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N353RK accident description

Missouri map... Missouri list
Crash location 39.463889°N, 92.427500°W
Nearest city Moberly, MO
39.418369°N, 92.438237°W
3.2 miles away
Tail number N353RK
Accident date 05 Dec 2002
Aircraft type Hughes OH-6
Additional details: None

NTSB Factual Report

On December 5, 2002, at 0900 central standard time, a Hughes OH-6, N353RK, operated by Freelance Air, Inc., lost tail rotor effectiveness during cruise flight and executed a run on landing at the Omar N Bradley Airport (MBY), Moberly, Missouri. The helicopter was not damaged and the pilot was not injured. The 14 CFR Part 91 repositioning flight departed the Spirit of St. Louis Airport (SUS), St. Louis, Missouri, at 0810, en route to Trenton, Missouri. Visual meteorological conditions prevailed. No flight plan was filed.

The pilot reported that he was in cruise flight for approximately one hour when the helicopter "entered an estimated 20 degree uncommanded yaw to the right." Applying pressure to the anti-torque pedals had no effect. The pilot reported that there had been no advance auditory indications or unusual vibrations prior to the yaw occurring. The pilot reported he applied left cyclic and maintained airspeed. He flew to MBY, which was about 12 nautical miles away. Once he arrived at MBY, he executed a successful run on landing to runway 31.

The inspection of the helicopter revealed that the tail rotor bellcrank pivot pin was dislodged from the non-rotating swashplate and was rotated inboard towards the boom. The non-rotating swashplate spherical bearing boss was found damaged and there appeared to be fine ferrous material around the bellcrank area. The non-rotating swashplate had abnormal play around the tail rotor gearbox output shaft. Excess vertical play was found in the bellcrank arm at the bellcrank attachment bolt.

The tailrotor gearbox and the non-rotating swashplate assembly were sent to Boeing Product Integrity, Mesa, Arizona, for further inspection and teardown. The inspection revealed the following:

1. The plain brass liner, which is a pressed liner internal to the non-rotating swashplate and surrounds the tail rotor gearbox output shaft, was broken into 15 identifiable pieces. No evidence of fatigue or abnormal wear was found on the pieces of the plain liner.

2. The splined brass liner within the non-rotating swashplate was damaged. Eight of the seventeen splines of the swashplate splined liner had fractured areas and one of the splines was cracked. No evidence of fatigue was found on any of the fractures. The splined liner was still riveted to the rotating swashplate and was still capable of bearing a load.

3. The bellcrank assembly was undamaged, however, both bellcrank bolt holes showed evidence of distortion or elongation.

4. After the removal of the outer boot, swashplate assembly and the bellcrank from the tail rotor drive shaft, both the tail rotor gearbox output shaft and in input shaft rotated freely without binding. (See Boeing Engineering Laboratory Report, 03L0312, included in the docket)

Maintenance records indicated that the last 100 hour inspection occurred on April 25, 2002, at 3,700.3 hours aircraft total time. The total time on the aircraft at the time of the accident was 3,781.1 hours.

The non-rotational swashplate was installed as an overhauled unit on November 23, 1999. The swashplate had 385.4 flight hours since the time of overhaul.

The tail rotor gearbox was replaced with an overhauled gearbox on October 25, 2002. The helicopter had a total time of 3,760.8 hours. The tail rotor gearbox had 29.4 flight hours since overhaul at the time of the incident. There was no record of any mechanical discrepancies to the tail rotor assembly, including the non-rotating swashplate, when the overhauled tail rotor gearbox was installed.

NTSB Probable Cause

The loss of tail rotor effectiveness during cruise flight due to the bellcrank pivot pin becoming separated from the non-rotational swashplate. The plain brass liner of the non-rotating swashplate and the bellcrank pivot pin were worn.

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