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To test for bugs, most aircraft manufacturers use a set of guidelines called the DO-78 B standard, which was created by the US-based Radio Technical Commission for Aeronautics, a collection of government, industry and academic organizations, and the European Organization for Civil Aviation Equipment. Recognized by the US Federal Aviation Administration (FAA), the standard rates software on how seriously it would compromise safety were it to fail, and then recommends different levels of testing depending on that rating.

The most rigorous “level A” test, reserved for software whose failure would cause a catastrophic event, is called ‘modified condition/decision coverage” (MCDC), and it places the software in as many situations as possible to see if it crashes or produces anomalous output.

But it isn’t clear whether the MCDC test includes enough different conditions to provide any greater protection than the level B tests done on less safety-critical software. So in 2003, UK Ministry of Defense contractor Qinetiq ran both levels on a range of software that is deployed in military transport aircraft. The MCDC should pick out many more flaws than the level B tests, but the Qinetiq team found that there was “no significant difference” between them.

Now Boeing is planning to get rid of the hydraulic wheel brakes on its 787 in favour of lighter electrically actuated ones and to shift from using pneumatic engine starters and wing de-icers to electrical ones. Airbus will also be adopting a “more electric” approach in its forthcoming A350, says the plane’s marketing director. “The addition of more electric systems will mean even more computer control,” says Martyn Thomas, a systems engineering consultant based in Bath, UK, and a member of the NAS panel that produced the software report last year.” It will mean more wires or shared data lines and so still more possibilities for errors to arise.

Why do software bugs arise and why can’t they be removed? Bugs are sections of code that start doing something different to what the programmer intended, usually when the code has to deal with circumstances the programmer didn’t anticipate. All software is susceptible to bugs, so it must be tested under as many very different circumstances as possible. Ideally, the bugs get discovered at this time and are removed before the software is actually used. This is very difficult in complex systems like aircraft because the number of possible scenarios —such as different combinations of air densities, engine temperatures and specific aerodynamics — is huge.

But the addition of software led to different problems. Some of these are documented in a report completed last year by the US National Academy of Sciences (NAS) . It lists a number of instances in which software bugs caused frightening problems during flight. In one instance in August 2005, a computer in a Boeing 777 presented the pilot with contradictory reports of airspeed. It said the aircraft was going so fast it could be torn apart and at the same time that the plane was flying so slowly it would fail to generate enough lift to stay in the air. The pilots managed to control the aircraft nonetheless, but it was a stark illustration of what can go wrong.

In another instance in 2005, this time in an Airbus 319, the pilots’ computerised flight and navigation displays as well as the autopilot, auto-throttle and radio all lost power simultaneously for 2 minutes. Another time, what the NAS calls “faulty logic in the software” meant that when the computer controlling fuel flow failed in an Airbus A34o, the back-up systems were not turned on.

In early aircraft, moving parts such as the rudder and wing flaps were linked to controls in the cockpit either by a system of cables and pulleys or by hydraulics. In the 1970s, aircraft makers realized they could get rid of much of this very heavy equipment and replace it with lightweight wiring and systems driven by electric motors in the wings and tail. Such “fly-by-wire’ (FBW) systems vastly increased fuel efficiency.

FBW had another advantage too: because it uses electrical signals, for the first time it allowed a computer to be placed between the pilot and the moving parts. The computers were programmed to modify the pilots’ instructions in certain instances: for example, to stop them moving the rudder too far or too quickly and so damage the plane. They also allowed the plane’s aerodynamics to be finely adjusted during flight in response to wind conditions, further improving fuel economy.

All software can, as does, go wrong and with aircraft containing even more of it, can we really be sure they will fly safely?
Cockpit displays plunged into darkness,engines that throttle back during take~off and then contradictory airspeed readings-----are just some of the problems caused in recent years by inexplicable failures in the software that controls aircraft. So far there are no known cases of such failures alone causing an accident.

Speculation that software problems led to the crash landing of the British Airways Boeing 777 at Heathrow airport, London, on 17 January 2008, remain unconfirmed. While software was implicated in the Korean Air jumbo jet crash in August 1997 on Guam, which killed 228 people, human error, not software design, was to blame. Software failures remain a risk, though, and with aircraft makers set to increase the proportion of aircraft functions controlled by software, experts are warning that they will become more frequent, increasing the chance that one will cause an accident.

