Vanderbilt Engineering to lead FAA helicopter reliability project

All it took to rip the roof off Aloha Airlines Flight 243 in 1988 was the gradual corrosion around rivet holes that had, over time, created tiny cracks in the Boeing 737’s fuselage that suddenly combined with fatal results.

That incident, which caused one death, 65 injuries and a traumatic open-air ride in an airplane convertible for the rest of the passengers, sparked two decades of national programs designed to make aging airplanes more reliable.

Now the Vanderbilt School of Engineering is leading a new Federal Aviation Administration program to apply and expand aging aircraft reliability techniques to helicopters. Although the project is focused on helicopters, researchers believe much of what is learned could be applied to other types of aircraft.* *The five-year, $1.5 million project will be kicked off in a project team meeting to be held Nov. 27-29 in Atlantic City, N.J.

Helicopter safety has become an increasing concern in recent years because the number of emergency medical service helicopter accidents in the U.S. nearly doubled from the mid-1990s to 2004. Although most of these accidents were caused by challenging weather, difficult terrain conditions and pilot error, the FAA wants to ensure that the equipment gives pilots every advantage.

“The margin for error in flying a helicopter, especially in rescue missions, is very slim,” says project principal investigator Sankaran Mahadevan. “We want to make sure that helicopter pilots don’t have to deal with equipment failure, such as metal fatigue, on top of the challenges of shifting winds, unseen obstacles like power lines, birds flying into the blades, and space limitations of maneuvering in tight spots.”

As the aviation industry moves towards using new, lighter materials in their designs, more understanding is needed about the characteristics and performance of these materials under various operating conditions, said Mahadevan, professor of civil and environmental engineering.

“Lighter materials can translate into fuel economies,” Mahadevan said, “But the industry needs more data on how these new materials will perform over time. We are going to help develop that knowledge base to guide rotorcraft design as well as maintenance schedules.”

Mahadevan and his team at Vanderbilt will work with subcontractor Bell Helicopter Textron, Inc. of Fort Worth, Texas, to test the mettle of the materials used in helicopter components.

“In addition to needing information about the materials, we need better understanding of how the entire structure of a helicopter functions under a variety of performance requirements,” Mahadevan said. “Helicopters have complex structural geometry and are subject to a variety of loading conditions, even without taking into considerations the unknowns involved with the new materials.”

The team’s first step will be to do controlled laboratory tests on new materials to get an idea about how and where cracks and other flaws will materialize under various conditions.

This data will be leveraged to predict the materials’ behavior throughout the life of the aircraft, using various computer models, including computer simulation and probability software. These and other computational tools will be used to determine how a helicopter would most likely to be affected by failures within various components.

“Finally, this information will guide us in recommending inspection and maintenance schedules,” Mahadevan said. “The FAA wants to create the optimal schedule of inspection and maintenance to ensure against catastrophic failure without wasting resources by redundant inspections.”

Mahadevan said that Bell Helicopter Textron will be an integral part of the project. The company, which produces a wide range of commercial and military helicopters, will contribute data on helicopter components and materials, as well as on how the defects grow in size. The company will also advise Vanderbilt researchers on useful and practical demonstration problems with which to test the research.

“This research differs from most studies on helicopter damage tolerance in that it will incorporate uncertainties in geometry, material behavior, mission operations and initial flaw distribution within the rotorcraft components,” Mahadevan said. “As in the Aloha Airlines accident, sometimes more than one flaw can interact, causing catastrophic results.”

The computing methods to be used in the research have been used in a variety of other risk and reliability applications for automotive, spacecraft and aircraft components, but this is the first time they have been applied to helicopter components and materials. In addition to providing a risk and reliability foundation for helicopters, the research project will refine and expand the computing methods, Mahadevan said.

“It has been shown that different models will yield very different predictions,” he said. “One possible way to reduce modeling error responsible for these variations is to develop and validate a more accurate and efficient model.

“In the past few years, our group has been successful in developing such models for complex fatigue and fracture, which have shown excellent performance for a wide variety of materials. The FAA project will build on this success and will have strong impact on design methods and risk management.”


Federal Aviation Administration Sankaran Mahadevan