Primary finding
Probable cause
A loss of engine power due to fuel starvation resulting from contamination of the fuel system during low-level flight.
Investigator assessment
Analysis narrative
The pilot reported that he had not flown the experimental amateur-built helicopter for 3 months, so he performed several practice liftoffs to a hover and landings. He then departed and flew a modified airport traffic pattern with the intent of terminating the approach to a hover. During the circling approach, about 50 ft above ground level (agl), he added power to increase the helicopter’s altitude and airspeed, but the engine lost total power. The pilot attempted to arrest the descent, but the helicopter landed hard, which resulted in substantial damage to the tailrotor driveshaft and tailrotor gearbox. Postaccident examination of the airframe fuel system revealed that the main fuel tank contained excess hardened sealant pieces found at the base of the tank, near the fuel pickup points that fed into the left and right fuel tanks. The size and shape of the excess sealant was too large to flow through the fuel lines and may have precipitated a fuel starvation event. Foreign object debris in the form of dirt-like particulates were observed from the cross-feed outflow fuel fitting to the fuel filter input. The left and right fuel tanks, fuel filter, and fuel collator all contained foreign object debris in the form of dirt-like particulates. Given the extraordinary quantity of foreign object debris in the airframe’s fuel system, foreign object debris accumulation, specifically at the fuel fitting entry points, likely resulted in fuel starvation and a loss of engine power during the pilot’s increased collective application and corresponding increased power demand. The source of the foreign object debris was undetermined.
Source record
Factual narrative
On October 1, 2023, about 1320 Pacific daylight time, an experimental amateur-built Helicycle, N519SP, was substantially damaged when it was involved in an accident near Dear Park, Washington. The pilot was seriously injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that he had not flown helicopter in over 3 months, and he performed several practice liftoffs to a hover and landings before departing to the northeast to fly a modified airport traffic pattern. He continued his left turn to the west with the intent of terminating the approach to a hover. The pilot reported that during the left turn and about 50 ft agl he “added power to increase airspeed and altitude” when the engine lost power. He lowered the collective, rolled out of the turn and attempted to arrest the descent, but the helicopter landed hard, resulting in substantial damage to the tailrotor driveshaft and tailrotor gearbox. NTSB investigators did not travel to the accident site. Postaccident examination of the helicopter’s fuel system, showed that the experimental amateur-built helicopter was equipped with three polypropylene fuel tanks. The main fuel tank was fabricated with a vent and two fuel lines at the base of the tank; one fuel line at the left base fed into a fuel tank on the left side of the helicopter, and one fuel line on the right base of the tank fed into a fuel tank on the right side of the helicopter. The left and right side fuel tanks were equipped with fuel quantity indicators, and a cross-feed fitting that fed into the fuel filter. The anodized aluminum fuel filter was intact, disassembled, and revealed an observable quantity of foreign particulates inside the body of the filter, as well as the inflow orifice. The left and right fuel tanks contained dirt-like debris, which was also found in the cross-feed outflow fuel fitting to the fuel filter input. The main fuel tank revealed hardened pieces of sealant affixed to arbitrary locations at the base of the tank. The pieces of sealant were too large to flow through the fuel lines into the subsequent left and right fuel tanks. The gascolator, filter screen, O-rings, and rubber seal were intact. However, an observable quantity of dirt-like debris was trapped inside the gascolator and staining was observed inside the fuel bowl. The fuel control assembly was affixed to the fuel pump, securely mated to the accessory case, and appeared unremarkable.