Corvallis, OR, USA
N719HT
SIKORSKY CH-54B
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.
On April 7, 2009, about 1530 Pacific daylight time, a Sikorsky CH-54B (Skycrane), N719HT, was substantially damaged following touchdown at the Corvallis Municipal Airport, Corvallis, Oregon, when the main rotor blades diverted from their normal rotation plane and severed the tail rotor drive shaft. The helicopter was operated by Helicopter Transport Services, Corvallis. Visual meteorological conditions prevailed at the time, and no flight plan had been filed. Neither the airline transport certificated pilot, who held a certified flight instructor (CFI) certificate, nor the second crewmember-pilot was injured during the instructional/proficiency flight. The flight was performed under the provisions of 14 Code of Federal Regulations Part 91 and originated from Corvallis about 1500. The CFI reported to the National Transportation Safety Board investigator that the helicopter had operated normally prior to the mishap, although a vibration was felt during ground operation. Therefore, the collective position was kept up about 2 inches, which seemed to reduce the ground vibrations. After practicing four or five landings from a hover, the pilot, who was receiving instruction, landed to adjust his seat position. He handed off the controls to the CFI. The CFI lowered the collective to the full down position, in preparation for the seat adjustment. The terrain on which he landed was sloped. The CFI reported that as he "lowered the collective to the full down position, a vibration was noticed, followed by a loud 'bang', followed by severe vibrations. An emergency shut down followed, with immediate application of the rotor brake." After exiting the helicopter, it was determined that the number 5 & 6 tail rotor drive shaft sections had departed the helicopter, with damage to 2 of the 6 main rotor blades. During the subsequent examination of the helicopter, the CFI verbally reported to the Safety Board investigator that the accident occurred when the main rotor blades diverged from their normal plane of rotation and contacted the tail rotor drive shaft. The operator's director of maintenance verbally reported to the Safety Board investigator that maintenance of the helicopter was by the company's mechanics. During maintenance, the bellcranks that connect to the control rods had been incorrectly installed (in a reversed manner) thereby allowing the swashplate to travel outside of its designed range. Federal Aviation Administration (FAA) personnel, who examined the wreckage and interviewed the operator's mechanics, stated they were unable to ascertain the date on which the subject bellcranks had been installed, but it appeared to have been several years prior to the accident date. To reduce the likelihood for recurrence of this type of accident, the operator modified its maintenance procedure regarding bellcrank installation.
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.
Source: NTSB Aviation Accident Database
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