Presently, the rotary wing-head and hub subassemblies of the Army’s Black Hawk helicopter require almost complete disassembly to inspect failure critical threads of the main spindle. Even with direct access to the threads, detection of fatigue cracks in the thread roots is very difficult using visual and penetrant methods. Therefore, the purpose of this project was twofold: (1) to demonstrate an improved nondestructive inspection method for the spindle threads applicable to routine teardown maintenance, and (2) to determine the feasibility of performing safety-of-flight inspections on the spindle with only minimal disassembly.

Recent projects funded by the Air Force have shown that the electric current perturbation (ECP) method is capable of detecting very small surface fatigue cracks in gas turbine engine disks1 and second layer defects in relatively thick structural wing sections.2 Based on these results, the ECP method was evaluated for its capability to inspect the spindle thread roots not only by scanning the outside diameter (crest of the threads), but also by scanning the hollow spindle bore under the threads and inspecting through the wall thickness for flight-critical cracks. With an ECP probe located on the crest of the threads, high sensitivity to very small defects in the thread roots was achieved and thumbnail shaped EDM slots as small as 0.53 mm long by 0.23 mm deep by 0.064 mm wide were detected. Inspection from the bore requires only that the rotary wing be removed so that a probe can be inserted into the spindle bore. Since this inspection is performed through the spindle wall, sensitivity is reduced and only larger defects are detectable. From the bore, detection of a thumbnail shaped EDM slot measuring 7.75 mm long by 2.21 mm deep by 0. 102 mm wide was successfully demonstrated.