AFRL manufacturing technology engineers, working with personnel from the 76th Maintenance Wing's Software and Propulsion Maintenance Groups at the Oklahoma City Air Logistics Center (OC-ALC) and Wyle Laboratories (formerly Veridian Engineering), delivered a major configuration upgrade and improved the inspection process for the Air Force (AF) Eddy Current Inspection System (ECIS) at OC-ALC, Tinker Air Force Base (AFB), Oklahoma. These ECIS improvements are part of AFRL's Engine Rotor Life Extension program. With investments exceeding $80 million, the ECIS program addresses an AFRL initiative to extend the useful life of turbine engine components and reduce the cost of replacing aging engine components in the AF's fighter and bomber fleets.
Eddy current inspection uses electromagnetic induction to detect flaws in conductive materials. The current ECIS station consists of a large electronic manipulator arm that rides on a stable granite block, a turntable to rotate the part throughout the inspection, and an electronic control console (see figure). An engine rotor disk clamped to the turntable rotates as directed by the inspection program. The manipulator arm carries an eddy current probe and maneuvers over, around, and through various areas of the disk. Using an assortment of probes, the system collects information from the engine disk and uses this data to indicate defects in the disk's different geometric features. AFRL's ECIS upgrade improves the speed of communications between the ECIS station computer and the robotic inspection head and thus enables faster and more efficient inspection of engine parts. New digital software signal processing filters replace the current hardware analog filters, significantly improving reliability and minimizing system downtime. The upgrade also consolidates the system's electronics and wiring from a three-bay cabinet to a single-bay cabinet. As a result of these enhancements, OC-ALC personnel have substantially decreased F100 and F110 engine inspection cycle times, which will allow them to accept future F119 and F135 engine inspection tasks.
Concurrent with the ECIS upgrade was the implementation of an inspection process known as the "sewing stitch." The sewing stitch inspection technique significantly increases the efficiency and productivity of inspecting dovetail slots on F100- GE-129 high-pressure turbine disks. Using this new technique during scheduled maintenance operations, OC-ALC maintainers demonstrated a 65% decrease in inspection time.
ECIS' original implementation occurred in the 1980s at the San Antonio Air Logistics Center, Kelly AFB, Texas, under the laboratory's Retirement for Cause program. Eddy current inspections are critical for early crack detection and ensuring safety throughout the AF fleet. Engineers upgraded ECIS in the late 1990s, replacing obsolete system components and upgrading the station computer. The system continues to provide the AF with the enhanced inspection capability needed for conducting safety inspections under the AF structural integrity program.
Prior to the 1980s, AF maintainers replaced all engine disks when they reached a predetermined number of operating hours—a necessity since eddy current inspection technology did not yet exist. Even after manufacturing processes improved the quality and durability of aircraft engine disks, the philosophy and time frame established for retiring the disks remained unchanged. Recent advances in sensor technology, computing speed, and computational analysis now enable engineers to determine conditions under which an engine disk can continue to operate safely. By gathering more accurate measurements and processing those measurements intelligently, ECIS allows the AF to safely increase the operating life span of engine components.
In addition to increasing engine component safety, ECIS' improved inspection techniques generate substantial cost savings. Since the price tag for one engine rotor disk can range from $25,000 to several hundreds of thousands of dollars, the AF may ultimately save hundreds of millions of dollars by keeping disks in service longer.
Dr. Howard Sizek and Dr. Paul R. Riggle (Anteon Corporation), of the Air Force Research Laboratory's Materials and Manufacturing Directorate, wrote this article. For more information, contact TECH CONNECT at (800) 203-6451 or place a request at http://www.afrl.af.mil/techconn_index.asp. Reference document ML-H-05-19.