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Engineers save nearly $300 million in engine repairs

by Pete Meltzer Jr.
Air Force Research Laboratory Materials and Manufacturing Directorate

3/10/2009 - WRIGHT-PATTERSON AIR FORCE BASE, Ohio (AFNS) -- Engineers at the Air Force Research Laboratory Materials and Manufacturing Directorate here saved nearly $300 million by reducing the costs associated with aircraft jet engine maintenance.

The Engine Rotor Life Extension program, called ERLE, demonstrated a life-cycle cost avoidance for engine rotor maintenance, repair and overhaul, or MRO for F-15 Eagle and F-16 Fighting Falcon engines, said Siamack Mazdiyasni, the lead project engineer of the directorate's Metals, Ceramics and Nondestructive Evaluation Division.

"ERLE is an integral part of the propulsion safety and affordable readiness initiative," Mr. Mazdiyasni said. "It is a cooperative effort between development, acquisition, maintenance, and manufacturing technology that safely extends the operational life of critical rotating turbine engine components by one additional depot interval while increasing depot inspection capability, reliability and efficiency."

Right now, the program's emphasis is on currently fielded fighter engines in the F-15 and F-16. But Mr. Mazdiyasni said the same technology will someday benefit other jet turbine engines, including legacy and future engines, along with military jet turbine engines, military transport aircraft engines, and commercial airline engines.

"The eventual payoff will be an increase in the operational life of fracture-critical turbine engine components and increased depot throughput, as well as improved flight safety and cost reductions per component. In reducing MRO expenditures, the ERLE program will increase potential funds for modernization," he said.

"Life management for fracture-critical aircraft engine components is too conservative due to outdated life models and supporting nondestructive inspection technologies. Life management practices make many worst-case assumptions. It follows that improvements in lifing models, using more accurate values in stress analysis and fracture mechanics solutions, will add more precision into calculating the predicted life spans of these components. Supported by advanced nondestructive inspection tools and techniques, these improvements will substantially increase current life limits while increasing overall safety," Mr. Mazdiyasni said.

Aircraft turbine engine disks represent an immense asset for the military, Mr. Mazdiyasni said. Discarding them prior to their expiration of full useful life poses a tremendous burden on gas turbine engine budgets.

"The current approach necessitates discarding 1,000 components to ensure the removal of the one that is theoretically predicted to be in a 'Failed State.' In other words, 99.9 percent of these components are being retired prematurely at 8,000 total accumulated cycles when they have at least one additional interval of life remaining," Mr. Mazdiyasni said.

Four thousand total accumulated cycles are equivalent to approximately 7.5 years.

"As the retirement date of the fleet stretches out, the additional operational intervals become an increasingly important factor associated with the cost of purchasing new turbine hardware. By increasing the life, the Air Force can reduce these costs while maintaining safety and operational readiness," he continued.

Doing this often necessitates the detection of smaller flaws during depot inspection, which can be challenging. Oklahoma City Air Logistics Center members are responsible for the depot inspection of the Air Force's fleet of turbine engines. Mr. Mazdiyasni said detection of smaller flaws with existing inspection capability would put an extreme burden on their ability to meet readiness requirements.

"However, increasing the confidence in the inspection process in locating smaller cracks and embedded flaws is critical to extend the life of fracture critical engine components," he said.

"ERLE developed several nondestructive inspection capability improvements for OC-ALC that are faster, less expensive, and more sensitive, helping reduce the heavy maintenance workload while increasing capacity. One such improvement is the development of a fully automated robotic ultrasonic inspection system for detecting buried flaws in engine components," he explained.

Since its inception about eight years ago, the ERLE program has achieved a number of successes, Mr. Mazdiyasni said. These include development and implementation of a comprehensive data management system, a fully automated laser fretting inspection system, an advanced eddy current probe for engine disk web inspections, and an advanced new "sewing and stitch" data collection system for inspecting dovetail features.

"Widespread implementation of the sew & stitch technology by OC-ALC on legacy engine part inspections continually increases the pool of parts from which cost avoidance credit can be taken," Mr. Mazdiyasni said.

As of March 2008, reliability and capability enhancements attained under ERLE have reduced depot shop floor costs at the OC-ALC by more than $1 million, he said.

"These important innovations and others developed under the ERLE program have significantly improved rotating engine parts inspections and reduced inspection workload, analysis, and scanning times," Mr. Mazdiyasni said.

"Full achievement of the ERLE program goals will be realized through implementation of advance life analysis and nondestructive inspection as part of the Air Force's engine life management plan," he added.

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