Stanford professor shapes safety specs for next-generation aircraft
A new generation of aircraft is taking flight on wings made from carbon composites, a new class of materials less expensive to manufacture than aluminum with the added benefit of embedding safety sensors into a plane’s frame and skin.
As these new materials have started coming into commercial use, researchers have been working to understand how carbon composites withstand the rigors of flight and demonstrate their safety.
Such studies benefit from the fact that composites are woven together from carbon filaments, a process that allows manufacturers to build in sensors that creating a central nervous system to monitor the aircraft‘s structural health.
Fu-Kuo Chang, a professor of aeronautics and astronautics at Stanford Engineering, has played a leading role in bringing structural health monitoring (SHM) technology into practical use, a development designed to ensure safe and affordable flying.
Since 2006 Chang has led an international team consisting of engineers from leading aerospace companies, scientists from universities and research institutes, and experts from regulatory agencies such as the Federal Aviation Administration and the European Aviation Safety Agency. This working group recently produced the world’s first comprehensive guidelines for using SHM to design, build and maintain commercial aircraft worldwide.
“It has taken time, but we’ve worked together to engineer intelligent materials to improve aircraft safety and make it easier to perform maintenance,” said Chang, who holds a courtesy appointment in mechanical engineering.
Participants in this process say SHM will reduce the unnecessary downtime and last-minute delays that result from current maintenance practices that take planes out of service on fixed schedules to assess structural health. Instead, built-in sensors will monitor the performance of aircraft while they remain in operation, benefitting air travelers by keeping more planes safely in flight.
It took many years of combined effort to make the case for this change in maintenance procedure, according to Holger Speckman, manager of inspection processes and test methods for the European aerospace consortium Airbus.
“Without a widely accepted standard it would have been difficult for a single manufacturer to ensure the authorities that SHM is a safe solution to ensure structural integrity,” he said,
But in order to set technical standards, aerospace rivals had to share many technical details they might normally consider proprietary, explained Professor Peter Foote of Cranfield University in the United Kingdom, requiring diplomatic as well as technical leadership.
“Key industrial leaders approached Professor Chang to convene the standards-setting effort because he is internationally recognized and respected as a focal point on the subject,” said Foote, an expert on composites who now chairs the team and played a leading role in drafting the SHM standards.
“This effort succeeded because of Professor Chang,” said Aydin Akdeniz, a technical fellow at Boeing and another SHM advocate. “He’s personable and persistent, and he had a vision and belief in SHM that helped hold the process together.”
At his Stanford lab, Chang explained that a key SHM technology is based on the principle of piezoelectricity – piezo being the Greek word for pressing or squeezing. Simply put, squeezing a solid object generates a measurable electrical effect. The reverse is also true: jolting an object with electricity can cause measurable perturbations.
Piezoelectricity has found many uses since its discovery more than a hundred years ago. Sonar was invented during World War I, when scientists found that underwater sound waves bouncing off a submarine generated a pressure that could be picked up by a piezoelectric receiver. Likewise, piezoelectricity is built into the pickup that acoustic guitarists use to amplify the vibration of their strings.
In the aircraft setting, SHM uses piezoelectricity in passive and active ways.
Sensors built into carbon composites can detect strains in these structural elements in a passive way. Actuators can be used to create an active diagnostic test. Think of a doctor striking your knee with a rubber mallet to test your reflexes, and imagine “tapping” components with jolts of electricity to ascertain their structural health.
Chang says technology is only part of the story. SHM has the potential to change the mindset behind aircraft maintenance.
Today structural maintenance is scheduled. After a certain number of flight hours, aircraft are taken out of service for inspection. A flaw that developed just before or after such an inspection could pose a danger. Meanwhile, structurally sound aircraft are needlessly taken out of the flight rotation. SHM can change things. The built-in sensors and actuators can provide feedback from operating aircraft, spot trouble earlier should it appear while leaving sound aircraft in operation longer than is presently the case.
“SHM allows us to shift from scheduled maintenance to condition-based maintenance,” Chang said.
Other efficiencies are also possible. Say an aircraft experienced a rough landing or other unexpected shock that raised questions about the structural integrity of an internal part. Today it might be necessary to disassemble sections of the plane just to inspect that part. That would take time and cost money. Disassembly also introduces the possibility of making an error in reassembling the plane after performing such a safety check. With SHM, operators could do at least a preliminary diagnosis of the part without disassembly, saving time while assuring safety, Chang said.
SHM will continue to evolve as new sensing or actuating technologies come into being and users gain experience at interpreting the data gleaned by tracking aircraft in flight. The standards document Chang helped to create will provide the basis for future efforts according to Ricardo Pinheiro Rulli, who manages the structural health monitoring effort at Embraer, the Brazilian aerospace concern.
“Improvement will happen, but the fundamentals are there and have been accepted by the SHM community,” Rulli said. “Passengers can benefit with SHM by getting safer aircraft and, perhaps, cheaper tickets.”
Tom Abate is the associate director of communications at the School of Engineering.
