The Deadliest Plane Crash

Room For Error by Peter Tyson

With airline flight increasingly automated, to what degree can human fallibility still creep in?

I had reason to ask this question one September day in 1998, as we all do at one time or another when traveling by air. I was on a flight to San Jose, Costa Rica, and as we approached its airport in the dark of early night, we entered a heavy rainstorm. My ears told me we had descended a long way, but out my rain-streaked window I could see nothing but thick gray clouds rushing past. Suddenly I could see lights on the ground below us, and they were alarmingly close.

Just then the pilot gunned the engine. As we climbed through the dense blanket of clouds, I thought, Isn't San Jose hemmed in by mountains? The whole cabin fell silent. Finally the pilot got on the intercom. He would try one more pass, he said, but if it didn't work we would fly to Managua, Nicaragua and wait till the visibility improved.

If it didn't work? That comment did not inspire confidence. With the clouds pressed almost to the ground, clearly no room for error existed. My thoughts focused on the pilot: how experienced was he? And I wondered how much human error can still factor into air travel today. Don't planes land themselves, clouds or no clouds? Aren't last-minute pilot decisions a thing of the past?

Well, not always, as I found out. Even as automation has made flying easier for pilots and safer for passengers—to the point that the chance that a flight taking off today in the U.S. or much of Europe will crash is less than one in two million—mistakes can and do still happen. "Human error is alive and well," one aviation safety expert told me. As another put it, automation has changed the nature but not the existence of error.

Great leaps forward

Commercial airline flight has come a long way since the Tenerife accident in 1977, the worst disaster in aviation history. In that tragedy, two 747s collided on a runway in the Canary Islands, killing 583 people.

While several advances grew directly out of that catastrophe (see Making Air Travel Safer), the greatest advances in the intervening decades have come on the technological front. The air traffic controllers at Tenerife could only guess where planes were on the runway that day because of thick fog. Today, most large airports have sophisticated radar equipment that pinpoints and identifies every plane in the air nearby and on the ground in real time.

And pilots today fly very different planes. Rather than a steering wheel and column connected to cables and pulleys that move wing flaps and tail elevators, jetliners today feature a "glass cockpit" with advanced computer displays. Pilots fly the plane electronically, or "fly-by-wire," rather than manually, and they have all manner of high-tech systems at their disposal, from autopilot to ground-proximity warning systems.

In theory, planes today can take off, navigate along a course, approach an airport, land, and roll to a stop all automatically.

Who's in charge?

While automation has improved many aspects of flight, some aviation safety experts worry that we may be putting too much trust in it. "Some people have a messianic view of software, meaning that it will save us from all our problems," says Michael Holloway, an aviation safety expert at NASA Langley Research Center. "And that's not rational, or at least it's not supported by existing evidence."

Holloway is one of a growing number of experts who suspect that pilots may be becoming overreliant on computers. Such overreliance can manifest itself in several ways. For one, it can cause complacency, particularly during the cruise portion of flight, when pilots have little or nothing to do. With nonstop, long-haul flights like Houston to Tokyo becoming more common, this risk is increasing, experts say.

Software glitches are becoming more common as airliners grow more automated.

Only six percent of errors occur during cruising, however, while 42 percent happen during descent and landing, says Robert Helmreich, who runs the University of Texas Human Factors Research Project. And here, in the most dangerous phase of flight, automation can paradoxically magnify the workload of a pilot, who often has to enter new commands from Air Traffic Control into the flight computer. "We've seen accidents where people were actually too busy trying to reprogram the computer when they should have been looking out the window or doing other things," Helmreich says.

Pilots and experts also fear that pilots are losing basic stick-and-rudder flying skills. This is not good news for tricky situations like that landing in Costa Rica. Or, worse, during bonafide emergencies, when one would hope not only that a pilot's skills are up to par but that he could override any automation and take complete control of the aircraft. As Helmreich told me, "Who do you want to make the ultimate decision, a very well-trained [pilot] or maybe a computer programmer who's sitting home having a beer?"

