There were a handful of aircraft hijackings in the 1930s and '40s but numbers increased after the end of the Second World War, especially after the Iron Curtain fell across Europe. People on the Soviet side hijacked passenger planes from the East, taking them to places like West Berlin or Denmark where they could apply for political asylum.
The liabilities associated with causing some kind of nuclear holocaust in Tennessee would clearly put them out of business forever.
As the number of hijackings escalated worldwide throughout the 1960s, it became a preferred tactic of many revolutionary groups. But although many hijackers made political claims, there were often other things in play, according to Brendan Koerner, the author of The Skies Belong to Us: Love and Terror in the Golden Age of Hijacking.
'Often if you scratched beneath the surface of their political motivations you would find personal grievances, personal difficulties, romantic prospects that had gone awry, professional difficulties, mental health problems,' he says.
'Eventually, in the early '70s, you saw a phenomenon on in the United States where people were actually demanding large amounts of ransom in exchange for the hostages' lives, hundreds of thousands and eventually millions of dollars.
'It really became more of an extortionate crime than a political crime.'
A calculated risk becomes a thorny dilemma
During the years when hijackings seemed commonplace, the airlines instructed their flight crews to go along with the hijackers' demands, says Philip Baum, the author of Violence in the Skies: a history of aircraft hijacking and bombing.
'The hijackers were never suicidal; they simply wanted to get to pastures they deemed greener,' Baum says.
'So it was a little bit of fuel spent, a little bit of time, but you reached your destination, there was some negotiation on the ground and then all the hostages and the crew were able to return to where they came from.'
Governments generally went along with what the airlines wanted. Koerner says airlines were seen as high-tech, and had a lot of political power.
'They actually did not want tighter security,' he says. 'They made a calculation that this could be a managed risk, that if you gave in to the hijackers' demands ... your passengers would be safe, your aeroplane would be intact, and oftentimes you would even get the money back.
'Their fear was that if they put in lots of new security measures on the ground in particular, they would scare away passengers. Passengers would choose to drive or take trains instead and would wreck their industry.'
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While the early hijackers were often after refuge or ransom, militant groups—especially in the Middle East—realised that hijacking a plane was a way to seize the media spotlight
A watershed moment in the history of aviation security was the simultaneous hijacking of four commercial jet airliners by a group called the Popular Front for the Liberation of Palestine in September 1970. Three of them were flown, with their passengers, to a disused airstrip in Jordan called Dawson's Field.
'This was a highly organised, premeditated attack,' explains Dr Simon Bennett, the director of the Civil Safety and Security Unit at the University of Leicester.
'The passengers were, over a period, released, leaving just the aircraft sitting in the desert sun. The PFLP then invited the world's press with their cameras and tape recorders to Dawson's Field to witness those three beautiful aircraft belonging to various flight carriers being blown up.'
Fear of a nuclear holocaust (and its liability cost)
Norman Shanks, a former manager of airport security at Heathrow Airport, says that although the events at Dawson's Field shocked the world, change to airport security systems was slow.
In the US, a hijacking in November 1972, where three men threatened to crash a plane into a nuclear reactor in the state of Tennessee, forced the airlines to act.
'The liabilities associated with causing some kind of nuclear holocaust in Tennessee would clearly put them out of business forever,' says Koerner.
'So they actually bent to the public and political will at the time and that led to the start in January 1973, of universal physical screening. All passengers from that moment forward had to walk through metal detectors and have their luggage searched before going on planes.'
But who should pay? The airlines wanted the creation of a government agency that would have salaried public servants, whereas the government wanted the security agents at the gate to be airline employees.
'A compromise was struck which was kind of the worst of both worlds,' says Koerner.
'The government let the airlines farm out their security to independent contractors. So third-party companies arose that were specialised in aviation security. They competed for contracts with airlines and airports, which always favoured the lowest bidder. And so you got progressively lower and lower pay and lower and lower skills for aviation screeners.
'You ended up with the airport screener who is making $12,000 a year whose training consisted of one hour of watching a videotape of how to do their job.'
Security increases after Lockerbie bombing
The introduction of the metal detecting arch and the x-raying of hand luggage contributed to the decline of hijacking into the 1980s, but radical groups found another way of bringing terror to the skies. An event over Scotland became another key moment in aviation security.
In December 1988, a Pan Am jumbo jet travelling from London to New York crashed into the small village of Lockerbie in southern Scotland. The plane, with 259 people on board, hit a petrol station, sending a fireball more than 100 metres into the air and spraying debris over houses.
Eleven people were killed on the ground. There had been a bomb in a bag on the plane.
'That was a turning point for us in the UK,' Shanks says.
'We introduced a measure whereby all baggage going into the hold of an aircraft, checked baggage, would be screened. Up until then it was only a small number of high risk flights would have their checked baggage screened.'
In the year following the attack, the UK also required all staff going to the air-side security areas to be screened to the same standard as passengers. Twenty-four years later, in 2014, that was adopted as a world standard by the UN's International Civil Aviation Organisation (ICAO).
'And yet it's not carried out at every airport today, even though it is a standard,' Shanks says.
A new chapter: 9/11 and beyond
Outrage and fear following the attacks on the United States on 11 September, 2001, opened a new chapter in aviation security, although Baum says the changes did not go far enough.
'The US government created the Transportation Security Administration because they decided that they didn't really want contract screening companies providing the security at airports—and that's something that I am quite sympathetic to,' he says.
'However, if you are just going to put the same people that used to work for contract screening companies in federal uniforms, you're not really going to recruit the right calibre of staff.'
Prior to 9/11, Shanks says, American aviation security had pretty low standards. The four flights hijacked that day were domestic flights, so were not covered by ICAO's international regulations.
'One of the changes after 9/11 is that domestic flights are now included within the same regime as all other international flights,' he says.
'But you could argue that 9/11 exploited a loophole allowing passengers to carry the types of makeshift weapons that they had and get into the cockpits of the aircraft.
'Up until then, doors were typically not locked. It was quite common for flight crews to invite passengers up to the flight deck during travel.'
Could more be done to make flying safer?
Over the last 15 years, further security breaches have illuminated areas of weakness in passenger screening.
Incidents involving the so-called 'shoe bomber' and the 'underpants bomber' have seen more items banned in carry-on luggage, and made going through security a kind of semi-striptease. You might even get a pat down or a quick zap in the body scanner.
Still, the crew on an EgyptAir flight from Alexandria to Cairo in March believed a passenger when he said he had a bomb.
'That probably comes down to their lack of confidence in the security measures at the airport they left,' says Shanks.
'If the crew had a high confidence in the way in which they were screened as they went through the airport ... then they probably wouldn't have taken that action.'
The plane was flown to Cyprus, where the passengers and crew were ultimately set free and the hijacker's supposed suicide vest turned out to be a fake.
Bennett says it would be possible to take security to a more extreme level—for example, by hand-searching every passenger—but that doesn't mean it should be done.
'In my opinion, if we did that we would be playing into the hands of the terrorists,' he says.
'One of the key objectives of Al Qaeda, Daesh, and similar organisations is to bring the western economy to its knees, and to cause great disquiet and discomfort to those of us who enjoy freedom and liberty.
'It's very important to understand that in free societies we need to preserve, as far as we reasonably can, our civil liberties.'
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