Anterograde amnesia is not that sexy. It doesn't allow you to slap a new identity on a person and until a few years ago it didn't sell scripts. Then, in 2000, came the film Memento. Guy Pearce plays Leonard, an insurance investigator who, following a blow to the head, is unable to remember anything that happens to him. Since he is looking for his wife's killer, it's important that he logs each new lead. So he compensates for his condition by taking snaps with a Polaroid camera, annotating them, and tattooing important facts on his body.
Della Sala says the scriptwriters force Leonard to repeat a howler: "He keeps saying he has a short term memory problem, but that's wrong, he has anterograde amnesia." In other words, it's not that he loses memories, it's that he can't form them in the first place.
Finding Nemo, on the other hand, is top of Della Sala's amnesia class. In this Walt Disney/Pixar cartoon about fish, the amnesiac Dory is advised to swim through a trench rather than over it, to avoid danger. When she arrives at the trench she says she feels compelled to swim through it, but doesn't know why. "They did their homework," says Della Sala. "She remembers the emotion linked to some particular event, but not the event itself, which is exactly what people with amnesia do."

As a fictional device, this kind of amnesia offers huge scope for invention. It can also catch out unwary scriptwriters who haven't done their homework, says Sergio Della Sala, an expert in memory disorders at Edinburgh University.
One film he gives full marks for scientific accuracy is Amateur, in which Isabelle Huppert plays an ex-nun and writer of pornography who befriends Thomas, an amnesiac. When Thomas lights up a cigarette she expresses surprise, because although he can remember nothing about his past he can remember how to smoke. "That's accurate," says Della Sala. "People with amnesia do not forget procedures. They can still ride a bike."
Retrograde amnesia is very rare. But there is another kind of forgetfulness that crops up in real life as often as the Gregory Peck variety does in the movies: anterograde amnesia. This is a core symptom of Alzheimer's disease, the most common of dementias, and people who suffer from it have difficulty learning new things. They get stuck in a rut of what they already know.

To achieve the vestal virgin's happily spotless mind will take more than pharmacology. It may even need such old-fashioned techniques as talking through the problem. Some years ago I ran an experiment which showed that for mildly depressed people psychotherapy could achieve the same effects on the body's biochemistry as could taking a standard anti-depressant drug. The brain/mind is after all a profoundly social organ, and memories are more than mere molecules.
This is why in the film, human attraction triumphs over erased memories and the ex-lovers, meeting again, fall once more for each other, and start over - wiser only in their knowing that they may well fail again.
Hollywood obsession
Hollywood is obsessed with memory, or the loss of it. And although forgetfulness can take different forms, film-makers tend to fixate on just one: retrograde amnesia.
This is when you forget information you already had stored, such as who you are and why you're being scooped out of the Mediterranean by strange fishermen, like Jason Bourne in The Bourne Identity. Or what you're doing posing as a famous psychiatrist about to take charge of the mental asylum: Gregory Peck in Alfred Hitchcock's Spellbound.

The trouble is that any such treatment is likely to be pretty toxic and the side-effects more serious than the PTSD they are trying to cure. However, I'd hazard a guess that if the Lacuna Inc name - lacuna means gap or space - is not yet formally registered as a company, it won't be long before some enterprising neurobiotechnologists adopt it and make themselves a tidy living. There are several US companies in the memory drug business, mainly searching for agents that will enhance memory, but their publicists have already been quick to point out that erasure is the flip side of enhancement, and that if they can achieve the one, a quick toss of the pharmacological coin will come up with its opposite.
But I suspect that even leaving the ethical issues to one side, a non-toxic drug is unlikely to have the desired effect. The animal experiments are run in controlled conditions in which both the initial learning experience and its recall are precisely managed by the experimenter. Just how to reactivate the terrors of the multiple experiences of the survivors of a train crash, still less the richness, pain and pleasure of a love affair gone awry beggars imagination. And because the memories concerned are vastly more complex than those involved in a rat anticipating an electric shock, they are going to engage many more neural processes.

All that would be necessary, if the animal experiments are right, would be to re-evoke it - perhaps by revisiting the scene - and then inject some biochemical blocker and it would be gone for good. So enthusiastic have some advocates become that even the US President's Bioethics Council has been obliged to consider the possibility.
In Kaufman's film, Lacuna Inc technicians perform the erasure by placing an imaging helmet on the potential erasee's head, locating the site at which the memory is stored, and then zapping it with some type of focused energy source. There are indeed techniques of transcranial brain stimulation being developed, largely funded by the US military, in which focused pulsed magnetic fields are directed at specific brain regions and which temporarily disrupt mental processes. These might serve Lacuna Inc's purposes, but are unlikely to be effective if only because in practice there are unlikely to be specific "sites" for specific memories - rather they are stored, if at all, in the form of distributed networks of synapses. In the film, the attempt at erasure runs into difficulties because other memories get entangled with those they are aiming at, and this is likely to be the consequence of any such attempt based on disrupting the brain's electrical communication systems. This is why the PTSD snake oil salespeople are thinking chemically, of a drug that disrupts neurotransmission across a wide region of the brain.