HISTORY OF FLIGHT On July 30, 2020, about 2045 Pacific daylight time, a Bell OH-58A helicopter, N901XV, sustained substantial damage when it was involved in an accident near Holtville, California. The pilot sustained a minor injury. The helicopter was operated as a Title 14 Code of Federal Regulations Part 137 agricultural flight. The pilot stated that after his initial departure, he touched down to pick up his first load. After he completed dispersing two loads, he returned to the loader and added another load. He made a normal takeoff and completed three passes over the field that he was spraying. As he approached wires, the pilot maneuvered the helicopter into a climb and prepared to make a fourth pass. As he was completing a left turn back to the field, the engine experienced a total loss of power. He performed an autorotation, and during the touchdown, the helicopter collided with haystacks. The fuselage sustained substantial damage. HELICOPTER INFORMATION The helicopter was manufactured in 1969 and had amassed 7,299.5 total hours at the time of the accident. It was equipped with an Allison (Rolls-Royce) T63-A-720 gas turbine engine, serial number AE-405061. According to maintenance records, the last compressor overhaul occurred on June 06, 2020, at a total engine time of 3,635.0 hours, equivalent to 21.5 flight hours before the accident. TEST AND RESEARCH Postaccident examination of the engine revealed a disconnect of the compressor rotor from the power turbine drive train. A complete engine teardown was conducted. Separating the compressor module from the accessory gearbox revealed significant damage in the area where the splined adapter joins the compressor-turbine shaft to the compressor’s splined fitting. The aft end of the compressor, the spur adapter gearshaft, and the splined adapter were all extensively damaged. The damage had disconnected the compressor from the power turbine, which would have resulted in an engine failure. During removal of the compressor from the accessory gearbox, the five sets of shims at the interface flanges between the gearbox and the compressor’s rear diffuser were examined, and an unusual number of shims were observed on the compressor to-accessory gearbox mounts. Per the Rolls-Royce Maintenance Manual, a maximum of 0.020-inch of shimming is permissible on any single mount. One mount was found to have 0.036-inch of shimming, and another mount had 0.022-inch of shimming. According to Rolls-Royce, a standard “shim card” is included in all engine log books, and it is standard practice to record the compressor’s shimming during module installation. There was no record of shimming found in the engine’s logbook. The shimming procedure provides correct alignment of the compressor rotor to the gas producer turbine rotor, reducing the possibility of accelerated spline wear on the compressor-to-turbine shafting components. As the height of the mounting pads varies, the angle of the compressor relative to the gearbox also changes; this change in angle changes the alignment of the engine. The alignment build procedures in the Operation and Maintenance Manual (OMM) must be followed to prevent misalignment conditions. At the request of the National Transportation Safety Board (NTSB) investigator-in-charge, the Rolls-Royce Materials Laboratory examined the compressor splined adapter, part number 23076559-1 (serial number 312392), and determined that it had failed in fatigue. There were multiple fatigue cracks on the compressor splined adapter originating from the radius aft of the external splines. Measurement of the inside diameter (ID) of the impeller’s stub shaft pilot revealed that it had been machined to fit a 23076559-3 compressor splined adapter with a larger pilot outside diameter (OD). According to Rolls Royce, there are three sizes of compressor splined adapters available, denoted by adding -1, -2 or -3 to the end of the part number, and they have progressively larger pilot diameters. The ID of the impeller must be machined to achieve the proper fit. This machining results in a progressively larger ID, necessitating a compressor splined adapter with a larger OD for a proper fit. If a -1 adapter is installed, a new impeller is required. The OMM and C20 Series Commercial Engine Bulletin (CEB) A-1392 describe the proper procedures for installing compressor splined adapters of the correct size to prevent fractures that could lead to a power loss event. According to the engine logbooks, in 2009, a -3-compressor splined adapter was installed; after this, there was no mention of the compressor splined adapter being removed/replaced in the engine logbooks. ADDITIONAL INFORMATION In June 2014, an accident (NTSB accident number WPR14TA236) occurred in a helicopter equipped with a Rolls Royce M250-C20R/2 engine where the spur adapter gear and the splined adapter were found extensively damaged. The NTSB determined that the probable cause of this accident was the loss of engine power during cruise flight due to the decoupling of the engine's turbine and compressor sections. Contributing to the decoupling was the excessive wear of the turbine-to-compressor coupling components due to maintenance personnel's placement of an incorrect shim during a compressor section overhaul and a latent misalignment within the exhaust collector.