Engineers work to improve A-10 APU insulation

The Ogden Air Logistics Center in Utah partnered with the Hill Center Test Authority in a component improvement project to identify material more suitable for the A-10’s Auxiliary Power Unit insulation.

The project is the result of the 419th Supply Chain Management Squadron’s continuing efforts in evaluating depot maintenance data and feedback from aircraft maintainers and using the information to modernize Air Force aircraft.

“Aircraft continually go through modifications to incorporate improved technologies. This project was identified as a venture that would offer significant cost savings and improve ease of maintenance,” said Eric Collins, an aerospace engineer with the 419th SCMS.

The APU, a small gas turbine engine that starts the main engines and provides hydraulic and electrical power during aircraft maintenance, can become very hot, requiring thermal insulation to protect other aircraft components from heat damage. Insulation material is molded around the hottest section of the APU to reduce the surrounding temperatures.

Jared Hainsworth (left), A-10 Special Program Office, and Stephan Hall, Honeywell test engineer, look into the APU bay of a A-10 Thunderbolt II June 23, 2021, at Hill Air Force Base, Utah. The engineers were determining where they would be placing thermalcouplers to determine temperatures inside the bay during a ground test. (Air Force photograph by Cynthia Griggs)

As a result of the high temperatures, this insulation material can fatigue, crack and degrade, causing many overhauled APUs to need to be reworked for insulation repair.

“This costs the government around $250,000 per year and results in reduced time of APUs on aircraft,” said Bobby Yen, an aerospace engineer with the 419th SCMS.
“Finding material with improved durability and integrity will keep fully operational APUs on aircraft longer, saving money and enhancing the A-10 mission.”

The SCMS team finished the first phase of the project, which was a baseline test to gather an insulation temperature profile of the existing APU to determine if modern insulation materials would improve performance.

The first phase wasn’t without challenges. According to Yen, in order to ensure accurate results, performance requirements for component testing should be equal to, or even more strenuous than, what the aircraft experiences. Mimicking an actual aircraft environment proved to be not only difficult, but also cost prohibitive.

“We really needed to simulate actual conditions surrounding the APU environment, but we do not have a test facility that measures the heat that the APU radiates onto nearby surfaces,” Yen said.

Engineers watch temperature readings during a A-10 Thunderbolt II engine test, June 23, 2021 at Hill Air Force Base, Utah. The engineers were determining temperatures inside the APU bay. (Air Force photograph by Cynthia Griggs)

To solve the problem, the team planned to travel to Davis-Monthan AFB, Arizona, to perform tests on an A-10 there.  However, that became unnecessary when the Center Test Authority, which led the test planning and safety review for the project, was able to secure an actual A-10 aircraft that was coming to Hill for regular maintenance.

The CTA coordinated with the A-10 aircraft depot scheduling team from the 571st Aircraft Maintenance Squadron and the A-10 System Program Office to schedule the A-10 to arrive several days early and allowing engineers to perform Hill’s first-ever pre-depot induction ground test.

“The outstanding collaborative support from many functions and organizations – such as the A-10 engineering, test and logistics team, the 309th Aircraft Maintenance Group’s security and ground support team and the 571st AMXG depot scheduling team – enabled effective test planning and execution to include early aircraft arrival for testing,” said Roger Beal, test engineer for the CTA.   

The savings for performing the tests at Hill was estimated to be about $70,000, according to Beal.

Teams are now in phase 2 of the project, which includes designing and ordering prototype insulation materials. Phase 3 will be to perform insulation qualification testing at the APU depot test facility and then perform a comparison test in the same manner as the baseline test in phase 1, but with the new thermal insulation material installed to ensure temperatures are at or below the operational thresholds from the baseline test.

A date for when the project will be completed is to be determined.  In addition to this project, the 419th SCMS is currently involved in more than 20 improvement projects for secondary power systems across multiple aircraft platforms, including B-1B, B-2, F-15, A-10, F-16, KC-135, C-130, and E-3.

Stephan Hall, Honeywell test engineer, and Calvin Blaser, Honeywell customer support engineer attach thermalcouplers up inside the APU bay of a A-10 Thunderbolt II June 23, 2021 at Hill Air Force Base, Utah. The thermalcouplers will determine temperatures inside the bay during a ground test. (Air Force photograph by Cynthia Griggs)

The engines of an A-10 Thunderbolt II can be seen from a ‘hush house’ tunnel. A hush house is a hanger where aviation maintainers can run engine tests where the sound would hushed. (Air Force photograph by Cynthia Griggs)


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