Army planners say they don’t want to have to wait for a routine maintenance cycle to spot potential problems in an aircraft. They also don’t want to spend time and money on scheduled maintenance that isn’t needed.

Researchers at the Army Research Laboratory (ARL) say they have successfully tested a possible fix ― a means of identifying damage as it happens.

“We are going to change the entire paradigm,” said Mulugeta Haile, an ARL research aerospace engineer. “We are going to have maintenance-free aircraft, an aircraft that can fly for over 720 hours with no maintenance and no down time. It’s highly ambitious but that is where we are headed.”

In 2016 the research team spent six months testing out a new sensor array on a $20 million conceptual composite UH-60 Black Hawk rotorcraft built by Sikorsky. They published their findings in a recent edition of the Journal of Structural Health Monitoring.

The ARL team suggest using acoustic sensors to effectively “hear” damage to the airframe as it occurs.

“These are passive sensors distributed over the entire aircraft, like a microscopic microphone. All they do it listen to what’s moving and what’s cracking,” Haile said. “If a part is breaking, if there is a microscopic crack in a solid, it emits a tiny amount of energy that disturbs the surface of the structure. If you measure that very small surface displacement, that signal will give you information about the location and the severity of the damage.”

The math here is staggering. Researchers want to hear tiny sounds amidst the rush and roar of a helicopter in flight, and they want to assign meaning to those signs: A crack like this is a rotor coming loose, a ping like that is a tweak in the fuselage.

“You are looking for a small sound in a noisy, vibrating environment, and then you need to use that to determine the damage in a big complex structure. It’s taken years to develop a method for this,” Haile said.

That method relies primarily on advanced signal processing techniques. Essentially the art lies in subtracting out known noises in order to hear anything unusual. “We have experience in what the frequency range of the engine vibration looks like,” Haile said. “Then we can tune the sensors so they won’t be sensitive to that information.”

Logistics burden

Researchers say that by identifying problems as they occur, rather than putting aircraft through cycles of routine maintenance, they can reduce the logistical burden around aviation support.

The Army spends close to 60 percent of funds for a given air platform on maintenance and sustainment. The Black Hawk undergoes maintenance every 10 flying hours, then at 40, 360 and 720 hours. “Most of this maintenance is performed not because of any incident or any damage, but just to fulfill the requirement,” Haile said. “It’s not sustainable to do this.”

Other across the armed services are likewise looking for ways to improve efficiencies around aircraft support.

Last fall the Army Aviation Branch reported it was revamping its training program after some 1,500 soldiers scored “dismal” results on a test of their basic knowledge around general aviation.

The Marine Corps in spring 2017 began setting up Wi-Fi connectivity in its hangers. By enabling ready network access, the service said, it could trim the time needed to order and receive parts, making aircraft maintenance more timely and efficient.

Others, meanwhile, are looking at predictive analytics, the ability to anticipate systems failures, as a means to streamline aviation support. This was a hot topic at a Defense Logistics Agency aviation leadership conference last year. “We need to track and act as the opportunities arise in order to move the readiness needle. We need to become more predictive and get in front of readiness issues before they impact the flight line,” said Navy Capt. Timothy Pfannenstein, Logistics and Industrial Operations executive director, Naval Air Systems Command in Patuxent River, Maryland.

Haile said his team’s acoustical sensors could help the military to achieve those aims. The technology will be in development for some years to come, but could be deployed on the next generation of vertical lift aircraft, Haile said.

In the longer term, he says, the sensors could do more than just diagnose problems in the airframe. They could help the aircraft heal itself.

The team is simultaneously developing a “damage adaptive controller,” a technology that adjusts how an aircraft runs based on damage-sensor information.

“Suppose you detect a crack in the bulkhead. The sensor would precisely detect where the damage is and how it is growing,” he said. “During flight there are things like speed and the angle of the rotor blade or the RPMs of the rotor blade. We want to be able to adjust these parameters in such a way that the crack does not grow any bigger.”

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