Anatomy of ingenuity and resourcefullness: Kingsley Airman leverages fabrication background to keep F-15s airborne
By Tech. Sgt. Jefferson Thompson, 173rd Fighter Wing
/ Published March 25, 2015
March 2015 -- KINGSLEY FIELD, Ore. -- What keeps the oldest fighter aircraft in the U.S. Air Force inventory airborne? The trend is increasing ingenuity and resourcefulness.
Picture a muscle car, vintage 1978, and what it would mean to take that car to the quarter-mile track and ask for every last ounce of power and torque possible from its engine, running the RPMs to the red, transferring all of that force through the drive-train and turning it into forward motion--everyday.
The F-15 airframe dates to 1978 and they do get "max performed" every time they take to the sky. Take gravitational forces, better known as G's; these aircraft usually pull up to nine Gs on a given mission, translated into everyday terms that means nine times the actual weight of the aircraft. According to the Boeing technical specifications a given F-15 weighs 45,000 lbs., so at nine Gs it is exerting 405,000 lbs. on the wings and the airframe.
Over the years, those forces sap the strength of even the most stalwart of metals; titanium reigns supreme in this environment for its strength-to-weight ratio--half the weight of steel and four times as strong. The modern F-15 aircraft uses this metal extensively but those forces worry away at the metal and eventually it cracks.
Necessity has prolonged the service life of this aircraft from 6,000 hours to 10,000 hours and more in some cases. There are precious few F-22s to augment the air superiority mission and the F-35 has yet to reach active service. Even when they do, plans call for the F-15 to remain in service through 2040. With that extended service metal fatigue issues arise.
To identify problems that will arise from this extended lifespan, Boeing has tested an F-15 airframe to absolute failure, well beyond the 20,000 hour mark.
Sure enough, this testing revealed that wing spars absorb tremendous stress and as a result will break. So the engineers had identified the problem and knew that units using these airframes should inspect them regularly, and with that, uncovered a larger problem.
The wing spar's location is such that it prevents inspection by means of X-ray or other normal techniques. Short of pulling the skin off the entire wing, which is not feasible at a normal flying unit, a method of ensuring the integrity of wing spars didn't exist.
The solution is an example of ingenuity and resourcefulness at the grass-roots level. Tech. Sgt. Jeff Childs, a sheet metal troop at Kingsley Field, has an extensive background in machining and fabrication. By extensive I mean that he designed and fabricated his own 1932 Ford T-Bucket automobile from scratch at one point.
"It took three years," he said of building the car. "The hardest part was modifying the lathe gearbox to get clearance for the wheels we machined from stock aluminum."
So when his structural shop supervisor, Master Sgt. John McAllister, asked him if he saw a potential solution to inspecting wing spars, he said "sure".
The result of that conversation is a block of machined aluminum called a fixture which allows a field unit to cut the skin away from the spar--without touching or damaging the spar. This small opening in the titanium skin allows for inspection via an eddy current tester.
It's a difficult proposition because the two are literally touching each other.
"I created a fixture that attaches directly to the spar and allows us to machine off a couple thousands of an inch at a time," he said, thus creating the opening in the titanium skin.
Although the fixture is fairly simple once designed, the procedure is not stress free. Anomalies in the spar itself cause the titanium skin to lie differently and on two occasions the cutting tool has cut through to the spar itself, scratching it slightly.
"When it happened I was almost sick to my stomach," said Childs. "We stopped everything and took pictures to send to Boeing engineers to see if a spar replacement would be required." And that would require the jet be sent to depot in Georgia with a price tag of over one million dollars.
The way Childs designed the process, successive cuts are made taking a small thickness of titanium on each pass, when the remaining piece separating the spar is only the approximate thickness of a human hair he takes his fixture off and uses a hand etching tool to remove the final paper-like bit.
"When I came up with this idea I thought I would put it in a box and send it to depot," he said.
But necessity intervened and depot and Boeing engineers said "go". While discussing the risk of the procedure, Childs does wear the worry on his face, but he says he's game and can get the 173rd FWs fleet all taken care of.
The week previous he traveled to Portland, Ore., to share the technique with sister unit 142nd Fighter Wing. Their alert mission requires jets be ready to take flight on a moment's notice and engineers at depot have stressed the importance of inspecting these spars as quickly as possible, so the 173rd was happy to send members on temporary duty to ensure their aircraft are mission ready.
"They needed to get three of their wings cut and we suggested to have Jeff [Childs] go up, help them build their fixture and show them the process," McAllister said.
Currently engineers at depot have supplied every F-15 unit with plans and instructions for manufacturing the fixture. In the future it is projected that other units will adopt his technique, which he continues to refine, and soon the F-15 fleet will fly with some field-grade ingenuity on every airframe courtesy of Tech. Sgt. Jeff Childs of the 173rd Fighter Wing.