Olympia, WA, USA
N111DR
BELL UH 1H
The purpose of the maintenance test flight was for a track and balance of the tail rotor and main rotor blades. The pilot reported that during his preflight check and walkaround, everything looked fine with no anomalies noted. After becoming airborne and performing a hover check, the pilot proceeded to transition to forward flight, when he experienced a loss of tail rotor thrust. This led to a near-vertical rotating descent and hard landing, which resulted in substantial damage to the tailboom and lower fuselage. On on-site examination found that both pitch change link bolts and the tail rotor pitch change horn were only finger tight and found no evidence of cotter pin installation. One of the two crosshead bolts found in the debris field showed thread wear damage but was observed to be relatively complete. The accident could have been prevented if, during the rigging process of the tail rotor assembly, the crosshead bolts and the tail rotor pitch change bolts had been properly torqued and cotter pins had been installed. Additionally, had company quality control personnel identified the improper installation of the tail rotor assembly before certifying the helicopter for flight, or had the pilot detected the missing cotter pins during his preflight inspection, the anomalies could have been corrected.
HISTORY OF FLIGHTOn May 25, 2018 about 0947 Pacific daylight time, a Bell UH-1H, N111DR, was substantially damaged when it was involved in an accident near Olympia, Washington. The pilot and the sole passenger sustained serious injuries. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 maintenance test flight. The pilot stated that he arrived at Olympia Heliport (0WN4) about 0930. He performed a preflight walk-around of the aircraft, then he received a briefing from the passenger/mechanic about the forward flight track and balance checks that needed to be performed during the flight. The pilot stated that, during his preflight walk-around, everything looked good on the aircraft, so he and the passenger/mechanic boarded the helicopter and started the engine. The pilot stated that all indications were normal at idle and at 100% rpm. The pilot stated, “I picked up and hover taxied with a left pedal turn to the helipad. Hovering at the helipad, I got the ATIS [automatic terminal information service] information and aligned the helicopter into the wind.” In the hover, about 10 ft skid height, the passenger/mechanic took the hover measurements for the track and balance, which were within limits to allow for forward flight. The pilot reported that, about 0945, he contacted the tower for permission to take off and then departed to the south. Having received permission from the tower to take off, the pilot bang and felt a “violent right yaw.” The helicopter then increased power and nosed forward. The pilot stated that about 3 to 5 seconds later, while about 70 ft above ground level over a grass field and at an airspeed of about 20 knots, he heard “a loud squeal” with a violent right yaw. That was the last memory he had of the event until they were on the ground at rest upright in a grass field within the airport boundary. The helicopter sustained substantial damage to the tailboom and lower fuselage. The pilot recalled that, when the helicopter was on the ground, he shut the engine down, checked on the condition of the passenger/mechanic, and egressed the helicopter. There was no postcrash fire. The owner of the company observed the helicopter take off, climb to about 200 ft above ground level, start to spin, descend, and crash. He stated that the tail rotor did not appear to be turning. WRECKAGE AND IMPACT INFORMATIONThe main wreckage was located on flat terrain about 657 ft west of the 0WN4 heliport. 0WN4 is a privately-owned heliport situated in the southeast corner of Olympia Regional Airport (OLM), Olympia, Washington. A survey of the accident site revealed that the helicopter’s tail boom had partially separated about 4 ft aft of the engine. Drive train continuity was established from the transmission to the 90° gearbox input quill. The 90° gearbox housing/output quill assembly, tail rotor hub, and tail rotor blade assembly separated from the tail boom mounted input quill of the 90° gearbox. The bolts, washers, castellated nuts, and cotter pins that connect the tail rotor crosshead to the slider were not found on the tail rotor assembly. Cotter pins that go through the attach bolt castellated nuts that connect the pitch link rod end bearing to the tail rotor grips were not located in the wreckage; both nuts were finger tight. The shims between the retainer plate and the pitch change links had separated and were found about 100 ft east of the wreckage. Both bolt holes on five of the shims appeared to be intact. No deformation or elongation of the holes was noted. During the on-site investigation, a Federal Aviation Administration airworthiness inspector observed that both pitch change link bolts and the tail rotor pitch change horn were finger tight and found no evidence of cotter pin installation. Three days after the accident, the company chief inspector found one of the two crosshead bolts in the debris field (see figure). The bolt showed thread wear damage but was described by the inspector as relatively complete. Figure - Tail rotor hub, blade assembly, and pitch control mechanism diagram and photographs of a crosshead bolt from the accident helicopter Continuity of the tail rotor control system was confirmed from the left seat pedals to the tail rotor gearbox through the bell crank clevis. However, the crosshead bolts and pitch change link bolts were installed but not properly torqued, as they were only finger tight. Additionally, cotter pins had not been installed. ADDITIONAL INFORMATIONThe accident helicopter was operated by Northwest Helicopters. During the investigation, the company safety officer stated that, based on condition of parts in the wreckage, “it was reasonable to believe that the Crosshead Bolts and Pitch Change Link Bolts were installed but not torqued, nor were cotter pins installed to aid in the rigging process.” The safety officer stated that Northwest Helicopters was unable to determine who installed the tail rotor hub assembly on the aircraft or why the four bolts in question weren’t torqued.
A loss of tail rotor thrust due to the improper installation of the tail rotor assembly by maintenance personnel resulted in the pilot’s inability to maintain control of the helicopter as he began transition into forward flight from a hover, which resulted in a hard landing. Contributing to the accident was the failure of company quality control personnel to identify the improper installation of the tail rotor assembly before certifying the helicopter for flight, and the pilot’s inadequate preflight inspection.
Source: NTSB Aviation Accident Database
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