LAX01FA306

The failure of the mechanic to secure the No. 8 bearing thrust nut during maintenance work, which resulted in the total failure of the first stage turbine wheel shaft. Also causal was his misjudgment of his height during the flare resulting in a hard landing. A contributing factor was the high vegetation in the forced landing field, which resulted in a hard landing.

The pilot made a mayday call, shut off the fuel valve, and performed an autorotation into high grass after the single engine helicopter experienced a loss of engine power shortly after takeoff. The pilot had heard a bang followed by illumination of the engine out warning light, and the engine began smoking. During the terminal phase of the autorotation, the pilot assumed the grass height was 3 feet and flared accordingly; however, the grass was at least 6 feet tall. Post-accident examination of the engine revealed that the No. 1 turbine wheel was missing. The No. 8 bearing thrust nut exhibited a locking indentation and a notch on its inside surface. Additionally, indentations were on the threads of the threaded end of the first stage turbine wheel shaft. These marks suggest that the thrust nut was not fully engaged on the first stage turbine shaft. The location of the notch suggests that the edge of the nut protruded beyond the end of the turbine shaft. There were no indications of contact by the shaft threads on the inner surface of the deformed area to suggest that it had been displaced into a groove on the first stage turbine shaft. Review of the maintenance records and interviews with the mechanic, who performed the most recent maintenance work, revealed that the turbine section had been replaced approximately 30 hours prior to the accident. Due to excessive oil consumption, maintenance personnel removed the turbine and replaced the No. 5 carbon seal approximately 14 hours prior to the accident. The maintenance records indicated that the day before the accident, the sump cover had been removed for trouble shooting oil consumption. Though it is not detailed in the maintenance entry, it is interpreted that the "sump cover" referred to is the No. 8 Bearing sump nut since the thrust nut was loose.

