What happened
On June 27, 1998, a Schweizer 269D helicopter, registration PH-HTR, was climbing near the TT circuit in Assen when the engine unexpectedly failed. The pilot, while flying at approximately 480 feet, heard a loud bang followed by a loss of power and a leftward yaw.
In response, the pilot immediately initiated an autorotation procedure, turning the aircraft into the wind to facilitate an emergency landing. The pilot selected a grassy field for the touchdown, attempting to maintain a low forward speed to prevent a rollover. However, during the final phase of the landing, the rotor RPM dropped below the critical threshold. The helicopter struck the ground with a high vertical velocity, causing the aircraft to bounce once before coming to a halt. The impact caused significant structural damage to the airframe, and a small fire broke out, which was extinguished by a passenger.
The investigation
The investigation focused on the mechanical failure of the Rolls-Royce Allison 250 engine and the execution of the emergency landing. Investigators examined the pneumatic sensing line (Pc-line) connecting the turbine regulator to the fuel control unit. They also reviewed maintenance records, the pilot's training, and the physical impact of the landing on the occupants.
Findings
- The primary cause of the engine failure was the fatigue fracture of a pneumatic sensing line (Pc-line). This failure led to a loss of pneumatic sensing pressure, resulting in reduced fuel flow and total power loss.
- While the line showed some misalignment, this was considered a normal characteristic of the component and not a design flaw. However, the aircraft had a history of excessive vibrations from the starter-generator, which likely contributed to the fatigue of the line.
- The maintenance records were found to be in order, and the fracture was located in a position that would have been invisible during standard inspections.
- The severity of the injuries to the occupants was attributed to the high vertical impact speed. The investigation also noted that the use of thicker, non-original seat cushions may have contributed to the severity of spinal injuries due to a "submarining" effect during the high-G impact.
- The pilot's training and the Dutch examination requirements for autorotation were found to be adequate.