CEN25LA093

Improperly torqued intake manifold bolts that allowed unmetered air to enter the engine, resulting in an overly lean fuel-air mixture, eventual failure of the No. 3 cylinder intake valve, and a total loss of engine power.

The pilot reported that, while performing an aerial inspection of a fence, he heard a loud bang come from the engine and the helicopter began to shake and lose altitude. The shaking stopped momentarily but then started again and the engine lost total power. The pilot performed a forced landing, during which the helicopter impacted cedar trees and then rolled several times down a hill. The helicopter sustained substantial damage to the fuselage, tail boom, and main rotor gearbox. A postaccident inspection of the engine revealed that the No. 3 cylinder intake valve head was burned and chipped specifically on the combustion face, margin, and seat face. Additionally, the bolts that secured the intake manifold system to the engine block were loose. A review of maintenance records revealed that all four engine cylinders were removed for overhaul and reinstalled about 8 months before the accident due to low compression. According to the pilot, who was also the owner of the helicopter, about one month before the accident (about 360 hours after the cylinders were overhauled), the No. 1 cylinder was replaced due to the intake valve head being burned. According the engine manufacturer, when engine cylinders are removed and/or reinstalled, the bolts that secure the intake manifold system to the engine block need to be removed to manipulate the cylinders. It is likely that when the cylinders were removed 8 months before the accident for overhaul and reinstallation, the bolts that secured the intake manifold system to the engine block were not properly torqued when the cylinders were reinstalled. The loose intake manifold system likely allowed unmetered air into the engine, resulting in an overly lean fuel/air mixture, which resulted in damage to both the No. 1 and No. 3 cylinder intake valves. The damage to the valves would likely result in poor combustion and a decrease in engine performance.

On February 7, 2025, about 1640 central standard time, a Robinson R22 helicopter, N7191B, was substantially damaged when it was involved in an accident near Spearman, Texas. The pilot was not injured, and the passenger sustained minor injuries. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that they departed near Pampa, Texas, about 1020 for an all-day wildlife management hunting expedition and to inspect fences. While inspecting a fence, the pilot heard a loud bang coming from the engine and the helicopter began to shake. The shaking stopped and then started again, and the engine lost total power. The pilot was unable to find a suitable forced landing site due to uneven terrain, and the helicopter impacted cedar trees and then rolled several times down a hill. The helicopter sustained substantial damage to the fuselage, tail boom, and main rotor gearbox. A postaccident inspection of the engine revealed that the No. 3 cylinder intake valve head was burned and chipped specifically on the combustion face, margin, and seat face. Additionally, the bolts that secured the intake manifold system to the engine block were loose. A review of maintenance records revealed that all four engine cylinders were removed for overhaul and reinstalled in June 2024 due to low compression. According to the pilot, who was the owner of the helicopter, in January 2025, about 360 hours after the engine cylinders were overhauled, the No. 1 cylinder was replaced due to the intake valve head being burned. According to the engine manufacturer, when engine cylinders are removed and/or reinstalled, the bolts that secure the intake manifold system to the engine block need to be removed to manipulate the cylinders.