en-route to Christchurch.

A full report of the 2008 incident can be viewed in the video above.