Now, the rush is under way to develop measures to boost security and the confidence of the travelling public.

Among measures put forward are a system which instead of merely warning pilots the plane is about to hit a large object, commands the autopilot controls to steer the aircraft away from danger and a network which would shut down flight-deck controls during a hijacking and allow the aircraft to be controlled from the ground.

No-flight zones, which put city centres and other potential targets out of bounds for passenger aircraft, could be delineated in the computer databases of planes to prevent pilots steering towards them.

Research has already been conducted on these proposals in the US, but they are expected to take time to develop if the industry or authorities want them.

Perhaps the most likely, and relatively simple, idea among the measures being put forward is to arm passenger-jet pilots with stun guns.

A North American pilots' union is pushing to have at least two of the electrical guns - which can incapacitate a person from up to five metres - installed on aircraft.

'Before [September 11], we could scarcely have envisioned calling for protection in the form of weapons carried in the cockpit. However, the world has changed, and we must change with it,' said Captain Duane Woerth, president of the Airline Pilots' Association International, which represents aviators in the US and Canada.

He made an appeal to a US Senate committee on aviation security, which met in the wake of the attacks on New York and Washington, for help in implementing short-term measures, including use of stun guns.

Sophisticated stun guns were accurate and did not pose health risks, he said. The guns would be used only in extreme circumstances to protect lives and for the safety of passengers and crew.

It was not clear from his comments whether the guns could be used to subdue violent or aggressive passengers in 'air-rage' incidents. Carrying stun guns is understood to have been considered by the union for some time in relation to violent passengers.

Hong Kong pilots appear to be receptive to the arming of Cathay Pacific Airways and Dragon Airlines flights with the devices. An Aircrew Officers' Association spokesman said: 'We would welcome being consulted on any measure to implement stun guns.'

The North American union also wants stronger cockpit doors. 'Current cockpit doors are weak and flimsy, and can be easily compromised by a determined adult,' Mr Woerth said.

A dead-bolt lock which could not be opened with a key from the outside should be installed on the inside of doors. But the door must be able to be opened quickly when necessary, he said.

According to an aviation-security expert, airlines and aircraft manufacturers have been urged for the past decade to install reinforced doors and bulkheads to improve cockpit safety.

It was thought a thicker wall between the cockpit and cabin would be more effective in deflecting bullets and protecting the flight crew in the event of a small explosion in the passenger area.

Hill and Associates director Ian Gilchrist said the industry had largely turned its back on the proposal because it would make aircraft heavier, meaning fewer passengers, and less fuel and cargo could be carried.

'At the end of the day, I am sad to say that it was [not installed] due to money,' said Mr Gilchrist, who became aware of the proposal while working with the United Nations' International Civil Aviation Organisation.

Aircraft manufacturer Boeing said installing reinforced doors in current aircraft would be a major project requiring significant work.

'You can't just put in a heavier door; you need stronger hinges and locks, and you would have to redesign the bulkhead and floor beams,' Boeing's Asia spokesman Mark Hooper said.

It would take 'some time' for the work on each aircraft to be done, and the aircraft would have to be out of service for the duration. Doors were currently designed to open should the aircraft suffer decompression and were 'unjammable' in an emergency.

Boeing had designed a more secure door for an international airline several years ago, but Mr Hooper refused to name the carrier or provide more details, citing security restrictions.

The airline is believed to be Israel's El Al, but El Al would not comment. The carrier has faced one of the biggest security threats in the world because of Middle East tensions, but Mr Gilchrist said its aircraft had never been hijacked.

El Al spokesman Nachman Klieman said cockpit doors were locked from passenger boarding until after landing. 'At no time is the cockpit accessible during the flight,' he said. Special procedures were adopted for toilet breaks and flight-crew meals, but he refused to disclose them.

Hong Kong's Civil Aviation Department said on Wednesday that it had ordered Cathay Pacific and Dragonair to keep cockpit doors locked and ban passenger visits to flight decks.

In the US, cockpit doors are required to be locked, but this measure did not prevent the recent hijacking disasters.

Mr Gilchrist attributed El Al's avoidance of hijackings to a security-conscious culture among ground personnel, the airline's relatively small size, and stringent security checks requiring passengers to check in four to five hours in advance.

The best way to avoid threats was to place an aircraft in a 'totally sterile bubble' - checking it thoroughly for hidden weapons after it had been cleaned and putting passengers through tough security checks, he said.

Other ideas which had been explored included gas to knock out passengers during an attack, which would also neutralise hijackers, but this was viewed as being too shocking for travellers.

Mr Gilchrist said arming pilots was an 'over-reaction' and worried about putting weapons - including stun guns - in the hands of 'non-professionals' inside the aircraft.

Cathay Pacific has stepped up security, along with measures implemented by the Airport Authority at Hong Kong International Airport, but is unlikely to rush into adopting new measures.

'Our view is that, in general, good security starts on the ground. That's the place to concentrate,' said corporate-development director Tony Tyler.