ERA23LA272 · Air Tractor AT802 A

The pilot was returning to land following an aerial application flight when the engine lost power. The pilot performed a forced landing in a clearing, during which the airplane sustained substantial damage to the fuselage and empennage. Examination and a test run of the engine revealed that the fuel control unit (FCU) drive shaft splines and the corresponding fuel pump drive gear splines were worn sufficiently to allow for disengagement of the FCU drive shaft from the fuel pump. The surfaces in the vicinity of the FCU-to-fuel-pump interface were wet with a brown residue, similar to iron oxide. Removal of the fuel pump from the accessory gearbox revealed a similar residue on the accessory gearbox interface and drive shaft. Functional testing of the fuel pump showed leakage at the FCU side carbon seal, consistent with the wetness observed on the surfaces. Testing also revealed that the fuel flows were below minimum limits, consistent with wear of the pump gears; however, the fuel pump is designed to deliver more fuel to the engine than required for operation (most of the fuel is bypassed by the FCU and returned to the fuel pump inlet). Disassembly of the fuel pump showed pitting on the fixed bearing dams and on the housing gear bore inlet surfaces, both evidence of exposure to prolonged cavitation. The drive gear journals exhibited surface corrosion. The drive and driven gear teeth showed a high level of wear on the working flanks of the teeth. Bench testing of the FCU permitted a simulation of a loss of speed input condition. When the input rpm was decreased from 6,500 rpm to 0 rpm, the output fuel flow decreased 34%, from a rate of 632 lbs per hour to 416 lbs per hour. According to the manufacturer, the FCU receives the engine gas generator rotational speed (Ng) signal via the drive shaft of the fuel pump. This allows fuel modulation in relation to Ng, among other inputs. The presence of iron oxide on the FCU fuel inlet strainer suggests possible fuel contamination, which may have reduced the fuel lubricity and caused the observed corrosion, wear, and cavitation on the fuel pump’s internal components. It is probable that this lower fuel lubricity may have introduced vibrations in the drive gear, which induced the wear observed on the FCU and fuel pump splines. Based on the available evidence, the disengagement of the fuel control unit drive shaft from the fuel pump resulted in a decrease in fuel flow and a subsequent loss of engine power.

Verbatim NTSB analysis and probable cause from the NTSB dataset