ERA16FA144

An inflight loss of engine power due to a failure of the engine fuel pump, which resulted in a collision with trees and terrain during the subsequent autorotation. The failure of the engine fuel pump resulted from the absence of adequate grease leading to accelerated spline wear within the fuel pump.

The commercial pilot and four passengers departed in the helicopter on a local air tour flight. A witness observed the helicopter at a low altitude in a descent and noted that it sounded unusual. He then heard the engine go silent, which was followed by sounds associated with impact. Examination of the accident site revealed that the helicopter initially impacted trees near the top of a ridge at an elevation of about 1,100 ft mean sea level and came to rest in a wooded area near the bottom of the ridge. The main wreckage was mostly consumed by a postcrash fire. The main and tail rotor blades exhibited signatures of low rotational energy consistent with unpowered ground impact damage. Disassembly of the engine fuel pump revealed anomalous and accelerated spline wear that was severe enough to prevent the fuel pump from delivering fuel to the engine, resulting in a total loss of engine power. The wear on the splines was likely accelerated due to a lack of grease. Remnant material on the fuel pump splines was consistent with grease being present at some time on the drive gear and drive shaft splined connection, but it could not be determined if it was from the last overhaul which was performed about 8 years and 1,078 flight hours before the accident or from an earlier overhaul. Drive gear spline impressions on the drive shaft spacer were consistent with an erroneously selected spacer. The incorrectly-sized spacer could have resulted in a gap between the spacer and drive gear that provided a path for grease that was applied on the splines to escape. According to records from the most recent fuel pump overhaul and the overhaul facility, the spacer was not replaced during the last overhaul. Because overhaul records preceding the most recent fuel pump overhaul were not available, it could not be determined when the incorrectly-sized spacer was introduced into the fuel pump assembly. Following an inflight loss of engine power, an autorotative landing can be accomplished if the pilot successfully enters the autorotation, the helicopter has adequate altitude and airspeed, and rotor rpm is managed throughout the autorotative descent. The lack of crash-resistant data and cockpit image recorders, radar data, or surveillance videos precluded determination as to why the pilot was not able to successfully complete an autorotation. Based on the described injuries in the autopsy reports, the accident was likely survivable for the pilot and one passenger had they received immediate emergency treatment and not been subject to a postcrash fire. One of the passengers would not have survived based on their injuries. The investigation was unable to determine whether two of the passengers would have been able to survive their injuries had they not been subject to a postcrash fire. The helicopter was not equipped, nor was it required to be equipped, with a crash resistant fuel system. Crash resistant fuel systems are intended to reduce the risk of fuel spillage and provide additional egress time for occupants. At the time of the accident, a modification to the accident make and model helicopter was available from the manufacturer (at a cost) to incorporate a fuel system with improved crash resistance. This modification was not certified to the current airworthiness standards but was tested to similar standards and would have improved the crash resistance of the helicopter's fuel system. Due to the lack of recorded data, such as a flight data recorder or videos capturing the crash, the conditions at the time of impact, such as helicopter attitude and airspeed, are unknown. Therefore, the effectiveness of a crash resistant fuel system, either one that met the current airworthiness standards or the available manufacturer modification, in preventing a postcrash fire could not be determined in this investigation.

