Skip to content

How Plane Crash Forensics Lead to Safer Aviation Read more: Part 2 – Air France Flight 447 – Plane Crash Forensics – Popular Mechanics

April 23, 2012

All the attention given to a crash like Air France 447’s can obscure an important truth: Commercial air travel is incredibly safe—and getting safer. In 2008, the U.S. fatality rate was fewer than one death per nearly 11 million passenger trips. This impressive record is the result of more than a century of incremental improvements that have been amassed through painstaking forensic analysis.

After each plane crash, investigators study the wreckage, analyze flight data and examine clues regarding flight conditions. Once they have determined a cause, they often help create recommendations that prevent the problem from recurring.

The FAA is determined to cut the already minuscule airliner fatality rate in half by 2025. With this in mind, the agency recently developed a new approach to make safety improvements. In 2007, it began working with airlines to sift through the masses of data that planes record about their normal flight operations, looking for safety improvements that could preempt accidents before they happen, instead of learning these lessons after a plane crash occurs.

The sophistication of aircraft makes this strategy possible. Modern planes are studded with environmental sensors that record flight conditions, while other sensors constantly assess the health of the airplane’s subsystems. This information is fed to a central computer, forming a network that resembles the neural system of a primitive organism. At the end of each flight, maintenance crews can easily download the data for analysis. Airlines have been using this information to improve their safety performance since the early ’90s, but two years ago the FAA began collecting these records as part of its Aviation Safety Information Analysis and Sharing (ASIAS) system.

This year, the FAA opened the Accident Investigation and Prevention Service to scrutinize the ASIAS data. “We’re having many fewer accidents, but the ones we do have are being caused by threats that are much harder to detect,” says Jay Pardee, the director of the new office. As an example of the kind of problem that ASIAS data could prevent, consider Comair Flight 5191, which was scheduled to take off from Lexington, Ky., in August 2006. Thinking they were on 7000-foot Runway 22, the pilots failed to get their aircraft airborne before they ran out of asphalt on the runway they were actually on—3500-foot Runway 26. The airplane’s wheels clipped an airport perimeter fence and the plane plowed into a grove of trees 1800 feet from the end of the runway. All 47 passengers and two of three crew members were killed. After the accident, the FAA reviewed 25 years of data and discovered that 80 commercial aircraft around the country had either taken off or tried to take off from incorrect runways. “Nobody connected the dots,” Pardee says.

Following the AF 447 disappearance, other Airbus 330 operators studied their internal flight records to seek patterns. Delta, analyzing the data of Northwest Airlines flights that occurred before the two companies merged, found a dozen incidents in which at least one of an A330’s airspeed indicators—­4-inch-long, pressure-sensing pitot tubes located on the fuselage under the cockpit—had briefly stopped working. Each time, the flights had been traveling through the Intertropical Convergence Zone, the same location where Air France 447 disappeared.

In the case of the Northwest A330s, the pitot tube malfunctions had been brief and harmless. But what if a severe version of the problem had struck Air France 447 amid more unforgiving circumstances?

At last, on June 6, the multinational search effort began to find evidence of the crash. The Brazilian military recovered bodies and debris floating approximately 40 miles north of the last automatic Aircraft Communications transmission. Over the next two weeks, search vessels retrieved 51 corpses from a stretch of ocean 150 miles wide, along with bits of wreckage—a section of the radome, a toilet compartment, part of a galley—that collectively added up to less than 5 percent of the aircraft. The largest single piece was the tail fin, marked with the distinctive blue and red stripes of the French national carrier.

The most important piece of the wreckage, however, remained missing. More than a month after the plane went down, despite the joint efforts of the French and U.S. navies, the black box still hadn’t been found. Given the huge search area, the ruggedness of the undersea terrain and the depth of the water (up to 15,000 feet), locating the recorder, let alone retrieving it, was proving to be an enormous task. Once the unit’s acoustic pinger passed its 30-day certified life span, the chances of recovering the black box became virtually nil.

