What happened
On September 2, 1975, an Enstrom F-28A helicopter, registration HB-XEA, was performing private flight operations at the Bad Ragaz airfield. After completing several short passenger flights, the pilot initiated a final approach toward a landing area located between the control tower and the paved runway.
During the descent, at an altitude of approximately 70 meters and a forward speed of 50 mph, the pilot attempted to increase engine power to manage the descent rate and stabilize rotor RPM. Despite increasing power and adjusting the pitch, the rotor speed continued to decay. The pilot eventually found the engine unable to maintain power, forcing the aircraft into an autorotative state. While attempting a flare at a low altitude, the tail rotor boom struck the ground with significant force. The impact caused a main rotor blade to sever the tail rotor and the rear section of the fuselage, which was thrown onto the side of the runway. The aircraft came to rest upright on a nearby meadow. The accident resulted in two injuries to passengers and heavy damage to the aircraft, including the main rotor, tail rotor, engine, and landing gear.
The investigation
Investigators examined the HIO-360-CIA engine and found that the valve keepers had struck the valve guides on at least three separate occasions, suggesting the engine had likely exceeded its maximum allowable RPM of 3200. This overspeeding caused radial cracks in the valve spring retainers for cylinders 2 through 4, while the retainer for cylinder 1 fractured completely. This failure allowed an intake valve to enter the combustion chamber, causing severe damage to the cylinder head and piston.
While the metallurgical analysis of the steel used in the retainers showed no inherent manufacturing defects, the physical evidence pointed to mechanical stress from previous overspeed events as the primary driver of the fatigue cracks.