The helicopter was not equipped nor was it required to be equipped with any crash-resistant data or image recorders. Fuel Pump Overhaul A discussion with IGS personnel about their overall experiences with overhaul of the fuel pump revealed that they had seen wear similar to the accident fuel pump drive shaft spline wear but not to the severity of the wear observed on the drive shaft from the accident fuel pump. An IGS mechanic who specialized in overhauling fuel pumps similar to the one on the accident helicopter stated that he had seen drive shaft spline wear "about 4 or 5 times" since 2008, and a lack of lubricant or corrosion was associated with the spline wear. Additionally, the IGS mechanic stated he had only observed wear on the smaller splines of the drive shaft and had not observed wear on the larger splines of the drive shaft (the splines that mate to the engine accessory gearbox drive gear). According to IGS, fuel pumps that are submitted for overhaul are visually examined for evidence of spline or gear tooth wear; if wear is observed, the part is rejected. Fuel System Crash Resistance On October 3, 1994, the FAA introduced improved fuel system crash resistance standards for normal category helicopters via Amendment 27-30 to Part 27 of the Federal Aviation Regulations. The standards for fuel system crash resistance, 14 CFR 27.952, are intended to minimize fuel spillage near ignition sources in order to increase the evacuation time available for crew and passengers to escape a postcrash fire. The improved crash resistance standards were not retroactively applicable to either existing helicopters or newly manufactured helicopters whose certification basis and approval predated the effectivity of Amendment 27-30. The Bell 206L model was type certificated in September 1975 under type certificate data sheet No. H2SW. The accident helicopter was not manufactured, nor was it required to be manufactured, with a fuel system meeting the provisions of 14 CFR 27.952. According to Bell Helicopter, the 206L helicopter was originally equipped with a thin-wall bladder-type fuel cell. These original fuel bladders were not nor were they required to be drop-tested. Additionally, although frangible structural interfaces were used at major interfaces, such as the filler cap adapter, the vent and fuel feed lines were rigid tubes. In March 1994, Bell Helicopter released Service Instruction No. BHT-206-SI-2043 to improve the crash resistance of the original fuel system for 206L and 206L-1 helicopters via incorporation of a retrofit kit. While this retrofit kit was not certified to the provisions of 14 CFR 27.952, it introduced fuel bladders with a higher puncture resistance than the original fuel bladders, replacement of rigid interconnect tubes with stainless steel braided hoses, and frangible structural attachments to the fuel bladders. Additionally, the improved fuel bladders were drop-tested at 80% capacity from a height of 50 ft. According to Bell Helicopter, the retrofit kit has been available since 1994, but they did not have a record of selling one of these retrofit kits. A review of the helicopter's maintenance history revealed no record of modifications to the original fuel system to improve its crash resistance. Autorotation Information According to the FAA Rotorcraft Flying Handbook, "…an autorotation is a descending maneuver where the engine is disengaged from the main rotor system and the rotor blades are driven solely by the upward flow of air through the rotor. In other words, the engine is no longer supplying power to the main rotor. The most common reason for an autorotation is an engine failure…At the instant of engine failure, the main rotor blades are producing lift and thrust from their angle of attack and velocity. By immediately lowering collective pitch, which must be done in case of an engine failure, lift and drag are reduced, and the helicopter begins an immediate descent, thus producing an upward flow of air through the rotor system…" The Bell 206L rotorcraft flight manual states that in the event of an engine failure in flight, an autorotation should be performed by adjusting collective pitch control to maintain a main rotor speed of 90-107% rpm and cyclic control to obtain the desired autorotative airspeed for the condition (with a stated "normal autorotation airspeed" of 70 mph or 61 knots). The 1615 weather observation at Gatlinburg-Pigeon Forge Airport (GKT), Sevierville, Tennessee, located about 3 nautical miles northeast of the accident site, reported wind from 220° at 10 knots, 10 statute miles visibility, clear skies, temperature 24°C, dew point 2°C, and an altimeter setting of 29.93 inches of mercury. The Regional Forensic Center Knox County, Knoxville, Tennessee, performed autopsies on the pilot and the four passengers. According to the autopsy reports, all five helicopter occupants suffered blunt force injuries, but three of the five died primarily from thermal injuries; one died as a result of combined thermal and blunt force injuries (pilot); and one died as a result of blunt force injuries. [For additional information pertaining to the occupants' injuries, see the NTSB Injury Factual Report in the public docket for this investigation.] Toxicological testing performed by the FAA Bioaeronautical Science Research Laboratory, Oklahoma City, Oklahoma, on specimens obtained from the pilot were negative for all tested drugs. According to Federal Aviation Administration (FAA) airmen records, the pilot held commercial pilot and flight instructor certificates with ratings for rotorcraft-helicopter and instrument-helicopter. He reported a total flight experience of 550 hours, with 300 hours accumulated during the preceding 6 months on the application for his most recent FAA second-class medical certificate, which was issued on April 21, 2015. According to company records, the pilot was hired in April 2015. He satisfactorily completed a factory Bell Helicopter 206L pilot transition course on April 10, 2015 and received a logbook endorsement that noted satisfactory completion of a flight review in accordance with 14 CFR section 61.56 on that date. Review of the pilot's logbook revealed that, as of March 25, 2016, he had logged about 1,310 hours of total flight experience, which included about 875 hours in Bell 206-series helicopters. According to the operator's helicopter log, the pilot flew the accident helicopter on several occasions between March 25 and the accident flight; however, the flight time that was accumulated by the pilot during these flights could not be determined. The seven-seat helicopter was manufactured in 1977 and issued an FAA standard airworthiness certificate in the normal category on March 1, 1977. It was equipped with a two-blade main rotor system and a two-blade tail rotor system that were powered by a 420-horsepower Rolls-Royce (formerly Allison) 250-C20B turboshaft engine. Review of maintenance information revealed that, at the time of the accident, the helicopter had accumulated about 22,562 total hours, and the engine had accumulated about 8,550 total hours. The helicopter had been operated for about 40 hours since its most recent 100-hour and annual inspections, which were signed-off concurrently on March 4, 2016. According to engine records, the engine fuel pump, model number 386500-5, serial number T103542, was manufactured by the Power Accessories Division of TRW, Inc., later known as Argo-Tech Corporation and now a part of Eaton Corporation. The fuel pump was installed on the engine on June 23, 2009, at an engine total time of 7,472.0 flight hours. A maintenance record entry stated that, at the time of installation, the fuel pump had 0 flight hours since overhaul. Based on the engine total time at the time of the accident, the fuel pump had accumulated about 1,078 flight hours since its last overhaul. According to the Rolls-Royce M250-C20 series maintenanc