Without the box’s data, the only physical evidence of the airplane available to investigators was the mangled wreckage. From the way it had been deformed—in particular, the way the floor of the crew’s rest compartment had buckled upward—French investigators determined that the fuselage hit the water more or less intact, belly first, at a high rate of vertical speed. Added to the ACARS messages and the satellite weather data, the evidence began to conform to a possible scenario.

By 10:45 pm, 10 minutes after the last radio transmission, the plane hit the first, small storm cell in the Intertropical Convergence Zone. Fifteen minutes later, it hit a larger, fast—growing system. And then, just before its last ACARS transmissions, the plane hit a whopper, a multicell storm whose roiling thermal energy rose more than 3 miles higher than AF 447’s altitude. Buffeted by turbulence, near the heart of a strong thunderstorm, the pitot tubes froze over. Lacking reliable speed indicators, the airplane’s computerized Flight Management System automatically disengaged the autopilot, forcing the co-pilots to fly the airplane manually.

Without autopilot, the pilots had no envelope protection restrictions, which are designed to keep the pilot from making control inputs that could overstress the aircraft. This is particularly dangerous for airliners at high altitudes. The thin air demands that airplanes fly faster to achieve lift, but they still must remain below speed limits. Flying too fast can create a phenomenon known as mach tuck, when supersonic shock waves along the wings shift the aircraft’s center of pressure aft and can make it pitch into an uncontrollable nose-dive. Flying too slow can cause a plane to stall.

AF 447’s flight crew, disoriented in the storm, uncertain about their speed and buffeted by turbulence, could easily have taken the A330 outside its flight envelope. “The fact that they didn’t transmit a mayday would seem to indicate that whatever happened to them happened quickly,” says William Waldock, a professor of safety science at Embry-Riddle Aeronautical University in Arizona.

Without more data, this kind of scenario can never be verified completely. But the global aviation community has already taken steps to prevent another accident like AF 447. Within days, Air France replaced pitot tubes on its Airbus planes with ones made by another company, and in July Airbus advised other airlines to do the same. Three months later the FAA turned the recommendation into a regulation.

To be sure, the pitot tubes are not the definitive cause of the crash. Even if they had failed, that alone should not have been enough to bring down an airliner. As in virtually every fatal air crash, what doomed AF 447 was not a single malfunction or error of judgment, but rather a sequence of missteps that crash investigators call the accident chain. “There’s always a series of events,” the FAA’s Pardee says. “That means there are multiple opportunities to intervene and break that accident chain.”

In the case of AF 447, the error chain included the co-pilots’ decision to fly too close to severe thunderstorms—bad weather that several other pilots, flying similar routes that night, had chosen to give a wide berth. There were certainly other links in the accident chain that pushed AF 447 beyond its limits. But unless the black box is found, we may never identify those links. And that means safety officials might never learn the full lessons of the disaster. To prevent a similar loss of forensic evidence, executives at Airbus say they are now studying alternatives to physical black boxes.

It is feasible to create a system that could broadcast not only text messages like ACARS but comprehensive data about the status of every aircraft, in real time. The aircraft would continuously transmit data to VHF stations within a radius of 125 miles, or by satellite if the plane is farther away.

Airliners in flight could one day stream all sorts of high-speed data, sharing information directly with one another. “It would be a network in the sky,” says Bob Smith, chief technology officer at Honeywell, which manufactured AF 447’s ACARS. “Aircraft could pass not only information about their location and where they’re headed,” he says, “but whole data sets. An airliner over Seattle could send its weather radar picture to a plane inbound from Dallas. And the guy from Dallas could pass it along to five other aircraft.” Military aircraft already use a similar system; it is not clear if civil aviation will adopt it.

The disquieting truth is that we don’t really know precisely what happened to Air France 447, and perhaps never will. The same links in the accident chain could someday take down another unlucky airliner. If they do, improved technology might provide investigators with the data they need to make sure that the next time is the last time.

No comments yet

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s