NDI: Seeing the unseen

  • Published
  • By Master Sgt. Mark C. Olsen
  • 108th Wing Public Affairs
     They're known as the NDI shop and its Airmen have a simple mission: make the invisible, visible.

     The job of making sure the 108th Wing's KC-135R Stratotankers are structurally and mechanically sound falls on the four Airmen of the Non-Destructive Inspection shop. They take safety seriously.

     The Air Force NDI Program was established in 1958. And just like its name implies, it all about inspecting and checking aircraft parts and surfaces - the aircraft's skin - in a way that does not damage the part. Common inspections are the aircraft's high load and vibration areas, the aircraft skin and landing gear.

     During the latest inspection, two if its members - NDI craftsman Staff Sgt. Dean C. Schwaner and journeyman Senior Airman Lucas Derflinger - were busy preparing their equipment to find out if there was anything wrong with some parts that could potentially threaten lives and the Wing's mission.

     "NDI shop members attend a 10-week course in Pensacola, Fla.," said Master Sgt. Raymond J. Joubert, NDI noncommissioned officer in charge. "They must then complete 16 months of on-the-job training before they can work on any part of the aircraft without supervision."

     NDI also works on the flight line performing scheduled and unscheduled aircraft maintenance. Scheduled maintenance is based on trends of past airframe hours that result in structural defects. The unscheduled maintenance occurs when a crew chief or a maintenance technician suspects there is a possible defect during a routine inspection.

     At the Wing, NDI Airmen use a variety of methods for testing: Eddy current, ultrasonic testing, fluorescent dye penetration, magnetic particle and optical inspection. In addition, they also inspect engine oil lubricant for abnormal-wear metals. They are also trained to conduct x-ray inspections, but that is done working with the 87th using their equipment.

     "Some of the setup procedures can take hours or even a day," said Schwaner. "In some cases, the actual inspection may take only 30 minutes."

     The shop, which is located in the corner of the maintenance hangar, has two rooms where the testing is done. In one room, Schwaner gets ready to perform an eddy current inspection on a set of tire rims for the Stratotankers nose landing gear. Next door, Derflinger is checking the magnetic particle inspection unit to make sure it is properly calibrated before inspecting aircraft parts.

     "With the magnetic particle inspection unit, the part gets magnetized, creating north and south poles," said Derflinger.

     Once the part is magnetized, it is then bathed in a suspended particle bath. This oil based bath contains fine iron particles which coat the part. The liquid seeps into the defects and when it is exposed to ultraviolet light - black light, causes the defects to become florescent and visible.

     "This enables the technician to determine depth and length of the defect," said Derflinger.

     The lights are turned off, plunging the room into near darkness. The only light comes from some red-lit switches on the inspection unit. Derflinger bathes a ketos ring - a round flat doughnut-shaped piece of metal used to calibrate the machine - with the suspended particle bath. He then turns on a hand-held black light unit and the ring glows in a sickly pale green color. Derflinger increases the magnetic level and the calibrated defects on the ring appear brighter. The unit is now ready to test parts.

     One of the items that gets a lot attention are bolts.

     "We do 6,200 bolts using the bath," said Derflinger.

     In the other room, Schwaner checks the eddy current tester by placing the probe on a test piece, which is shaped exactly like the Stratotanker's nose landing gear tire rims. The probe sends out a current, which flows through the object creating a magnetic field. The test piece has built in flaws which appear as jagged lines on the readout screen of the test unit. A straight line means there are no flaws.

     The tester checks out and Schwaner begins testing the three rims. Each rim is made out of solid aluminum and weigh close to a hundred pounds. The rims come in two pieces, so there are actually six pieces to test.

     Schwaner lifts one of the rims and places it on a revolving table that looks like an oversized lazy susan. Placing the probe along the curved edge of the rim, Schwaner revolves the rim while watching the tester for any disturbing readouts. None appear and he proceeds to repeat the process with the next five pieces.

     While the process is tedious, it is paramount to the safety of the air crew and to the success of the mission.

     "It has to be right on the money," said Schwaner.