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How materials scientists investigate plane crashes

Accident investigators often study aircraft components’ materials to identify the root cause of failure and find ways to ensure safety

by Mitch Jacoby
November 27, 2022 | A version of this story appeared in Volume 100, Issue 42
An old black-and-white photo of a Wright brothers plane that crashed.

Credit: Associated Press | Moments after a Wright brothers airplane crashed in 1908, onlookers rushed to help the two injured aviators, one of whom soon died.


In brief

Modern materials and the engineered parts made from them have helped advance aircraft since the days of the Wright brothers’ hand-carved wooden propellers. As high-tech alloys capable of withstanding extreme conditions became available, engineers designed and built ever faster, lighter, and more maneuverable planes. But in some cases, these materials have failed, setting in motion a series of events that ended in disaster or a narrowly averted one. Accident investigators, including materials experts, analyze destroyed engines and other aircraft components, searching for microscopic clues in the materials that point to the cause of failure and ways to prevent it from recurring.

On Sept. 17, 1908, Thomas E. Selfridge became the first person to die in an airplane crash. Selfridge, an early military aviator and a first lieutenant in the US Army, was a passenger and trainee that day, flying over Fort Myer, Virginia, in a plane piloted by Orville Wright. Just 5 years earlier, the Wright brothers, Orville and Wilbur, succeeded in making the world’s first powered and controlled flights. Now they were demonstrating the capabilities of their flying machines to the military as throngs of onlookers watched.

As the plane maneuvered through its fourth loop over an open field just after 5:00 p.m. that day, a piece of one of its wooden propellers snapped off and slammed into the rear rudder, which is the part that steers the plane. The impact damaged the steering system, causing the plane to plunge nearly 25 m to the ground, killing Selfridge and seriously injuring Wright, according to eyewitness accounts reported in newspapers and collected in aviation records.

One of those accounts came from Octave Chanute, a technical adviser to the Wright brothers, who examined the broken propeller blade and reported to the Army Signal Corps that “the wood was brittle and over seasoned, or kiln dried.” Chanute’s finding meant that a flaw in the propeller’s material, possibly caused by the processing method, triggered the crash. The flaw left the wooden blade incapable of withstanding the forces acting on it as it rotated rapidly and caused it to fracture midflight.

For more than a century, scientists and engineers have pushed the limits of aviation, producing airplanes that fly faster and higher, remain airborne for longer, and carry more passengers and heavier loads—all while being lighter, more fuel efficient, and safer than earlier models. Airplane components made of high-tech materials enabled many of those improvements. For example, aerospace engineers have selected strong, lightweight alloys that stand up to extreme temperatures and pressures while providing peak flying performance. Yet, like the first fatal airplane accident, many disasters resulted from a chain of events that started with an in-flight failure of those materials.

A replica of a Wright brothers airplane.
Credit: Mitch Jacoby/C&EN
This highly accurate replica of an early Wright brothers airplane sits on display at the National Museum of the United States Air Force in Ohio.

To reconstruct the cause-and-effect sequence following a crash or other flying mishap, teams of experts comb through debris at the accident site, collect broken pieces of the aircraft, and analyze them. Sometimes they uncover minuscule details that indicate that the root cause was a materials phenomenon—a physical or chemical process that generated a microscopic weak spot in an alloy, for example. Armed with that information, these research teams work with aerospace companies to come up with ways to improve the material; modify manufacturing, maintenance, and inspection methods; and prevent future mishaps.

A piece of a broken propeller from 1908.
Credit: Mitch Jacoby/C&EN
This is an original fragment of the Wright brothers propeller that broke midflight in September 1908.

One such team of materials forensic experts is based at the Air Force Research Laboratory (AFRL) at Wright-Patterson Air Force Base in Ohio. Like crime scene investigators for aviation mishaps, this group is called on to solve crash cases and other dangerous problems mainly involving military aircraft. Here are a few stories of accidents that researchers such as those at the AFRL have investigated. Each illustrates the role played by basic materials science in aviation and the lessons learned from accident investigations.

