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Tinker shop looking for trouble

Tech. Sgt. Steven Smith, 507th Air Refueling Wing non-destructive inspection lab shop chief inspects parts off a KC-135 Stratotanker during the final stage of a penetrant test.  The black light shows any cracks that are not visible to the human eye.  The NDI lab inspects about 300 parts either in the lab or with their portable units every month.  (U.S. Air Force photo by Senior Airman Mark Hybers)

Tech. Sgt. Steven Smith, 507th Air Refueling Wing non-destructive inspection lab shop chief inspects parts off a KC-135 Stratotanker during the final stage of a penetrant test. The black light shows any cracks that are not visible to the human eye. The NDI lab inspects about 300 parts either in the lab or with their portable units every month. (U.S. Air Force photo by Senior Airman Mark Hybers)

TINKER AIR FORCE BASE, Okla. -- There's one shop in the 507th Maintenance Squadron that spends its time looking for trouble.

Trouble in the form of cracks -- metallurgic imperfections that can play havoc with high-stress machinery if not detected. The forensic specialists in the squadron's Non-Destructive Inspection lab here marry science and chemistry to inspect and detect troublesome cracks in parts on the venerable KC-135 Stratotanker.

Each morning, Tech. Sgt. Steven Smith and the five-person NDI shop begin their "crack-finding" mission by sifting through a request list from the sheet metal shop. Everything from the smallest bolt to an aircraft wing gets the NDI shop's special attention.
"NDI plays a big role in the readiness of the aircraft," said Senior Master Sgt. Larry Spradling, 507th fabrication flight chief. "The inspections they perform show defects that are not detectable by the human eye. It's a very important part of the process."

The NDI shop uses five tests to check for cracks -- penetrant, magnetic particle, ultrasonic, eddy current and x-ray -- each designed for specific types of parts. Sergeant Smith uses a computer program and technical order to determine which test to perform.
"The TO breaks down everything you need to know about the inspection, which of the five tests to perform, details about the part or parts being tested, calibration settings and much more," Sergeant Smith said.

Some inspections, like the penetrant test, require a great deal of preparation. Once cleared of oils and dirt, the part is dipped into a penetrant solution and then sits to dwell (set up). Dwell times vary depending on the part. Once the penetrant soaks into the part, it's rinsed with water, and then rinsed with an emulsifier, a soap-like substance. The part is rinsed again with clean water and then dipped into a developer for about a minute then placed a large drying machine. From there, it's off to an enclosed area to be inspected under a black light.

Cracks appear as a green line under black light testing.

Not all parts are easily removed from the aircraft for testing, however. For these parts the NDI lab is equipped with portable equipment. That mobility comes in handy during Isochronal Inspections, an in-depth inspection that tests an aircraft from top to bottom, inside and out.

"The NDI guys are able to come out to the aircraft during an ISO and perform bulkhead tests," said Master Sgt. Jason Lawson, 507th Aircraft Repair/Reclamation shop chief. "They perform inspections on all of our hardware, on or off the plane. They really do a lot for us."

The 507th NDI team performs roughly 300 inspections per month. "We take this stuff very seriously," Sergeant Smith said. "If we miss something, it could be catastrophic."