Security is still based around a very human system

David Learmount Flight International

The checks start as passengers check in and are handled by the airlines.

Responsibility then shifts to the airport operator which, in the case of Heathrow, Gatwick and Stansted is BAA.

It is in charge of security control as people enter the pre-boarding area - known in the industry as "airside" - to wait for their flights.

Passengers have to show their boarding passes and have their hand luggage X-rayed, all of which is done by BAA staff.

The responsibility for security then shifts back to the airlines. Even though we do not see it, all luggage in the hold is X-rayed before it is loaded into the plane's hold.

BAA supplies all the security-checking equipment to do this, but the airlines either employ their own staff to do the X-raying or contract a security company to do it for them.

Vetting

When it comes to boarding passengers onto the plane, airline staff feed boarding cards through a computer to make sure everyone who has checked in with luggage is on the plane.

If not, they will evacuate the plane and remove every suitcase to find the piece of luggage has been left. This is called baggage reconciliation.

But passengers are not the only people to go near planes. Crew, catering staff and maintenance staff are among others who have access to an aircraft and baggage.

Everyone employed by airlines are subject to security checks imposed by the Department for Transport, Local Government and the Regions (DTLR), which is responsible for airport security.

Fallible

It is up to the airlines to make sure staff are vetted and they are subjected to their own checks from the DTLR to see if they are doing what is required.

To make sure things are up to standard airlines also employ their own security consultants to expose any weakness in their procedures.

The technology to carry out checks is constantly improving but despite the advances good security still relies on humans, according to Operations and Safety Editor of Flight International, David Learmount.

Because human make mistakes several layers of checks need to be in place to catch anyone who might slip through the net, he added.

Mr Learmount said: "Security is still based around a very human system.

"It requires humans to do repetitive tasks which we are usually not very good at.

"That is why lots of layers are important. If someone gets through one layer, another will hopefully get them."