Mechanical v human: Why do planes crash?
Mystery still surrounds the disappearance of Malaysia Airlines flight MH 370 but the speculation going on reveals something about lay people's assumptions of air crashes.
What is likely to be the main cause of a passenger plane crashing?
Mechanical failure? Or human error?
There are many people whose first assumption - after terrorism or hijacking is discounted - when a plane is lost is that some physical part has failed catastrophically. But mechanical failures alone account for only a small proportion of airliner crashes.
For fatal accidents, one calculation puts the primary cause as "pilot error" in 50% of all cases.
One of the most common scenarios for a plane crash (more than a fifth of all fatal accidents between 2006-11, according to the International Civil Aviation Organization) is known as "controlled flight into terrain" (CFIT), referring to aircraft that were piloted into the ground, water, mountains or other terrain.
The cause could be bad weather leading to poor visibility, a navigation mistake or a more fundamental pilot error. CFIT was coined by aircraft manufacturer Boeing, which argued that pilot error was the most common cause.
The most recent confirmed example was the Sukhoi Superjet 100 that crashed into Mount Salak in Indonesia in 2012. The aircraft, which was on a demonstration flight for potential buyers and journalists, had a terrain awareness and warning system (TAWS) but this had been switched off by the pilots who assumed the computer was malfunctioning. Unaware that they were near the mountain, the crew were also distracted by conversations with potential buyers in the cockpit. All 45 people on board died.
Another egregious example was Eastern Airlines flight 401 which smashed into the Florida Everglades in 1972. The crash happened after all three members of the crew became obsessed with finding why a single indicator light had not come on when the landing gear was lowered. While they investigated, and not noticing that the autopilot had been accidentally switched to a setting that allowed a steady descent, they did not realise they were heading for disaster until it was too late.
Another incident in 1978, the crashing of United Airlines flight 173, led to a fundamental change in the way pilots were trained. The captain allowed it to run out of fuel while circling Portland, Oregon. It crashed with the loss of 10 lives. As the fuel was running down, the crew was investigating whether the landing gear had deployed properly after a noticeable jolt and the plane turning to the right.
The aftermath led to a heightened recognition of the need for crews to properly communicate and interact as a team, known as Crew Resource Management. If a subordinate notices that fuel is running dangerously low, but for whatever reason does not communicate that to the captain, there is a problem. If the captain ignores a warning from a subordinate about drifting off course, that is a problem.
Passengers might think of pilot error as a single person's mistakes, but in a team of two or three (and occasionally even four) flyers, errors are compounded within the structure of a team.
These team problems can be exacerbated by cultural factors. This was blamed for Korean Air's appalling safety record during the 1980s and 1990s. An excessively hierarchical culture left captains ignoring their subordinates, and the subordinates sometimes afraid to speak up when they had noticed something was wrong. The result was a string of incidents. When Korean Air tackled the cultural issues, improvement was dramatic.
There have been CFIT accidents caused by human errors back on the ground. Aeroperu flight 603 crashed into the sea off Peru in 1996 after the instruments stopped working and the computer bombarded the crew with a series of baffling emergency messages. A maintenance worker had forgotten to remove tape from "static ports" needed to feed data to the instruments. At night over the sea, it was impossible for the crew to know their altitude and a wing hit the water. All 70 people on board died.
There are other crashes that investigators have laid firmly at the door of mechanical or material failure. The deadliest single-plane crash was in 1985 when a Boeing 747, Japan Airlines flight 123, crashed into a mountain, killing 520 people. It was caused by mechanical problems - a badly repaired rear pressure bulkhead ruptured leading to total loss of control.
But while there are crashes that are manifestly pilot error or manifestly mechanical failure, most experts emphasise that there's usually a range of interconnected and complicated factors that contribute to any crash.
The deadliest ever incident happened when two Boeing 747s collided on the runway in Tenerife. The 1977 accident in which 583 people perished occurred in thick fog with limited communication between the control tower and the two planes. It shows the chain of events that leads to accidents, says Graham Braithwaite, professor of safety and accident investigation at Cranfield University.
If there hadn't been a bomb attack at nearby Gran Canaria, planes would not have been diverted there, crowding the runway. The airport was built in a place that was prone to sea fog, he notes. The communication system from the control tower to the planes was poor. Sometimes the best people can make the worst mistakes, Braithwaite suggests.
The air accident most commonly linked to the current Malaysia Airlines situation happened in June 2009. Air France flight 447, an Airbus A330-200 from Rio de Janeiro to Paris, disappeared over the Atlantic Ocean. It took days for the wreckage to be found and three years for the mystery to be solved.
The plane had stalled after pilot error. A transcript of the pilots' conversation shows just how confused they had become.
But it's possible to see it another way. There was a violent storm which iced over the pitot tubes, or speed sensors, in the build-up to the accident. The autopilot switched itself off, but the pilot - the most junior member of the flight crew - reacted inappropriately to the turbulence the plane was going through.
Although pilot error was clearly the main factor, bad weather and momentary instrument failure contributed to the panic. And what resulted was a failure in crew resource management, the plane stalling and the death of all 228 on board. Critics have pointed to design, and particularly to the feedback from the control sticks as a major factor.
The relationship between the pilot and the plane has changed. Computer involvement has steadily increased, automated systems have come to the fore, and the array of sensors has multiplied.
It is perhaps understandable that when part of that system is not functioning normally, pilots can be disproportionately unsettled. "Catastrophic failures don't happen as often but they are more catastrophic when they do. If something went wrong in the 1970s there was a chance you could land it," says Joe Pappalardo, senior editor at Popular Mechanics magazine.
The pilots in the Air France accident did not do what they should have in the circumstances, particularly with regard to communication between them. But one also has to recognise the bafflement at the situation they found themselves in.
Pilots are extremely well-trained and immensely experienced people by the time they find themselves controlling an airliner, but they are not infallible.
It's all too easy to blame pilot error when things go wrong, says aviation writer and light aircraft pilot Sylvia Wrigley, author of Why Planes Crash. "It's a really, really easy scapegoat. The pilot is either dead or fired. The real problem is a dozen different things have gone wrong."
But Andrew Brookes, a former RAF pilot who has written about flight safety, says the technology is so good today that pilots are not really necessary. "The technology exists now for an airliner to fly without a pilot from London to Beijing. Today planes hardly ever fail - I can't think of a [recent] accident caused by engines failing or wings dropping off."
There's a reason people continue to hold pilots in such an exalted position. "It goes back to daring young men in their flying machines," says Brookes. Pilots look and sound authoritative.
Some experts disregard the term "pilot error" completely, regarding it to be nebulous to the point of uselessness.
But the most important thing is that flying is getting safer. In 1985 - the worst year for aviation deaths - there were more than 1,900 fatalities involving airliners. In 2012 there were nine fatal accidents involving airliners causing 372 deaths, according to the International Civil Aviation Organization.
But every single accident that is successfully investigated provides data that can be used to improve planes, notes Pappalardo. "Aviation is still on a learning curve. It is important to find out what happened - there are always so many variables."
The result is improvement for both planes and pilots.