173rd Maintenance pioneers critical inspection
By Tech. Sgt. Jefferson Thompson, 173rd Fighter Wing
/ Published March 25, 2015
March 6, 2015 -- KINGSLEY FIELD, Ore. -- Airmen at the 173rd Fighter Wing have pioneered a critical fix preserving airworthiness for the U.S. Air Force F-15C Eagle aircraft as the airframes approach 40 years-in-service, Feb. 26, 2015.
Master Sgt. Dusty McAllister, supervisor of the 173rd Sheet Metals shop asked one of his Airman, Tech. Sgt. Jeff Childs, with a background in machining, if it were possible to engineer an inspection point for wing spars, which are inaccessible at field locations. This inspection was in response to stress testing conducted by Boeing in which an airframe was subjected to stresses proportionate to more than 20-thousand flight hours. Those tests revealed a vulnerability in some wing spars.
"There was a catastrophic failure on the stress test aircraft on the intermediate spar," said McAllister. "[The engineers] said they need to find out if we can remove some skin in an area and check these, and we said, 'yes, we will take a look at this'."
He, Childs and other members of the shop spent several weeks of trial and error and created a fixture machined from a block of aluminum that holds a carbide cutting tool and allows them to cut through the titanium skin precisely enough to preserve the spar it touches.
"We take off thin layers in successive cuts until we get close, then we hold our breath for the last cut," said Childs, the one who physically performs all the cutting operations.
The operation has worked well enough that he visited the wing's sister unit, the 142nd Fighter Wing, and showed them how to make the fixture and helped them cut five wings.
McAllister says that depot maintenance thinks the design is excellent and is sending it to all F-15 units to help them accomplish their inspections.
"A rough drawing of the fixture and a new TCTO has gone out to all the F-15 units to begin working the fix," he said.
He adds that they continue to refine the process and are looking at using an ultrasound scan to help measure the thickness between cuts to provide a wider margin of error.