Science: Down to earth - safely: Parachutes that enable damaged small planes to land safely could be developed to save plummeting jumbo jets. Tony Newton reports

There are two key ingredients that keep an aircraft flying: the first is the engine, the second the airframe. Lose the first and, with good judgement, a lot of luck and somewhere reasonably long and flat to set down, a pilot will be able to land the aircraft with minimal injury to its occupants.

But do substantial damage to the flying surfaces of the airframe - as might occur in a mid-air collision or because of structural failure or explosion - and you are left with a pile of very heavy metal that wants to point in one direction only - straight down, with little hope for anyone on board.

But a device developed by Ballistic Recovery Systems (BRS) in Minnesota could change that.

The concept is simple: a parachute is attached to the aircraft. Parachutes have been used for years to drop military equipment but BRS has produced one made of up-to-the-minute lightweight material which is compressed by means of a hydraulic press and operated by hi-tech rocket motors. These can 'sense' the speed of the aircraft and ensure that the parachute opens immediately at low altitudes.

The hugely energetic sports aviation community in the United States has welcomed the BRS parachute. More than 10,000 have been installed in hang-gliders, microlights and kit-built aeroplanes and by this February, according to BRS figures on parachute use, 74 lives had possibly been saved as a result.

For some time, the parachute was available only for 'non-certificated' aircraft - those outside the jurisdiction of the Federal Aviation Authority (FAA). But in August 1993, following extensive trials, the FAA allowed a BRS parachute to be installed on the Cessna 150 and 152.

The FAA is now considering approval for other light aircraft. This could save at least 600 of the 1,000 lives lost each year in aviation accidents.

Only minor modifications to the aircraft are necessary for installation of the BRS parachute, which is simple to operate. No special training is needed - something of a benefit to a non-flying passenger when the pilot is incapacitated and has slumped forward over the controls.

All the pilot (or passenger) has to do is reach overhead to the cabin roof, remove the handle cover and pull the handle. A fifth of a second later, the rocket motor has fired, penetrated the fabric covering of its container and taken with it the deployment bag containing the parachute canopy and suspension lines. In less than a second, the rocket motor has pulled the deployment bag away from the main canopy, which now begins to fill, starting the deceleration. If initial aircraft speed is low, the canopy inflates fully almost immediately; if initial speed is high, a 'slider device' restricts the amount of air that can ram into the main canopy, so preventing the rupturing of seams until canopy drag has reduced speed to a level that permits full inflation.

The aircraft returns to earth at 21ft per second under a 40ft main canopy - about the same landing force as jumping off a 7ft wall.

But what does it mean for the rest of the human race - those who fly only for the purposes of business or vacation, and for whom the concept of 'flying for fun' is a contradiction in terms?

'People often ask us if we can handle large aircraft like airliners,' says Boris Popov, the president of BRS and a founder of the company, 'and the answer is 'yes, but not yet'. It's only geometry: parachutes lower the space-shuttle rocket motors to earth intact for reuse, and they weigh over 50,000lb]'

It may be only geometry, but to suspend a jumbo jet under a canopy would be an enormous undertaking. An old rule of thumb states that for every pound to be carried by a parachute, you need one square foot of cloth. As the 747-400 weighs in excess of a million pounds at takeoff, BRS calculates that this would require as many as 36 canopies, each comprising enough fabric to cover a football field. But it is theoretically possible, and it would probably save many lives.

But the saving of lives and avoidance of injury often seem to come second to commercial reality. Paradoxically, it is here that BRS devices may score highest in the minds of aircraft operators and insurers, because with such devices it is not just lives that are saved; an aircraft is an expensive piece of kit, and anything that can bring it to earth with a minimum of damage has to be a good idea.

(Photograph omitted)