C-130 military transport plane

Since the 1950s, the Lockheed C-130 Hercules has been flown by many militaries, mainly to transport troops and equipment. The plane has also been used in a variety of civilian operations. The C-130 is powered by four engines, two on each wing. The inner engines are closer to the fuselage—the airplane body’s central portion—and the outer engines are farther.

On July 10, 2017, one of these aircraft took off from a military depot at Cherry Point, North Carolina, headed to California. The plane never made it. It crashed in Leflore County, Mississippi, killing all 16 people on board. A detailed accident investigation, which included analysis of the way wreckage was distributed near the crash site, revealed the sequence of events that brought down the transport plane.

Roughly 2 h after takeoff, as the plane was cruising at an altitude of about 6,000 m (about 20,000 ft), one of the four blades on one of the inner engines flew off its propeller assembly at high speed and slammed into the fuselage. The impact transferred forces to the other side of the plane, ejecting the propeller from the other inner engine, which then rammed that side of the fuselage. The combined forces sliced through the forward section of the plane, which included the flight deck, completely severing that portion from the airplane. The disintegrating plane plunged and crashed.

A C-130 military transport plane.
Credit: Shutterstock
The C-130 is a four-engine transport plane that has been used since the 1950s, mainly for military operations.

The ejected propeller blade triggered the catastrophic sequence of events. But why did the blade fly off the propeller assembly? Together with researchers from industry and other military labs, Bob Ware, a specialist in metallurgical failure analysis at the AFRL, examined the C-130’s failed blade and the 15 other blades that remained intact. The team determined that the root cause of the blade’s failure was a fundamental electrochemical process known as galvanic corrosion, a form of damage that can occur between electrochemically dissimilar metals that are in direct contact. In that process, the more electrochemically active metal functions as an anode, and the less active metal as a cathode. Differences in the metals’ electrochemical properties drive a reaction that dissolves, or slowly corrodes, the anode.

According to Ware, the trouble began where the section of the blade known as the spar, which is made from an aluminum-​based alloy, couples to a sleevelike piece called a bushing, composed mainly of copper. “The more noble copper doesn’t corrode,” Ware says. “It’s the structurally important aluminum piece that’s the victim in this galvanic pair.”

Early in the galvanic corrosion process, microscopic pits form at the interface. As the damage progresses, the metal in that region forms tiny cracks and weakens. In the case of the C-130 crash, as the propeller rotated at high speed, the strong bending forces that act on the blades during normal operation caused the fatigued part to crack and fail.

Materials systems aren’t perfect. Now we’re figuring out ways to make them last longer while maintaining safety.
Eric Lindgren, expert in nondestructive analytical methods, Air Force Research Laboratory

Having identified the cause of the problem, the investigation team came up with detailed procedures to ensure the reliability of the thousands of C-130 blades currently in service. One step calls for field inspectors to scrutinize the potential trouble spot—where the spar meets its bushing­—with an eddy-current probe, a handheld electromagnetic sensing device that can detect variations in electrical signals caused by pitting and other microscopic flaws that form before cracks appear. The team also developed a process that combines ultrasonic examination techniques and the use of fluorescent dyes that can reveal the presence of otherwise-invisible cracks and other flaws. In addition, the procedure calls for treating the interface between the two metals with a chromium-​containing epoxy and other anticorrosion coatings. Manufacturers currently use other materials that don’t undergo galvanic corrosion to make new propeller blades.

“In this investigation, we learned more than we expected to about the nature of galvanic corrosion between these two components [the bushing and spar], which have been in service together at least since the 1950s,” Ware says.

So if these parts were manufactured roughly the same way and from the same materials since the 1950s, why didn’t more of them fail? First, it’s rare for all variables and conditions to be just right to initiate the electrochemical corrosion process and for it to progress far enough to form significant fatigue cracks. Another critical factor is the length of a plane’s service life, says Eric Lindgren, an AFRL expert in nondestructive analytical methods.