HISTORY OF FLIGHT On September 29, 2001, at 1423 Hawaiian standard time, a Bell 206B single engine helicopter, N206KS, experienced a loss of engine power and made a forced landing about 5 miles south of Hilo, Hawaii. The helicopter was registered to and operated by K&S Helicopters, Inc., Hilo, as an on-demand sightseeing flight under the provisions of 14 CFR Part 135. The commercial pilot and two passengers were not injured; however, the remaining two passengers sustained minor injuries. The helicopter was destroyed in the collision sequence and post-accident fire. The local flight departed Hilo about 1420. Visual meteorological conditions prevailed, and a company VFR (visual flight rules) flight plan had been filed. The accident site was located at 19 degrees 38.425 minutes north latitude and 152 degrees 02.088 minutes west longitude. The Federal Aviation Administration (FAA) inspector, who traveled to the accident site, interviewed the pilot and examined the wreckage on scene. The pilot reported that she heard a bang followed by illumination of the engine out warning light, and the engine began smoking. She made a mayday call, shut off the fuel valve, performed an autorotation, and landed in high grass. The pilot thought that the grass was 2-to 3-feet tall; however, the grass was actually 6 feet tall. She and the passengers evacuated the helicopter, and a couple of the passengers received minor burns from molten plastic or metal that was dripping from the engine deck, which was on fire. The pilot and passengers experienced difficulties as they attempted to gain distance from the helicopter in the tall grass; however, they all managed to obtain a safe distance from the burning helicopter. PERSONNEL INFORMATION The pilot held commercial helicopter and single engine airplane pilot certificates with instrument airplane and helicopter ratings. She also held an instructor rating for helicopters. She was issued a second-class medical certificate on November 6, 2000, with a limitation that she wear corrective lenses. The pilot obtained her last biennial flight review on June 29, 2001, in a Bell 206 helicopter. According to the Pilot/Operator Aircraft Accident Report (NTSB form 6120.1/2), the pilot had accumulated 2,955 hours of total aircraft flight time, of which 2,795 hours were obtained in rotorcraft, and 1,255 hours were obtained in the same make and model as the accident helicopter. AIRCRAFT INFORMATION The single engine helicopter utilized a 420 shaft horsepower Allison 250-C20B turboshaft engine (serial number CAE831174). The engine is a two-shaft turboshaft engine with a combination compressor (6-stage axial compressor attached to a 1-stage centrifugal compressor). It has a reverse-flow annular combustor, a two-stage high-pressure turbine (also referred to as the gas producer turbine or the N1 turbine), and a two-stage low-pressure turbine (also referred to as the power turbine or the N2 turbine). The gas path of the Allison 250-C20B engine flows into the inlet, through the compressor, and then is taken to the combustor at the rear of the engine through two external transfer tubes. The gases then turn (180 degrees) forward and flow through the gas producer turbine and the power turbine. Finally, the gasses are directed out the exhaust ducting. The flow of hot gases coming from the combustor is such that the first stage turbine nozzle shield is the first thing encountered, behind which, is the No. 8 bearing sump nut. The No. 8 bearing sump nut resembles a cover or cap; not what one would picture as a typical nut. Behind the bearing sump nut is the No. 8 bearing thrust nut, No. 8 bearing, and No. 8 bearing thrust plate. A review of the helicopter's maintenance records revealed the helicopter underwent its last annual inspection on July 1, 2001, at an engine total time of 5,190.0 hours, and underwent its last 100-hour inspection on September 20, 2001, at an engine total time of 5,441.5 hours. A review of the daily flight logs revealed that during the period of September 18 through September 27, the helicopter had been flown for 11.1 hours, and 12.5 quarts of oil were added. On September 27, the helicopter flew for 0.6 hours with 1 quart of oil consumed. The engine operation and maintenance manual prescribes a maximum oil consumption of 1 quart of oil per flight hour. The following is the oil addition information and flight times (at the beginning of each day) taken from the flight logs: DATE: TOTAL TIME AT START OF DAY: OIL ADDED: September 18, 2001 5,438.2 Hours 2 quarts September 22, 2001 5,444.0 Hours 1 quart September 24, 2001 5,447.5 Hours 5 quarts September 25, 2001 5451.0 Hours 3.5 quarts September 26, 2001 5453.6 Hours 0.5 quarts September 27, 2001 5454.7 Hours 1 quart On September 8, 2001, at a helicopter total time of 5,426.1 hours, a flight log maintenance entry indicated that the turbine (serial number CAT 37394) was removed for overhaul and No. 1 and No. 2 wheel change. Another turbine (serial number CAT 36018) was installed at a turbine total time of 4,410.1 hours. That same entry indicated that the installed No. 3 and No. 4 turbine wheels had accumulated 74.0 hours since new, and the No. 1 and No. 2 turbine wheels had accumulated 0.0 hours since new. After this turbine work was complete, the helicopter was flown on a 0.2-hour check flight and was signed off with no discrepancies noted. The helicopter accumulated approximately 30 hours after this maintenance work was performed. On the September 18, 2001, flight log (helicopter total time of 5,441.5 hours), a maintenance entry, dated September 20, 2001, indicated, "engine seem to consume more oil than normal (not smoking). Removed turbine, replaced No. 5 carbon seal, installed turbine." This work took place approximately 14 hours prior to the accident. Review of the turbine assembly inspection - maintenance - overhaul records for serial number CAT 36018, revealed that maintenance personnel replaced the No. 1 and No. 2 turbine wheels with new wheels and a new tie bolt (part number E6898784, serial number A63-128) on September 1, 2001. On September 16, 2001, they replaced the "No. 3 and No. 4 turbine wheel package and carbon seal" for "smoking and high operating temp for trouble shooting." On September 27, 2001, they "replaced sump cover for trouble shooting oil consumption 1 quart." The FAA inspector, who traveled to the accident site, spoke with the mechanic who performed the aforementioned work. The inspector asked if there had been any recent maintenance performed on the accident helicopter. The mechanic reported on the replacement of the turbine wheels, and approximately 4 hours prior to the accident, he replaced a turbine oil seal because of smoking and high oil consumption. The FAA inspector was told that a test flight was conducted after the oil seal replacement, a relocation flight was conducted, and the helicopter was returned to service. The helicopter then underwent a tour flight earlier on the day of the accident, and was on its second tour flight since the seal replacement when the accident occurred. On the morning of the accident, the flight log listed the helicopter total time as 5,455.3 hours. WRECKAGE AND IMPACT INFORMATION The FAA inspector observed that the tail boom separated from the fuselage at a point just aft of the baggage compartment. The rearmost portion of the tail boom, aft of the horizontal stabilizer, separated from the forward portion and was within 10 feet of the main wreckage; the middle section was about 50 yards away. Visual external examination of the thermally damaged engine disclosed that the first stage turbine wheel was missing. The FAA inspector found a section of what he thought to be turbine wheel rim in the wreckage, in an area that was formerly the baggage compartment. The engine and the recovered turbine wheel secti