Accident Overview
On July 30, 2020, about 2045 Pacific daylight time, a Bell OH-58A helicopter (registration N901XV) collided with haystacks during an autorotation landing near Holtville, California. The helicopter was operating as a Title 14 Code of Federal Regulations Part 137 agricultural flight. The pilot sustained a minor injury. The aircraft was substantially damaged.
According to the pilot, he had completed two dispersal loads and was returning to the field after a normal takeoff. While making a left turn to begin a fourth pass, the engine experienced a total loss of power. He performed an autorotation, and during touchdown the helicopter struck haystacks.
Helicopter Information
The helicopter was manufactured in 1969 and had accumulated 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. The most recent compressor overhaul occurred on June 06, 2020, at a total engine time of 3,635.0 hours, approximately 21.5 flight hours before the accident.
Engine Examination
Postaccident examination revealed that the compressor rotor had become disconnected from the power turbine drive train. A complete teardown showed extensive damage to the aft end of the compressor, the spur adapter gearshaft, and the splined adapter. The damage had disconnected the compressor from the power turbine, resulting in engine failure.
Examination of the compressor-to-accessory gearbox interface flanges found an unusual number of shims. One mount had 0.036-inch of shimming, and another had 0.022-inch, exceeding the Rolls-Royce Maintenance Manual's maximum of 0.020-inch per mount. No record of shimming was found in the engine logbook.
The Rolls-Royce Materials Laboratory determined that the compressor splined adapter (part number 23076559-1, serial number 312392) had failed in fatigue, with multiple cracks originating from the radius aft of the external splines. Measurement of the impeller stub shaft pilot inside diameter revealed it had been machined to fit a -3 adapter, which has a larger pilot outside diameter. According to Rolls-Royce, three sizes of compressor splined adapters exist (-1, -2, -3) with progressively larger pilot diameters. The proper installation procedures are described in the Operation and Maintenance Manual and Commercial Engine Bulletin A-1392.
Engine logbooks indicated that in 2009, a -3 compressor splined adapter was installed, with no subsequent record of removal or replacement.
Maintenance Findings
The alignment build procedures in the Operation and Maintenance Manual (OMM) must be followed to prevent misalignment conditions. The shimming procedure provides correct alignment of the compressor rotor to the gas producer turbine rotor, reducing the possibility of accelerated spline wear. The source noted that excessive shimming was present at the compressor-to-accessory gearbox mounts.