Humans remain far better than computers at rapidly assessing unexpected developments and improvising. In 1989, a DC-10 cruising at 37,000 feet had a catastrophic engine failure that incapacitated all hydraulics, making the aircraft almost uncontrollable. To steer the plane, the flight crew cleverly varied thrust on the remaining engines and were able to crash-land at a municipal airport in Sioux City, Iowa. While 112 people on board died in the crash, 184 survived, due in large measure to the flight crew's ingenuity—something a computer, however advanced, does not possess.


As jets become more automated, safety experts worry about another type of human error that has nothing to do with flight crews or air traffic controllers: software problems. With the latest jets bearing over five million lines of code, compared to less than a million in older planes, it is getting increasingly difficult for manufacturers to test for every eventuality.

And bugs do creep in. In August 2005, as reported in the Wall Street Journal (5/30/06), a Malaysian Airlines Boeing 777 flying from Australia to Malaysia suddenly ascended 3,000 feet, with no input from the flight crew. The pilot disengaged the autopilot and pointed the nose down to avoid a stall, but the plane went into a steep dive. When he throttled back on the engines to reduce the speed, the plane arched into another climb. The flight crew eventually got things under control and returned their 177 passengers safely to Australia. Investigators determined that a faulty computer program recently installed on all 777s had provided incorrect information about the plane's speed and acceleration, confusing flight computers.

The problem has since been corrected, but software glitches like that, while extremely rare and as yet not blamed for any major commercial jet crash, are becoming more common as airliners grow more automated. The Airbus A380 superjumbo jet, expected to start passenger service later this year, and Boeing's long-range 787 Dreamliner, slated for rollout in 2008, will forgo independent hardware and software systems for each task for redundant central computers that will run everything. The computers will have safeguards to ensure that different software programs don't contradict one another, as on that Malaysian Airlines flight.

Human error had no role to play that day, only human skill.

The risk, again, is that humans are increasingly left out of the loop. "We now have systems talking to each other, exchanging information," says Nadine Sarter, an expert on human error at the University of Michigan. "How do you ensure that the human knows about that, that he understands, 'Oh, that ground-based system and that airborne system, they've just come up with some decision about what they're going to do next'? Technology-to-technology communication and coordination will become more of an issue as the system evolves."

Power to the pilot

In the meantime, the airlines, safety experts, and pilots themselves are working hard to ensure that pilots remain confidently in charge. For one thing, pilots are trained more extensively than they have ever been in the past. "We're doing scenarios that require some out-of-the-box thinking," says retired pilot Bruce Tesmer, Manager of Cockpit Safety at Continental Airlines. "We try to teach our pilots the advantages of having a high attitude of vulnerability, letting yourself know that it can happen to you as well as to the next guy." With this in mind, pilots today often deliberately turn off parts of the automation to challenge themselves, just to keep those hard-earned skills in place.

I'd certainly like to have known that during that landing in Costa Rica. After we'd circled around, we approached the airport again. The pilot alternately revved and relaxed the engines, as if trying to ease down just beneath the cloud cover and catch sight of the runway. Lights appeared suddenly again; they were so nearby I could see their shapes.

Then, unbelievably, we were on the ground. Applause leapt up from the cabin. Human error had no role to play that day, only human skill—just what we want to rely on in such situations.

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Plane in clouds

Jet flight remains one of the safest ways to travel—and yet, as one aviation safety expert says, "Human error is alive and well."

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777 Flight deck

Today's electronic "glass cockpits" are a far cry from traditional cockpits with their arrays of mechanical gauges. Here, the flight deck of the Boeing 777.

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Airbus a380

Airbus's 555-seat A380 will take another step down the road of greater automation—a trend that has some experts concerned.

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Accident data chart

Human error—whether during pilot training or maintenance or the flight itself, among other steps involved in aviation—still plays a role in most aviation accidents, as these Boeing data show.

Peter Tyson is editor in chief of NOVA online.

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