These designs date from World War II, when planes were in active service for less than a year on average, Lindgren says. “In the last few decades, we’ve extended the [service] life. So now they may be in the inventory for 40 years or longer,” he says. “Materials systems aren’t perfect,” Lindgren acknowledges. But these mechanical parts weren’t designed initially to last that long. “Now we’re figuring out ways to make them last longer while maintaining safety.”

F-16 Fighting Falcon

The F-16 is a highly maneuverable fighter jet designed for air-to-air combat and air-to-surface attack missions. The plane, which is powered by a single turbojet engine, is used by the air forces of the US and several other countries.

In the early 2000s, the pilot of an F-16 ejected safely as the plane crashed near Luke Air Force Base in Arizona. (Sources for this story asked that the incident date and other details be omitted.) An accident investigation team, which included researchers from the AFRL, determined that the jet crashed because the engine stopped working. It also identified the material failure that caused the incident.

A turbojet is a type of engine that converts chemical energy stored in a fuel to mechanical thrust to propel a plane (or anything else). The engine performs four main functions, one in each section of the engine, in assembly-line fashion. It draws air through an inlet, strongly compresses the air with a fan-type device, mixes the compressed air with fuel to drive combustion, and expels the hot, fast-moving combustion exhaust past another fanlike device—the turbine—and out through a nozzle. The backward direction of the high-speed exhaust gas propels the plane forward.

Cutaway view of a jet engine.
Credit: Shutterstock
Many modern jet engines contain a fan and air inlet (far left) as well as compressor, combustor, and turbine sections, each consisting of stages with a central disk and numerous blades.

The fanlike sections consist of subsections known as stages. Each stage is similar to a ceiling fan, with a central disk and a set of radial blades. In the F-16 that crashed, the fourth stage of the turbine had 60 blades. By collecting and examining these blades from the debris, the team found that one of them had a weakened region that had started to crack because of fatigue. The cracking propagated to the point that the blade broke and was ejected from the rotating disk. The projectile blade likely caused a cascade that dislodged the other blades on that stage, causing the engine to seize and shut down. The pilot ejected after trying unsuccessfully to restart the engine.

Turbine blades must withstand exhaust-​gas temperatures often in excess of 1,000 °C while spinning at tens of thousands of revolutions per minute. The material of choice for turbines such as those on F-16s is a nickel superalloy. This family of materials, composed mainly of nickel, iron, and cobalt, is strong and good at resisting creep, a permanent type of deformation that occurs to materials continuously subjected to major mechanical stress and high temperatures. Manufacturers commonly add aluminum and chromium to makes these alloys corrosion resistant.

The performance of turbine blades made from these nickel superalloys depends strongly on the alloy’s lattice structure, not just its composition. Initially, manufacturers made the blades from polycrystalline versions of the alloy, meaning the microscopic crystalline regions, or grains, of the superalloy were randomly oriented, and the material had many grain boundaries. Because of the alloy’s strength, the blades were somewhat resistant to creep. But they weren’t sufficiently resistant because grain boundaries can function like microscopic perforations, making it easy to deform the material at those spots.

An F-16 Falcon fighter jet.
Credit: Shutterstock
The F-16 Fighting Falcon is a highly maneuverable, single-engine fighter jet.

So manufacturers devised ways of processing the alloys—using proprietary heating and cooling treatments, for example—that aligned the grains along select directions, reducing the number of detrimental grain boundaries. Blades made from these directionally solidified superalloys are more resistant to creep than polycrystalline ones. But with continued use, they too can deform and weaken, similar to the way a brick wall can collapse because of weak mortar joints even if the individual bricks are tough enough to resist crumbling. The investigation team found that it was a blade made from this type of alloy that was the root cause of the F-16 crash in Arizona, Ware says.

The team researching this accident developed inspection techniques that highlight grain boundaries in these alloys, allowing maintenance crews and manufacturers to identify which blades should be removed from service or fail quality control. These procedures have helped mitigate future disasters. Now the aerospace industry makes single-crystal (grain-boundary-free) nickel superalloy turbine blades for the most demanding applications to further avoid the failure that caused the F-16 crash.

