Accident Overview
On October 13, 2019, about 1700 Pacific daylight time, a Hiller UH-12E helicopter, registration N131HA, was substantially damaged in an accident near Susanville, California. The pilot sustained minor injuries. The helicopter was being operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight.
Pilot Account
The pilot reported that he was finishing an aerial application in a remote area with steep logged forest terrain when the helicopter experienced two loud compressor stalls along with a slight yaw to the left. He reduced engine power and turned the helicopter downslope to allow engine recovery, but continuous compressor stalls began and engine rpm decayed. The pilot continued downhill in an attempt to regain engine speed. When he realized recovery was not possible, he turned the helicopter uphill and applied collective pitch to soften the landing. The main rotor blades struck the terrain, and the helicopter rolled onto its left side. The pilot stated the engine was still running when he exited.
Postaccident Examination
Examination of the recovered airframe revealed no mechanical malfunctions or failures that would have precluded normal operations. The recovered engine showed that the stage 4 turbine wheel would not rotate with moderate hand force, and small metallic debris was found in the exhaust plenum.
The engine was shipped to a Rolls-Royce facility for further examination. Disassembly revealed that the stage 1 turbine wheel exhibited thermal damage with about 50% of the blade length absent, and a crack extending into the web. The stage 1 nozzle and nozzle shield both had a soot coating. The stage 2 turbine wheel showed thermal damage with about 25% of the blade length absent. Both stage 1 and stage 2 nozzles exhibited thermal damage with metal splatter and dirt throughout the gas path, including the stage 3 and 4 nozzles and their respective wheels. The stage 3 turbine wheel had a crack in the outer rim, while the stage 3 nozzle had no visible damage. The stage 4 nozzle exhibited flaking metal in the stage 3 blade track, and the stage 4 turbine wheel had no visible damage.
Engine Component Examination
The stage 1 and stage 2 turbine wheels and nozzles were further examined at the Rolls-Royce Corporation Materials Laboratory. The stage 1 turbine wheel crack extended radially inboard from the rim into the web, measuring about 1.12 inches. The crack initiation point showed signatures consistent with thermal fatigue, transitioning to an interdendritic fracture consistent with over-temperature operation. The fracture had two distinct regions, consistent with two separate over-temperature events. Additionally, 31 visually apparent cracks were found on the trailing edge side of the stage 1 turbine wheel rim, and cracks between every airfoil on the leading-edge side, consistent with an over-temperature event during startup.
Examination of the turbine outlet temperature gauge using a calibrated test set indicated that the gauge displayed values higher than the applied test temperature. The engine thermocouple had a resistance of 0.57 ohm. According to the Rolls-Royce M250-C20 series operations and maintenance document, the acceptable internal resistance for harness part number 23034926 is 0.50 to 0.60 ohm at room temperature.
Maintenance Records
Review of the helicopter maintenance logbooks showed that the most recent 100-hour inspection was performed on September 26, 2019, at 68.2 tachometer time and 12078.4 aircraft total time, with no defects noted. Additional documents revealed that the turbine section had been repaired on September 9, 2014, due to low power and high operating temperatures. No entries regarding previous hot starts or over-temperature operations were found in the logbooks.