alaysian Prime Minister Najib Razak's announcement that the Aircraft Communications Addressing and Reporting System (ACARS) and transponder on flight MH370 were deliberately disabled and the plane diverted towards the southern Indian Ocean or north towards Kazakhstan suggests that one (or both) of the pilots or one (or more) of the passengers was responsible for the loss of the aircraft. It has created hope among relatives of the crew and passengers that survivors could be found somewhere on land or in a life raft on the Indian Ocean.

This new Malaysian revelation again raises the spectre of terrorism. Terrorism certainly cannot be ruled out, but seems less likely than other possibilities. Terrorism is by definition politically motivated with a strategic outcome in mind. If terrorism was the motivation you would expect that the perpetrators would have already used the plane as a weapon against a possible target, such as Mumbai or Colombo, would have made political demands, or would have attempted to put pressure on a target government.

It is not always the case that terrorist groups claim responsibility for their acts, but that is usually true when there is little doubt about who they are and what they want, whether it be Hamas fighters rocketing southern Israel from Gaza, or al-Qaeda in the Arabian Peninsula deploying underwear bombs against US aircraft. There is little point in terrorists hijacking MH370 if no one knows why they did it or what they are trying to achieve.

In the modern era since 2000, for most people hijacking conjures up images of September 11, 2001 - where four hijacked aircraft were seized by 19 hijackers to be used as flying bombs. Before that there had only been one occasion when terrorists hijacked a large passenger aircraft to use as a weapon. In December 1994 Algerian terrorists hijacked Air France flight 8969 which they allegedly planned to crash on Paris.

Before that, particularly since 1968, there had been many terrorist hijacks; many of them by Palestinian groups trying to gain international recognition for their territorial rights. The usual routine was to hold the plane on the ground somewhere to get publicity and then release the passengers. Since the Oslo Accords in 1993 there has been a decline in the number of terrorist aircraft hijacks or attempted hijacks.

Internationally, since 2000 there have been only 18 hijacks or attempted hijacks of large passenger aircraft. Of these, seven were by passengers wanting to get to a destination to seek asylum, one was criminally motivated to steal the cargo, six were by mentally ill persons, and four were politically motivated (counting September 11 as one incident). Some of the perpetrators could probably be re-categorised from the "seek asylum" category to the mentally ill category, judging by how they went about it or their intended destinations. Mental disorders seem to account for most of the lone-perpetrator hijacks and attempted hijacks.

This is certainly true of Australia where since 1960 there have been at least 11 attempts to hijack aircraft, some of which have been successful. None was terrorism related. All were by lone perpetrators, most of whom seemed to be suffering from mental illness.

Since 2000 there have only been two hijack attempts in Australia.

On February 19, 2003, a TAFE student, Kelly Witchard, armed with a knife, hijacked a Cessna 210 at Hedlow Airport near Rockhampton, and forced the pilot to fly 300 kilometres to Mackay. There were no passengers on the aircraft. Witchard was arrested on arrival at Mackay Airport. The pilot was not injured. Witchard later received a four-year jail term.

On May 29, 2003, David Robinson, a passenger on Qantas flight 1737, a domestic flight from Melbourne to Launceston, tried to overcome the flight crew with wooden knives that he had taped to his legs to pass through a Melbourne Airport metal detector. Instead, he was overpowered by the crew and passengers. He later admitted attempting to hijack the

plane to crash into the Walls of Jerusalem National Park in Tasmania - an action intended "to release the Devil from his lair and bring about Armageddon". In July 2004, the Supreme Court of Victoria found Robinson not guilty of attempted hijack due to reasons of mental impairment.

Turning back to flight MH370, the most likely cause for the MH370 diversion seems to be mental illness on the part of one of the pilots, or a passenger who gained access to the cockpit and was able to force a pilot to fly in a particular direction - or was able to fly the aircraft himself (hijackers rarely being women). One would also have to question his mental state.

The MH370 incident highlights six mainly aviation security problems that will need to be addressed.

The first was on the ground, with lax passport checking against the Interpol stolen passport database - although that probably would not have affected the outcome here.

Second was information management which raised hopes and dashed them again, and created confusion about what to believe and what not to believe. For example, for several days we were told by Malaysia Airlines that there were five no-shows for flight MH370 whose baggage was offloaded. Malaysian police later said this was incorrect and there were no no-shows. Malaysia withheld for several days vital ACARS and transponder information that could have helped the countries searching for the aircraft, and so on. Malaysia was not helped by the international media continually publicising advice from technical experts, leaks from inside the investigation, and sideline sniping.

Third, we need to revisit the cockpit door issue. Reinforcing the door was a good idea to keep out 9/11-style hijackers, but undermined by the door being opened for toilet needs, refreshment breaks, and crew handovers on long-haul flights. Now someone on the cockpit side of the reinforced door can keep out anyone trying for the right reasons to gain access to the cockpit. This happened as recently as February 17 this year when the co-pilot of an Ethiopian Airlines plane flying from Addis Ababa to Rome locked the pilot out of the cockpit and flew the plane to Switzerland to seek asylum.

Fourth is the mental health of the pilots, which should be regularly reviewed, possibly every six months. I have heard from pilots that flying long distance on autopilot is excruciatingly boring and can cause fixation and stress problems. There seem to be inadequate studies on the effects of frequent long-haul flights on pilots, or for that matter the health effects on flight crew of regular high-altitude cosmic and solar radiation exposure.

Fifth, no one on an aircraft should be able to disable the ACARS or transponder.

Sixth is the issue of air security officers or flight marshals on international flights. Governments, including Australia's, have been cutting back funding in this area and it is probable now that not more than 5 per cent of international flights have air security officers on board. An officer on flight MH370 could have made a difference - provided of course he or she was able to open the cockpit door.

Clive Williams is a visiting professor at the ANU's Centre for Military and Security Law and an adjunct professor at Macquarie University's Centre for Policing, Intelligence and Counter Terrorism.