Sikorsky CH-54B N719HT accident near Corvallis, Oregon

Probable cause

The improper assembly of main rotor blade control components by the operator's maintenance personnel, which resulted in the main rotor blade's divergent path and their contact with the tail rotor drive shaft during ground operation following landing.

NTSB analysis

The pilot and certified flight instructor departed for a local area instructional flight during which the pilot practiced a series of takeoffs and landings. During ground operations, the crew stated that they felt an unusual vibration, which was partially mitigated by keeping the collective pitch raised about 2 inches from the full down position. The pilot reported that he decided to adjust his seat position so he landed and handed off the controls to the certified flight instructor (CFI). The CFI lowered the collective to the full down position in preparation for the seat adjustment operation. At this time he noticed a vibration, which was followed by a loud "bang." The subsequent helicopter examination revealed that two of the six main rotor blades had diverged from their normal plane of rotation and impacted the tail rotor drive system, resulting in the number 5 and 6 tail rotor drive shaft sections separating from the helicopter. The investigation and examination of the helicopter found that during maintenance performed at an undetermined date by the operator's mechanics, the bell cranks that connect to the main rotor blades' control rods had been incorrectly installed in a reversed manner. This allowed the swash plate to travel outside of its designed range.

Text from the NTSB. Open the NTSB docket for WPR09LA184 for the full record.