Safety of flight, cost savings hallmark of nondestructive inspections

Published March 9, 2012
By Tech. Sgt Brannen Parrish
931st ARG Public Affairs

MCCONNELL AIR FORCE BASE, Kan. -- Whether a KC-135 Stratotanker is defying gravity to achieve flight or static on a flightline, virtually every part of the aircraft undergoes stress that can cause surface and subsurface cracks. These cracks present a serious danger to aircraft, equipment, personnel and mission readiness.

At McConnell AFB, the responsibility for identifying and determining the severity of a crack falls on the shoulders of an integrated Total Force section known as Nondestructive Inspections. The section is manned by five active duty Airmen from the 22nd Maintenance Squadron along with one Air Reserve Technician and three traditional Reservists from the 931st Maintenance Squadron.

"We provide a level of crack detection that can't be replicated anywhere else in the maintenance complex. We are able to see things other people can't see because of the technology we have," said Tech. Sgt. Mark Teusch, NCOIC, Nondestructive Inspections, 22nd Maintenance Squadron.

A fully loaded KC-135, weighing more than 320,000 pounds, requires more than 85,000 pounds of thrust to become airborne. With a ceiling of 50,000 feet, the aircraft is exposed to outside air temperatures of -40 to -50 degrees Celsius. The process of cooling and warming, as well as speed, bird strikes and other factors can result in fractures and cracks.

When aircraft maintainers identify a fracture in an aircraft or an aircraft part, NDI specialists are called in to determine the size and depth of the flaw.

"We have things like the fluorescent penetrant that looks for surface cracks. Sure, you may be able to find that same crack with a magnifying glass and a flashlight but we would be able to verify exactly how long it is because as the crack propagates it gets smaller and smaller," said Teusch.

Fluorescent penetrant is a dye that can seep into cracks and crevices. When applied to a piece of equipment and placed under a black light, it reflects green light waves. Under a blacklight the penetrant-filled crack will glow like a vein of kryptonite.

Once NDI has determined whether a crack is present, maintainers can make a determination about how the part should be replaced or repaired.

Some cracks are so small that the naked eye cannot detect them. Depending upon the thickness and type of metal involved, NDI technicians may use an eddy current probe or an ultrasonic transponder to locate flaws in a part. When attempting to identify surface or near-surface defects on conductive materials, a technician will usually use an eddy current probe. The device emits a harmless magnetic current. As long as the material maintains uniformity the current will remain stable. If a fracture is present or if a defect exists within the metal, the device will register a change in the magnetic field.

For non-conductive metals, or to locate subsurface cracks in thicker pieces of metal, the inspector will usually use an ultrasonic transducer.

The transducer emits a high-frequency sound wave through the material and analyzes the signal in much the same way a sonogram shows expecting mothers the size and gender of their child. The device even requires the use of ultrasound gel.

Prepping and testing an area requires painstaking attention to detail.

"Looking for cracks on an aircraft can be a tedious process," said Staff Sgt. Bryan Tyler, NDI specialist, 22nd Maintenance Squadron. "You have to be patient and methodical but it's gratifying when you find a flaw because you may have saved lives."

According to Staff Sgt. Amanda Mitchell, NDI specialist, 931st Maintenance Squadron, the NDI section takes pride in preventing small problems from endangering lives. "We don't ever want there to be any cracks but we always get excited when we find something because we know we may have prevented a major catastrophe."

In addition to checking aircraft, NDI can identify cracks in parts and tools used by maintainers. They also certify the welds metals technologists make, to ensure they will withstand the rigors of flight.

In the maintenance world, it's all about ensuring aircrews receive a dependable, well inspected aircraft, that's free from defects.

"With the Maintenance Squadron there is a trust factor involved," said Lt. Col. Barry Jones, a pilot and the Commander of the 931st Operations Support Squadron. "When I check out an aircraft, I trust them to give me an aircraft that's safe and well-maintained and they trust me to return it to them in the condition I received it. We really put our lives in their hands."

The NDI section is in the business of ensuring safety of flight. But they also provide cost savings to the Air Force.

In one year the section will inspect nearly 5,000 aircraft bolts. A maintainer can't just drop in at the local hardware store to purchase a new one. Aircraft-grade bolts are manufactured under stringent requirements to withstand the strains of flight, and can cost between $8 and $100. According to Teusch, the cost savings can be immense.

"On average these bolts run about $55 a piece, so the cost savings to the Air Force is significant," Teusch said. "Our two goals or purposes are safety of flight and saving money."