What happened
During ground testing at a heliport, a Schweizer 2-69C-1 helicopter, registration C-FZQF, experienced a catastrophic mechanical failure. The aircraft, operated by Hélicraft 2000 Inc., was undergoing functional tests following a 100-hour inspection and a recent replacement of the main transmission gearbox.
During the second test run, the pilot and a ground engineer noted an unidentified noise while reducing engine rpm. During a third test, the noise was traced to the main transmission gearbox. As the pilot reduced power, the gearbox suddenly seized, causing the bolts securing the main rotor to the shaft to shear. The main rotor lifted to roughly 150 feet above the ground before landing on the heliport apron approximately 100 feet from the helicopter. There were no injuries during the event.
The investigation
The investigation focused on the recent maintenance performed on the main transmission gearbox. The gearbox had been in operation for only 12 minutes since its last inspection. Investigators examined the components and found that the input pinion bearings had been deprived of necessary lubrication.
Further analysis revealed that the input quill bearing housing had been installed incorrectly. This improper positioning obstructed the necessary oil flow to the bearings. Additionally, investigators noted that the force needed to shear the rotor head attachment bolts is lower than the force required to shear the main rotor shaft, which allowed the rotor to separate from the shaft during the sudden stoppage. The investigation also found that a previous independent inspection had failed to identify the assembly error.