Airbus A380 passenger plane

On Sept. 30, 2017, 497 passengers and 24 crew members boarded Air France flight 066 at Paris–Charles de Gaulle Airport headed to Los Angeles International Airport. As the Airbus A380 approached the coast of Greenland at cruising altitude (roughly 33,000 ft, or 10,000 m), one of its four engines began disintegrating. Many parts of the engine’s outer section, including the fan hub (the spinning central disk easily seen from an airport terminal window) and air inlet, separated from the engine and fell to the ground. The plane made an emergency landing at Goose Bay Airport in Canada. There were no fatalities or injuries.


The Bureau of Enquiry and Analysis for Civil Aviation Safety (BEA), the French aviation accident investigation bureau, led the investigation. The team formulated an initial explanation of the mishap after examining the damaged engine along with some debris recovered in Greenland. But almost 2 years passed before the missing fan hub was recovered. It was buried deep under ice and snow in a remote part of Greenland. Eventually the BEA issued a report explaining that the engine failed because of a crack in the fan hub. The titanium alloy disk formed the crack through a process known as dwell fatigue.

“We really thought we understood how these titanium alloys behave because we have been putting them in engines for 70 years,” says the AFRL’s Sean Donegan, a specialist in alloys and analytical methods. He notes that years ago, titanium-based disks failed on rare occasions for a different reason—as a result of an impurity, such as a microscopic tungsten particle, that formed in the disk’s titanium matrix. The impurity, which weakened the alloy and initiated its failure, likely came from tungsten electrodes that were used to melt the titanium during manufacturing.

An Airbus plane that suffered major damage to an engine.
Credit: Bureau of Enquiry and Analysis for Civil Aviation Safety
On a flight from Paris to Los Angeles in 2017, one of the engines on this Airbus A380 began breaking up midair.

Donegan says the aerospace industry figured out how to fix that problem—for example, by modifying the way the electrodes were used. And airplane operators were confident that their inspection and maintenance methods would safely catch this problem at the earliest stage.

“Then all of a sudden, around 10 years ago we started seeing failures in titanium disks before our first scheduled inspection,” when planes were new and thought to be free of potentially dangerous wear and tear, Donegan says. The dwell-fatigue problem that triggered the Airbus A380 damage has also been the cause of concern and investigation in some military planes.

When it comes to accident investigations, it takes a large team of people with a wide variety of skill sets to figure out what started the chain of events, how to detect it, how to correct it, and how to prevent it from happening again.
Andy McMinn, private consultant in air safety

Dwell fatigue likely results from a specific arrangement of microscopic grains or crystallites in the titanium alloy referred to as a microtexture region. According to Donegan, this type of feature forms when a large group of adjacent grains aligns in roughly the same orientation and that grouping is abutted by a small grain aligned differently. When that happens, it is possible that the grains are in just the right relative orientations to weaken that spot, leaving it susceptible to form a fatigue crack that can quickly propagate and lead to failure before it’s detected. Analysis of the Airbus A380’s fan hub revealed microtexture regions. Donegan and others have recently developed microscopy methods and other techniques to spot these microtexture regions early to avoid these types of failures.

Materials and analysis advances like these have helped make aircraft safer. In the 1960s, up to 70% of airplane crashes resulted from components that failed in flight as a result of a materials problem. Now, that number has fallen precipitously—to 3–5%, according to Andy McMinn, a specialist in forensic metallurgy and failure analysis. McMinn has more than 30 years of experience working for the US military and the Federal Aviation Administration. Now a private consultant in air safety, McMinn attributes the improvement in failure rate to the problem-solving collaborations among large numbers of researchers in military, industrial, and government labs.

“When it comes to accident investigations, it takes a large team of people with a wide variety of skill sets to figure out what started the chain of events, how to detect it, how to correct it, and how to prevent it from happening again,” he says.

If the past 100 years are any indication of the future, the aerospace industry will continue designing ever-more-capable flying machines. To actually build them and get them airborne, scientists and engineers will need to continue coming up with even more sophisticated high-tech materials that perform reliably and safely.


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