Since the terrorist attacks on 11 September 2001, anti-aircraft missile batteries have been installed to protect buildings in Washington DC and other US cities. Less drastic solutions have also been suggested.

Aerospace company Northrop Grumman, for instance, has proposed installing the electronics from its Global Hawk pilotless plane in passenger aircraft to allow ground control to take over a hijacked plane and land it remotely. Others say automatic landing systems could steer planes to safety without human intervention.

All these solutions have disadvantages, says Edward Lee at the University of California in Berkeley. They require radio links between the plane and air traffic control, and these can be jammed, or hacked into. They could even allow planes to be hijacked from the ground if terrorists managed to take over air-traffic control sites.

Lee and his colleagues have an alternative. They propose modifying the avionics in aircraft so that the plane would fight any efforts by the pilot to fly into restricted airspace.

So if a plane was flying with a no-fly-zone to the left, and the pilot started banking left to enter the zone, the avionics would counter by banking right. Lee's system, called "soft walls", would first gently resist the pilot, and then become increasingly forceful until it prevailed.

To the pilot, it would feel like fighting an external force, such as a strong wind. "When you reach a certain critical point, the pilot is banking as hard to the left as the aircraft will go- as far as he can tell- and that is only just cancelling the force, so the aircraft is still going straight," says Lee.

The system would include an on-board database of the GPS coordinates of the no-fly zones. If it sensed an attempt to jam GPS signals it would switch to other navigation aids such as airport beacons. Being independent of ground control means soft walls would be immune to hacking.

For modern fly-by-wire aircraft, installing soft walls would only require software changes. Lee's team has developed algorithms to control the aircraft and carried out some testing. "But no pure software simulation is going to be sufficient to convince any pilot," says Lee.

To take it to the next level, Lee is collaborating with aircraft manufacturer Boeing. Don Winter, director of R&D at Boeing's Phantom Works research division in St Louis, Missouri, says Boeing has asked the Pentagon for more research funding for soft walls. "We'd like to take the technology investigation to the next stage, which is evaluation of the algorithms in high-fidelity simulators," he says.

He has yet to convince the people who fly the planes. "In general, pilots are openly hostile," he says. "Frankly it surprises me, because of all of the options that they are facing right now- including being shot at or commandeered from the ground- this is their best one." Anil Ananthaswamy, Berkeley

This article is based upon a report by Paul Parsons which appears in the July 5 issue of New Scientist

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