TINKER AIR FORCE BASE, Okla. --
The Oklahoma City Air Logistics Complex recently performed its second double-skin removal this year on a KC-135 Stratotanker, the refueling aircraft which can carry up to 83,000 pounds of cargo.
Although single belly skin removals are fairly common, the 564th Aircraft Maintenance Squadron performs less than 10 double-skin removals annually.
In fact, OC-ALC conducted zero removals in years 2015 and 2016.
If you’re not knowledgeable about the KC-135 Stratotanker, a double-belly skin removal is a complex process requiring eight mechanics for each side of the aircraft’s fuselage.
Upon arrival, planes are inspected for defects which could drive mechanics and maintainers to change the belly skins, or fuselage skin of the aircraft. Each side of the aircraft demands four mechanics on the inside and four on the outside, totaling eight for one side of the belly removal.
What drives the change? What are some of the defects that lead to a double-skin removal, rather than just a single removal? Ty Brock, a sheet metal supervisor with the 564th Aircraft Maintenance Squadron, explained that corrosion is the most common indicator for change, but having dents too close to the static ports and deterioration issues with protruding sensors can force a change also. Like a sports car, aerodynamics are important to aircraft.
“Skin Waviness Measurement tests are performed to determine how the air comes over the fuselage,” Brock said. Rippling or dents in the fuselage affect how air flows along the aircraft and past the sensors, which indicate critical information like airspeed and altitude.
Brent Bacon, a sheet metal mechanic sub-section chief with the 564th Aircraft Maintenance Squadron, further elaborated on the process of a double-belly removal and how its efficiency has evolved in the past two decades.
“Formerly, this process used to take up to 24 days, non-concurrently,” Bacon said. “Now, a mechanic- centric Art of the Possible approach has enabled the process to be performed concurrently in as little as 16 days.”
The last 15 years have seen the implementation of concurrent tasking, which the second level supervisor added, allows for more work to be done simultaneously. Mechanics are able to work on additional areas of the aircraft such as the P-2 upper wing skins and belly skins, for example, at the same time. The work itself to remove the skins can be strenuous, so the ability to tackle multiple areas at once is a critical point in the OC-ALC efficiency plan.
“We prepare the new skin first,” Brock explained. The process of removing the skins themselves is very tedious, requiring highly skilled mechanics to drill roughly 2,500 fasteners per side to free the skin. “Once the skin is removed, we remove every underlying stringer, which is primary structural support the skin is attached to. There are 10 stringers and we remove five at a time to maintain structural integrity.”
The supervisor said the skin can be drilled off in a day, depending on the manpower available, but both skins are typically removed within a day and a half. The lengthiest portion of the process, Brock noted, is the removal and reattachment of the stringers, which can take between six and eight days if everything goes well. After building and removing the second half of the stringers, hanging of the new skin can proceed.
Building the stringers may be the most time consuming, but Bacon clarified that physically the most daunting task is hanging the new skin.
Skins are organized into three different sections, and have to be precisely lined up, put in place and then riveted to the aircraft. The largest of the belly skins weighs roughly 150 pounds and measures 21 feet long and 8 feet wide.
Precision is critical, as once complete, the fuselage of the aircraft must be air tight and able to withstand the multiple pressure cycles, experienced during takeoff and landing.
In conclusion, the double belly skin removal is a massive undertaking spanning several days and consuming thousands of man-hours. However, it is only one of more than a dozen separate major structural repairs potentially encountered during the KC-135’s programmed depot maintenance visit to the Oklahoma City Air Logistics Complex. Thanks to the engineering skills and technical know-how of its talented workforce, the world’s greatest maintenance and repair operation is able to return this 50+ year old aircraft to the warfighter faster than ever before and ready to immediately meet its mission requirements.